2nd Year Chemistry: Periodic Classification Of Elements And Periodicity PDF

CHAPTER Periodic Classification Of 1 Elements And Periodicity Animation 1.1 : Periodic Table Source and Credit: eLearn.Punjab 1. Periodic Classification of Elements and Periodicity eLearn.Punjab I...

CHAPTER Periodic Classification Of 1 Elements And Periodicity Animation 1.1 : Periodic Table Source and Credit: eLearn.Punjab 1. Periodic Classification of Elements and Periodicity eLearn.Punjab IN THIS CHAPTER YOU WILL LEARN 1. To describe the periodic table in terms of groups and periods. 2. To describe and explain periodicity in physical and chemical properties. 3. To describe the position of hydrogen in the periodic table. 1.1 INTRODUCTION To achieve a thorough understanding of a complex subject like chemis- try, it would be highly desirable to fit all the facts into a simple logical pattern. The periodic table of elements has served the purpose to systematize the properties of the elements for well over 100 years.The development of pe- riodic table is one of the most significant achievements in the history of chemical sciences. The Periodic Table provides a basic framework to study the periodic behaviour of physical and chemical properties of elements as well as their compounds. In previous classes, you have learnt about the periodic classification of elements. This chapter describes in more detail the periodic table and the periodicity of elements. 1.1.1 Historical Background The early history of ideas leading up to the Periodic classification of elements is fascinating but will not be treated in detail.Those who made memorable contributions in this field are Al-Razi,Dobereiner,Newland and Mendeleev. Al-Razi’s classifications was based on the physical and chemical properties of substances. Dobereiner, a German chemist in 1829, arranged then known elements in group called Triads, as each contained three elements with similar properties. Newland who was an English chemist , in 1864, classified 62 elements, known at that time , in increasing order of thier atomic masses. He noticed that every eighth element had some properties in common with the first one. The principle on which this classification is based was called the Law of Octaves. 2 1. Periodic Classification of Elements and Periodicity eLearn.Punjab In 1871, a Russian Chemist, Dmitri Mendeleev, gave a more useful and comprehensive scheme for the classification of elements. He presented the first regular periodic table in which elements of similar chemical properties were arranged in eight vertical columns called Groups.The horizontal rows of the table were called Periods. Mendeleev also started by arranging the elements in ascending order of their atomic masses and found that elements having similar chemical properties appeared at regular intervals. This significant observation was called Periodic Law. Mendeleev left some gaps in his table for elements, which had not yet been discovered, and by considering their positions in the periodic table, he predicted properties of these elements. For example, germanium was not known at that time, but Mendeleev was confident that this element must exist so he predicted its properties. A few years later, germanium was indeed discovered and a remarkable agreement was found with Mendeleev’s predictions. 1.1.2 Improvements In Mendeleev 's Periodic Table In order to make the periodic table more useful and accurate, a few improvements were made in Mendeleev s periodic table. After the discovery of atomic number by Moseley in 1911, it was noticed that elements could be classified more satisfactorily by using their atomic numbers, rather than their atomic masses. Hence, the periodic table was improved by arranging the elements in ascending order of their atomic numbers instead of their atomic masses. This improvement rectified a number of confusions present in the old periodic table.The modern Periodic Law states that: “if the elements are arranged in ascending order of their atomic numbers, their chemical properties repeat in a periodic manner” Another improvement was the addition of an extra group (group VIIIA) at the extreme right of the periodic table. This group contains noble gases, which had not been discovered in Mendeleev’s time. 3 1. Periodic Classification of Elements and Periodicity eLearn.Punjab Another confusion in Mendeleev’s table was that elements like Be, Mg, Ca, Sr, Ba and Zn, Cd, Hg were placed in a single vertical group, while according to their properties they belonged to two different categories. The same was true for so many other elements placed in the same vertical group. In modern periodic table, the confusion was removed by dividing the elements in two types of vertical groups, A and B. In modern periodic table, Be, Mg, Ca, Sr and Ba are placed in group IIA and Zn, Cd, Hg in group IIB. 1.2 THE MODERN PERIODIC TABLE In modern periodic table (see periodic table) all the elements are arranged inascending order of their atomic numbers. Followings are the essential features of the periodic table. 1. Group and Periods Elements with similar properties are placed in vertical columns called Groups. There are eight groups ,which are usually numbered by Roman numerals I to VIII.Each group is divided into two subgroups, designated as A and B subgroups. The subgroups, containing the representative or normal elements are labelled as A subgroups, whereas B subgroup contain less typical elements, called transition elements and are arranged in the centre of the periodic table. The horizontal rows of the periodic table are called Periods.The essential features of periods are as follows: a) There are 7 periods in the periodic table numbered by Arabic numerals 1 to 7. b) The period 1 contains only two elements, hydrogen and helium. c) The periods 2 and 3 contain eight elements each and are called short periods. All the elements in these periods are representative elements and belong to A subgroup. In these periods, every eighth element resembles in properties with the first element. As lithium and beryllium in the 2nd period resemble in most of their properties with sodium and magnesium of the 3rd period, respectively. Similarly, boron and aluminium both show oxidation state of +3, fluorine in 2nd period has close resemblances with chlorine of 3rd period. 4 1. Periodic Classification of Elements and Periodicity eLearn.Punjab Table 1.1 MODERN PERIODIC TABLE OF THE ELEMENTS 5 1. Periodic Classification of Elements and Periodicity eLearn.Punjab d) The periods 4 and 5 are called long periods. Each long period consists of eighteen elements. Out of these, eight are representative elements belonging to A subgroup similar to second and third periods. Whereas the other ten elements, placed in the centre of the table belong to B subgroups and are known as transition elements. In these periods, the repetition of properties among the elements occurs after 18 elements. As after 19K (having atomic number 19) the next element with similar properties is 37Rb. e) The period 6 is also a long period, which contains thirty-two elements. In this period there are eight representative elements, ten transition elements and a new set of fourteen elements called Lanthanides as they start after 57 La. Lanthanides have remarkably similar properties and are usually shown separately at the bottom of the periodic table. f ) The period 7 is incomplete so far. It contains only two normal elements 87Fr and 88Ra, ten transition elements and fourteen inner transition elements. The inner transition elements of this period are called Actinides, as they follow 89 Ac.The actinides are also shown at the bottom of the periodic table under the Lanthanides. Due to their scarcity, the inner transition elements are also called rare earth elements. 2. Some More Families in the Periodic Table: While studying about periods you have noticed that certain rows of elements with similar properties have assigned common names such as transition elements, Lanthanides, Actinides or Rate Earth elements.Similarly, due to their peculiar characteristics, some typical elements belonging to sub-groups A, have also been assigned family names. For example,elements of the group IA are called Alkali Metals, because of their property to form strong alkalies with water. 2Na +2H2O ——-—> 2NaOH + H2 Similarly,due to their presence in Earth’s crust and alkaline character,the elements of group IIA are known as Alkaline Earth Metals. Another important family in the periodic table is Halogen family. The name “Halogens” is given to the elements of group VIIA, due to their salt forming properties. As the gases of group VIIIA ‘are least reactive they are called “Noble Gases”,These family names are useful for a quick recognition of an element in the periodic table. 6 1. Periodic Classification of Elements and Periodicity eLearn.Punjab 3. Blocks in the Periodic Table Elements in the periodic table can also be classified into four blocks. This classification is based upon the valence orbital of the element involved in chemical bonding. According to this classification, elements of IA and IIA subgroups are called s-block elements because their valence electrons are available in s orbital.The elements of IIIA to VlllA subgroups (except He) are known as p-block elements as their valence electrons are present in p orbital. Similarly in transition elements, electrons in d-orbital are responsible for their valency hence they are called d-block elements. For Lanthanides and Actinides valence electrons are present in f- orbital hence these elements are called f-block elements. This classification is quite useful in understanding the chemistry of elements and predicting their properties especially the concept of valency or oxidation state. 4. Metals, Non-metals and Metalloids Another basis for classifying the elements in the periodic table is their metallic character. Generally, the elements on the left hand side, in the centre and at the bottom of the periodic table are metals, while the non-metals are in the upper right corner of the table. Some elements, especially lower members of groups, III A, IVA and VA(as shown in Table 1.1) have properties of both metals as well as non-metals. These elements are called semi-metals or metalloids. In the periodic table elements of groups IVA to VIIIA, at the top right hand corner above the stepped line, are non-metals. The elements just under the “steps’ such as Si, As, and Te are the metalloids. All the remaining elements, except hydrogen, are metals. 1.3 PERIODIC TRENDS IN PHYSICAL PROPERTIES As you have studied so far that in modern periodic table the elements are arranged in ascending order of their atomic numbers and their classification in groups and periods is based on the similarity in their properties. Yet, due to the gradual increase in the number of protons in the nucleus and electrons in outer shells the physical and chemical properties of the elements steadily vary within a group or a period. Here, we study some trends in physical properties. 7 1. Periodic Classification of Elements and Periodicity eLearn.Punjab 1. ATOMIC SIZE a) Atomic Radius: Atoms are so small that it is impossible to see an atom even with a powerful optical microscope. The size of a single atom therefore cannot be directly measured. However, techniques have been developed which can measure the distance between the centres of two bonded atoms of any element. Half of this distance is considered to be the radius of the atom. In the periodic table, the atomic radius increases from top to bottom within a group due to increase in atomic number. This is because of the addition of an extra shell of electrons in each period. In a period, however, as the atomic number increases from left to right, the atomic radius decreases. This gradual decrease in the radius is due to increase in the positive charge in the nucleus. As the positive nuclear charge increases, the negatively charged electrons in the shells are pulled closer to the nucleus. Thus, the size of the outermost shell becomes gradually smaller. This effect is quite remarkable in the elements of longer periods in which “d” and “f ” subshells are involved. For example, the gradual reduction in the size of Lanthanides is significant and called Lanthanide Contraction. b) Ionic Radius: When a neutral atom loses one or more electrons, it becomes a positive ion. The size of the atom is decreased in this process because of the two reasons. 8 1. Periodic Classification of Elements and Periodicity eLearn.Punjab First the removal of one or more electrons from a neutral atom usually results in the loss of the outermost shell and second, the removal of electrons causes an imbalance in proton-electron ratio. Due to the greater attraction of the nuclear charge, the remaining electrons of the ion are drawn closer to the nucleus. Thus, a positive ion is always smaller than the neutral atom from which it is derived. The radius of Na is 157pm and the radius of Na+ is 95pm. On the contrary, a negative ion is always bigger than its parent atom. The reason is that addition of one or more electrons in the shell of a neutral atom enhances repulsion between the electrons causing expansion of the shell. Thus, the radius of fluorine atom is 72pm and that of the fluoride ion (F ) is 136pm. In a group of the periodic table, similar charged ions increase in size from top to bottom. Whereas within a period, isoelectronic positive ions show a decrease in ionic radius from left to right, because of the increasing nuclear charge. The same trend is observed for the isoelectronic negative ions of a period; ionic size decreases from left to right. The variations in atomic and ionic radii of alkali metals and halogens are shown in Fig 1.1 and Fig.1.2. 9 1. Periodic Classification of Elements and Periodicity eLearn.Punjab 2. Ionization Energy The ionization energy of an element is the minimum quantity of energy which is required to remove an electron from the outermost shell of its isolated gaseous atom in its ground state. The ionization energy of sodium is 496kJ mol-1. Na(g) → Na+ (g) + e- i = 496 kJ mol-1 Elements with greater number of electrons have more than one values of ionization energy. So for magnesium, the first ionization energy value is the energy required to remove the first electron: Mg (g) → Mg+ (g) + e- i1 = 738 kJ mol-1 Similarly, the second ionization energy value is the energy required to remove the second electron. Mg+ (g) → Mg++ + e- i2= 1451kJ mol-1 a) Variation Within a Group: The factors upon which the ionization energy of an atom mainly depends are magnitude of nuclear charge, size of the atom, and the “shielding effect”. The shielding effect is actually the repulsion due to electrons in between the nucleus and the outermost shell. This effect increases, as the size of the atom increases due to addition of an extra shell successively in each period hence more number of electrons shields the nucleus. Fig. 1.3 Ionization energies of alkali metals 10 1. Periodic Classification of Elements and Periodicity eLearn.Punjab Going down in a group, the nuclear charge increases but as the size of the atom and the number of electrons causing the shielding effect also increases therefore ionization energy decreases from top to bottom. That is why in alkali metals, for example, it is easier to remove an electron from caesium atom than from lithium atom. The change in ionization energies of IA elements is shown in Fig. 1.3. b) Variation Across a Period: Generally, smaller the atom with greater nuclear charge, more strongly the electrons are bound to the nucleus and hence higher the ionization energy of the atom. By moving from left to right in a period, the outer shell remains the same, while the nuclear charge increases effectively that makes the removal of an electron difficult and hence the value of ionization energy increases. Fig. 1.4 Ionization energies of elements of short periods. 11 1. Periodic Classification of Elements and Periodicity eLearn.Punjab Although, the number of electrons also increases in this case but the shielding is not very effective within the same shell. The trend of ionization energies of short periods is shown in Fig.1.4 The figure also reveals that inert gases have the highest values of ionization energy because due to complete outermost shell in them, the removal of electron is extremely difficult. 3. Electron Affinity (E.A) The electron affinity is the energy released or absorbed, when an electron is added to a gaseous atom to form a negative ion. F (g) +e- → F-(g) E.A= -337 kJ mol-1 Energy is usually released when electronegative elements absorb the first electron and E.A. in such cases is expressed in negative figures, as in the case of halogens. When a second electron is added to a uninegative ion, the incoming electron is repelled by the already present negative charge and energy is absorbed in this process. O(g) + e- →O - (g) E.A1= -141 kJ mol-1 O- (g) + e- →O 2- (g) E.A2= +780 kJ mol-1 The absorbed energy is expressed as the electron affinity in positive figures. Electron affinity depends upon size of the atom, nuclear charge and vacancies in the outermost shell. Relatively smaller atoms with one or two vacancies in the outermost shell show large values of electron affinity. 12 1. Periodic Classification of Elements and Periodicity eLearn.Punjab Electron affinity generally increases with increasing atomic number within a period and decreases from lighter to heavier elements in a given group of the periodic table. Knowledge of electron affinities can be combined with the knowledge of ionization energies to predict which atoms can easily lose electrons and which can accept electrons more readily. 4. Metallic and Non-Metallic Character It has already been discussed in this chapter that elements of periodic table can be divided into metals, non-metals and metalloids. Chemically all the elements which have a tendency to form positive ions by losing electrons are considered metals. All metals are good conductor of heat and electricity. A characteristic property of metals is that they form basic oxides which give bases when dissolved in water. Na2O (s) + H2O (l) →2 NaOH (aq) As it becomes easier to remove the electron of an atom bigger in size, therefore metallic character increases from top to bottom in a given group of elements. On the contrary, it decreases from left to right across a period. The elements of group VIIA (the halogens) are least metallic in nature.The elements which gain electrons and form negative ions are called non-metals. All the gases are non-metals. The non-metals are normally poor conductor of heat and electricity. Non- metals form acidic oxides which yield acids on dissolving in water. SO3 (g) + H2O (l) →2 H SO (aq) 2 4 13 1. Periodic Classification of Elements and Periodicity eLearn.Punjab Non-metallic character of an element, decreases as the atomic size increases. Therefore in a group of non-metals like halogens, the non-metallic character decreases from top to bottom. The member at the top, fluorine, is the most non-metallic element of the periodic table. This trend can also be verified in the elements of groups VA and VIA. Nitrogen and oxygen are pure non-metals and usually exist in gaseous state while bismuth and polonium, the members at the bottom of these groups, are fairly metallic in nature. 5. Melting And Boiling Points Melting and boiling points of elements tell us something about how strong the atoms or molecules in them are bound together. (a) Variation in a Period Across the short periods, the melting and boiling points of elements increase with the number of valence electrons upto group IVA and then decrease upto the noble gases. The melting points of group IA elements are low because each atom in them provides only one electron to form a bond with other atom. Melting points of group IIA elements are considerably higher than those of group IA elements because each atom in them provides two binding electrons. Fig. 1.5 Variation of melting points with atomic number 14 1. Periodic Classification of Elements and Periodicity eLearn.Punjab Since carbon has the maximum number of binding electrons, thus it has a very high melting point in diamond in which each carbon is bound to four other carbon atoms. In general, the elements which exist as giant covalent structures have very high melting points, Fig. 1.5. An important change occurs when we move from group IVA to groups VA, VIA, VIIA as the lighter elements of these group exist as small, covalent molecules, rather than as three dimensional lattices. For instance, nitrogen,oxygen and fluorine exist as individual molecules which have very weak intermolecular forces between them. Consequently, their melting and boiling points are extremely low. (b) Variation in a Group The melting and boiling points of IA and IIA group elements decrease from top to bottom due to the increase in their atomic sizes. The binding forces present between large sized atoms are relatively weaker as compared to those between smaller atoms, Fig. 1.6. For elements of group VIIA, which exist in the form of molecules, the melting and boiling points increase down the group, Fig. 1.7. This is because large molecules exert stronger force of attraction due to their higher polarizabilities. Fig.1.6 Melting points of Group IIA elements. 15 1. Periodic Classification of Elements and Periodicity eLearn.Punjab 6. Oxidation State The oxidation state of an atom in a compound is defined as the charge (with the sign), which it would carry in the compound. In ionic compounds, it is usually the number of electrons gained or lost by the atom. As in the case of sodium chloride, the oxidation states of sodium and chlorine are + 1 and -1, respectively. In covalent compounds, it is decided on the basis of the difference in their relative electronegativities. For example, SnCl4 is a covalent compound. The oxidation state of tin is + 4 and that of chlorine is -1. The oxidation state of an element is zero in its free state. The oxidation state of a typical element is directly or indirectly related to the group number to which the element belongs in the periodic table. The elements of group IA to IVA have the same oxidation states as their group numbers are. Just as B, Al and Ga belong to group IIIA, hence, they always show oxidation state of +3. So, for the elements Fig.1.7. Bloiling (.---------) and melting points(;_______) of halogens. 16 1. Periodic Classification of Elements and Periodicity eLearn.Punjab of these groups, the oxidation state is same as the number of electrons present in the valence shells of the elements. However, for the elements of group VA, the oxidation states are either the number of electrons present in the valence shell (which is same as their group number) or the number of vacancies available in these shells. For example, N, P, As and Sb frequently show +3 as well as +5 oxidation states. Elements of group VIA show almost similar behaviour. In H2SO4, sulphur shows the oxidation state of +6, which is the number of electrons in its outermost shell whereas its oxidation state is -2 in H2S, which is the number of vacancies in the shell. In group VIIA elements oxidation state is mostly - 1, which is again the number of vacancies in their outermost shells. Group VIIIA elements, which are also called zero group elements, usually show zero oxidation state because there is no vacancy in their outermost shells. Transition elements, which are shown in B subgroups of the periodic table, also show the oxidation states equal to their group number as it can be seen for Cu(I), Zn(II), V(V), Cr(VI) and Mn (VII). But due to greater number of valence electrons available in partly filled d-orbitals these elements usually, show more than one oxidation states in their compounds. 7 Electrical Conductance One of the most familiar properties of metals is their ability to conduct electricity. This property is mainly due to the presence of relatively loose electrons in the outermost shell of the element and ease of their movement in the solid lattice. The electrical conductance of metals in groups IA and IIA, generally increases from top to bottom. However, the trend is not free from the individual variation in different atoms. Metals of group IB, which are known as coinage metals, have extraordinary high values of electrical conductance. Non-metals, on the other hand, especially of groups VIA and VIIA, show such low electrical conductance that they can be considered as nonconductors. 17 1. Periodic Classification of Elements and Periodicity eLearn.Punjab In the series of transition metals, the values of electrical conductance vary so abruptly that no general trend can be assigned to them. Carbon, in the form of diamond is non-conductor because all of its valence electrons are tetrahedrally bound and unable to move freely, while in the form of graphite, carbon is fairly good conductor because one of its four valence electrons is relatively free to move. The lower elements of group IVA, tin and lead, are fairly good conductors and their values of electrical conductivity are comparable with those of their counterparts in group IA. 8 Hydration Energy The hydration energy is the heat absorbed or evolved when one mole of gaseous ions dissolve in water to give an infinitely dilute solution. For example, when one mole of gaseous hydrogen ions are dissolved in water resulting an infinitely dilute solution, a large amount of heat is liberated: H+ (g) + H2O (l) → H3O+(aq) DHh = - 1075 kJ mol-1 Hydration energies of a few negative and positive ions are shown in the Table 1.2. Table 1.2 Hydration It is evident from the table that hydration Energies of Ions energies highly depend upon charge to size ratio of the ions. For a given set of ions, for example Hh Ion of group IA, charge to size ratio decreases from kJ mol-1 top to bottom in a group, the hydration energy Li+ -499 also decreases in the same fashion. On the Na+ -390 contrary, the hydration energy increases K+ -305 significantly by moving from left to right in Mg2+ -1891 a period as the charge to size ratio increases, as found in the metal ions of third period. Ca2+ -1562 Al3+ -4613 F- -457 Cl- -384 Br_ -351 I- -307 18 1. Periodic Classification of Elements and Periodicity eLearn.Punjab 1.4 PERIODIC RELATIONSHIP IN COMPOUNDS a) Halides: Halides are the binary compounds which halogens form with other elements. The physical properties of halides are largely determined by the nature of bonding present in them. On this basis, halides can be classified into twogeneral classes: ionic and covalent.In between the two, there is another class of halides in which the halogen atom acts as a bridge between the two atoms of the other element, such halides are termed as “Polymeric” halides.Strongly electropositive elements, having greater electronegativity difference with halogen atom, form ionic halides.The halides of group IA are considered purely ionic compounds, which are high melting point solids. Such halides have three-dimensional lattices consisting of discrete ions. Table 1.3 Melting Points of Chlorides of Period Three Elements and Their Bonding Character Name Property of compounds Melting Type of bonding point (°C) NaCl 808 Ionic MgCl2 715 Partly ionic AlCl3 192 Partly ionic SiCl4 -68 Partly covalent PCI3 -93 Partly covalent S2CI2 -80 Partly covalent 19 1. Periodic Classification of Elements and Periodicity eLearn.Punjab Among the pure ionic compounds, the fluorides have the highest lattice energies due to the small size of fluoride ion. Thus for ionic halides, the fluorides have the highest melting and boiling points which decrease in the order: fluoride > chloride > bromide > iodide. Less electropositive elements, such as Be, Ga and AI form polymeric halides having partly ionic bonding with layer or chain lattices. The lattice of SiCl4 consists of discrete molecules, which are highly polar. The bonds in PCI3, and S2Cl2 are less polar than those of SiCl4. On moving across the periodic table from left to right, the electronegativity difference reduces and the trend shifts towards covalent halides. The gradual change in bond type and melting points of the chlorides on moving across period 3 of the periodic table is shown in Table. 1.3. As the intermolecular forces in covalent halide molecules are weak van der Waal’s forces so they are often gases, liquids or low melting point solids. Physical properties of covalent halides are influenced by the size and polarizability of the halogen atom.Iodides, as being the largest and more polarizable ions, possess the strongest van der Waal’s forces and therefore have higher melting and boiling points than those of other covalent halides. The variation in bonding character is also present in descending from top to bottom in the halogen group. In general, for a metal the order of decreasing ionic character of the halides is: fluoride > chloride > bromide > iodide. For example, AlF3 is purely ionic compound having melting point 1290°C and fairly a good conductor, whereas AlI3 is predominantly covalent with melting point 198°C and electrically a non-conductor. In case of an element forming more than one halides, the metal halide in its lower oxidation state tends to be ionic, while that in the higher oxidation state is covalent. For example, PbCl2 is mainly ionic and PbCl4 is fairly covalent. This can again be explained by the high polarizing power of Pb4+ as compared to that of Pb2+. 20 1. Periodic Classification of Elements and Periodicity eLearn.Punjab b) Hydrides The binary compounds of hydrogen with other elements are called hydrides. According to the nature of bonding, hydrides may be broadly classified into three classes: ionic, covalent and intermediate. The elements of group IA and the heavier members of group IIA form ionic hydrides, which contain H- (Hydride) ion.These hydrides are crystalline solid compounds, with high melting and boiling points and which conduct electricity in molten state. The tendency towards covalent character increases by moving from left to right in the Periodic Table. Hydrides of beryllium and magnesium represent the class of intermediate hydrides. Their properties are in between the ionic and covalent hydrides. They have polymeric structures and covalent nature, Table 1.4. Table 1.4. Hydrides of the Elements of IA to VIIA and IIB Subgroups. IA IIA IIB IIIA IVA VA VIA VIIA LiH BeH2 BH3 CH4 NH3 H2O HF NaH MgH2 AIH3 SiH4 PH3 H2S HCl KH CaH2 ZnH2 GaH3 GeH4 AsH3 H2Se HBr RbH SrH2 CdH2 InH3 SnH4 SbH3 H2Te HI CsH BaH2 PbH4 BiH3 IONIC INTERMEDIATE COVALENT The covalent hydrides are usually gases or volatile liquids. They are non-conductors and dissolve in organic solvents. Their bond energies depend on the size and the electronegativity of the element. Stability of covalent hydrides increases from left to right in a period and decreases from top to bottom in a group.Fluorine forms the most stable hydride and the least stable are those of thallium, lead and bismuth. These hydrides are formed by elements with electronegativity values greater than 1.8 (Pauling Scale). Since the electronegativity of hydrogen is 2.1, most of these hydrides have polar covalent bonds in which hydrogen is carrying a slight positive charge. 21 1. Periodic Classification of Elements and Periodicity eLearn.Punjab On moving from left to right across a period the electronegativity of the other element increases and the hydrogen-element bond becomes more polar. Due to high polarity the hydrides like H2O and HF are capable of forming hydrogen bonds between their molecules. The boiling points of covalent hydrides generally increase on descending a group as shown in Table 1.5, except the hydrides like H2O, HF and NH3 which, due to hydrogen bonding, have higher boiling points than might be expected. Table. 1.5. Melting and Boiling points of Hydrides of Groups IV A and VI A Hydrides Property (Group IVA) Melting point Boiling point ‘ (°C) (°C) CH4 -184 -164 SiH4 -185 -112 GeH4 -165 -90 SnH4 -150 -52 (Group VIA) H2O 0.00 100 H2S -82.9 -59.6 H2Se -65.7 -41.3 H2Te -48 -1.8 c) Oxides Oxygen forms compounds, called oxides, with almost every other element in the periodic table. Since, many of these have quite unusual properties, there is an extensive and varied chemistry of the compounds of oxygen. Oxides can be classified in more than one ways: based upon the type of bonding they have as well as their acidic or basic character.We shall discuss here the classification based on their acidic or basic behaviour. In this chapter, you have already studied that metal oxides are basic in character as they yield bases in water and non-metallic oxides are acidic because they form acids in water.Basic oxides and acidic oxides react with one another to give salts, for example: 22 1. Periodic Classification of Elements and Periodicity eLearn.Punjab Na2O (s) + SO3 (g) → Na2SO4 (s) There is a third type of oxides, which show both acidic and basic properties, these oxides are called amphoteric oxides. The classification of elements which form oxides of acidic or basic properties is shown in Table 1.6. Table 1.6 Classification of Oxides Based on their Acid and Base Character IA IIA IIB IIIA IVA VA VIA VIIA Li Be B C N O F Na Mg AI Si P S Cl K Ca Zn Ga Ge As Se Br Rb Sr Cd In Sn Sb Te I Cs Ba Hg TI Pb Bi Po At BASIC AMPHOTERIC ACIDIC The oxides of alkali and alkaline earth metals except beryllium are basic and contain O2- ions. The O2- ion has high affinity for proton and cannot exist alone in an aqueous solution. Therefore, it immediately takes proton from water and forms OH- ion. Oxides of nonmetallic elements i.e. of C, N, P and S are acidic in nature. They generally dissolve in water to produce acidic solutions. Oxides of relatively less electropositive elements, such as BeO, Al2O3, Bi2O3 and ZnO are amphoteric and behave as acids towards strong bases and as bases towards strong acids. ZnO(s) + H2SO4 (aq) →ZnSO 4 (aq) + H2O (l) ZnO (s) + 2NaOH (aq) + H2O (l) → Na [Zn(OH) ] 2 4 (aq) 23 1. Periodic Classification of Elements and Periodicity eLearn.Punjab In a given period, the oxides progress from strongly basic through weakly basic,amphoteric, and weakly acidic to strongly acidic, e.g. Na2O, MgO, Al2O3, P4O10 , SO3 , Cl2O7.The basicity of main group metal oxides increases on descending a group of the periodic table, (e.g. BeO C18H38 Lubrication and greases Paraffin M.p. 50 - 60 C23H48 - C29H60 Wax products Asphalt, or Roofing, paving, fuel Solids Residue petroleum coke reducing agent It is refined to get different petroleum fractions. At present four oil refineries are in operation in our country. One oil refinery known as Attock Oil Refinery is located at Morgah near Rawalpindi. It has about 1.25 million tonnes oil refining capacity. Similarly, two oil refineries have been established at Karachi which have about 2.13 million tonnes of oil refining capacity. Another refinery known as Pak-Arab refinery is located at Mahmud Kot near Multan. 8 7. Fundamental Principles of Organic Chemistry eLearn.Punjab The crude petroleum is separated by fractional distillation into a number of fractions each corresponding to a particular boiling range, Table 7.1. 7.5 CRACKING OF PETROLEUM The fractional distillation of petroleum yields only about 20% gasoline. Due to its high demand this supply is augmented by converting surplus supplies of less desirable petroleum fractions such as kerosene oil and gas oil into gasoline by a process called cracking. It is defined as breaking of higher hydrocarbons having high boiling points into a variety of lower hydrocarbons, which are more volatile (low boiling). For example, a higher hydrocarbons CI6H34 splits according to the following reaction. Heat C16 H 34  700o → C7 H16 +3CH 2 =CH 2 +CH 3 -CH=CH 2 Alkane This is the process in which C-C bonds in long chain alkane molecules are broken, producing smaller molecules of both alkanes and alkenes. The composition of the products depends on the condition under which the cracking takes place. Cracking is generally carried out in the following ways. (1) Thermal Cracking Breaking down of large molecules by heating at high temperature and pressure is called Thermal Cracking. It is particularly useful in the production of unsaturated hydrocarbons such as ethene and propene. (2) Catalytic Cracking Higher hydrocarbons can be cracked at lower tem perature (500°C) and lower pressure (2 atm), in the presence of a suitable catalyst. A typical catalyst used for this purpose is a mixture of silica (SiO2) and alumina (AI2O3). Catalytic cracking produces gasoline of higher octane number and, therefore, this method is used for obtaining better quality gasoline. 9 7. Fundamental Principles of Organic Chemistry eLearn.Punjab (3) Steam Cracking In this process, higher hydrocarbons in the vapour phase are mixed with steam, heated for a short duration to about 900°C and cooled rapidly. The process is suitable for obtaining lower unsaturated hydrocarbons. Besides increasing the yield of gasoline, cracking has also produced large amounts of useful by-products, such as ethene, propene, butene and benzene. These are used for manufacturing drugs, plastics, detergents, synthetic fibres, fertilizers, weed killers and important chemicals like ethanol, phenol and acetone. 7.6 REFORMING The gasoline fraction present in petroleum is generally not of good quali- ty. When it burns in an automobile engine, combustion can be initiated before the spark plug fires. This produces a sharp metallic sound called knocking which greatly reduces the efficiency of an engine. The quality of a fuel is indicated by its octane number. As the octane number increas- es, the engine is less likely to produce knocking. Straight- chain hydrocar- bons have low octane numbers and make poor fuels. Experiments have shown that isooctane or 2,2,4- trimethyl pentane burns very smooth- ly in an engine and has been arbitrarily given an octane number of 100. The octane number of gasoline is improved by a process called reform- ing. It involves the conversion of straight chain hydrocarbons into branched chain by heating in the absence of oxygen and in the presence of a catalyst. 10 7. Fundamental Principles of Organic Chemistry eLearn.Punjab The octane number of a poor fuel can also be improved by blend- ing it with a small amount of additive like tetraethyl lead (TEL). Tetraeth- yl lead (C2H5)4 Pb, is an efficient antiknock agent but has one serious disad- vantage; its combustion product, lead oxide, is reduced to metallic lead which is discharged into the air through the exhaust pipe and causes air pollution. 7.7 CLASSIFICATIONS OF ORGANIC COMPOUNDS There are millions of organic compounds. It is practically not possi- ble to study each individual compound. To facilitate their study, or- ganic compounds are classified into various groups and sub- groups. They may be broadly classified into the following classes. 1. Open chain or Acyclic compounds. 2. Closed chain or Cyclic (or ring) compounds. (1) Open Chain or Acyclic Compounds This type of compounds contain an open chain of carbon atoms. The chains may be branched or non-branched (straight chain). The open chain compounds are also called aliphatic compounds. Straight Chain (or non- branched) Compounds Those organic compounds in which the carbon atoms are connected in series from one to the other. CH 3 -CH 2 -CH 2 -CH 3 H 2C= CH-CH 2 -CH 3 CH 3 -CH 2 -CH 2 -CH 2 -OH n-Butane 1-Butene 1-Butanol Branched chain compounds Those organic compounds in which the carbon atoms are attached on the sides of chain. 11 7. Fundamental Principles of Organic Chemistry eLearn.Punjab (2) Closed Chain Compounds or Cyclic Compounds These compounds contain closed chains or rings of atoms and are known as cyclic or ring compounds. These are of two types; (a) Homocyclic or carbocycli compounds (b) Heterocyclic compounds The classification of organic compounds into various classes is shown in Fig. 7.1. (a) Homocyclic or Carbocyclic Compounds The compounds in which the ring consists of only carbon atoms, Homocyclic or carbocyclic compounds. Homocyclic compounds are further classified as : 1. Alicyclic compounds 2. Aromatic compounds Fig:7.1 Classification of organic compounds. 12 7. Fundamental Principles of Organic Chemistry eLearn.Punjab (1) Alicyclic Compounds The homocyclic compounds which contain a ring of three or more carbon atoms and resembling aliphatic compounds are called alicyclic compounds. The saturated alicyclic hydrocarbons have the general formula CnH2n. Typical examples of alicyclic compounds are given below. One or more hydrogen atoms present in these compounds may be substituted by other group or groups. (2) Aromatic Compounds These carbocyclic compounds contain at least one benzene ring, six carbon atoms with three alternate double and single bonds.These bonds are usually shown in the form of a circle. Typical examples of aromatic compounds are given below. The aromatic compounds may have a side-chain or a functional group attached to the ring. For example: 13 7. Fundamental Principles of Organic Chemistry eLearn.Punjab The aromatic compounds may also contain more than one benzene rings fused together. (b) Heterocyclic Compounds The compounds in which the ring consists of atoms of more than one kind are called heterocyclic compounds or heterocycles. In heterocyclic compounds generally one or more atoms of elements such as nitrogen (N), oxygen (O) or sulphur (S) are present. The atom other than carbon viz, N, 0, or S, present in the ring is called a hetero atom. 14 7. Fundamental Principles of Organic Chemistry eLearn.Punjab 7.8 FUNCTIONAL GROUP An atom or a group of atoms or a double bond or a triple bond whose presence imparts specific properties to organic compounds is called a functional group, because they are the chemically functional parts of molecules. The study of organic chemistry is organized around functional groups. Each functional group defines an organic family. Although over six mil- lion organic compounds are known, there are only a handful of func- tional groups, and each one serves to define a family of organic com- pounds. The examples of functional groups are outlined in Table 7.2. TABLE 7.2 FUNCTIONAL GROUPS Class of compounds Functional group Example Formula Name C C None Alkane CH — CH3 C C Double bond Alkene H2C = CH2 C C Triple bond Alkyne HC=CH Halo (fluoro, chloro, -X(X=F,Cl,Br,I) Alkyl halide CH3-CH2-Cl bromo, iodo) OH Hydroxyl group Alcohol or alkanol CH3-CH2-OH NH2 Amino group Amine CH3-CH2-NH2 C NH Imino group Imine CH2=NH C O C Ether linkage Ether CH3-CH2-O-CH2-CH3 O O C Formyl group Aldehyde or alkanal CH3-C H H R CH3 C O Carbonyl Ketone or alkanohe C O R CH3 15 7. Fundamental Principles of Organic Chemistry eLearn.Punjab O O Carboxylicacid CH3-C C Carboxyl group (oralkanoicacid) OH OH O C X Acid halide Acid halide CH3 C Cl O C NH2 Acid amide Acid amide CH3 C NH2 O O R C Ester group Ester CH3-C OH OCH3 SH Mercapto Thioalcohol or Thiol CH3-CH2-SH Alkyl cyanide or alkane CH3-C N C N Cyano nitrile O N Nitro Nitro compounds C6H5NO 2 O 7.9 HYBRIDIZATION OF ORBITALS AND THE SHAPES OF MOLECULES Although the most stable electronic configuration of a carbon atom (having two partially filled 2p orbitals) requires it to be divalent, car- bon is tetravalent in the majority of its compounds. In order to explain this apparent anamoly, it is assumed that an electron from the 2s orbit- al is promoted to an empty 2pz orbital, giving the electronic configuration: Ground state electronic configuration of carbon = 1s2 2s2 2p1x 2p1y 2p°z Excited state electronic configuration of carbon = 1s2 2s1 2p1x 2p1y 2p1z The excited state configuration can explain the tetravalency of carbon but these four valencies will not be equivalent. Orbital hybridization theory has been developed to explain the equivalent tetravalency of carbon. 16 7. Fundamental Principles of Organic Chemistry eLearn.Punjab According to this theory the four atomic orbitals of carbon belonging to valence shell may be mixed in different ways to explain the bonding and shapes of molecules formed by carbon atoms. sp3 Hybridization In order to explain the bonding and shapes of molecules in which carbon is attached with four atoms, all these four atomic orbitals are mixed together to give rise to four new equivalent hybrid atomic orbitals having same shape and energy. This mode of hybridization is called tetrahedral or sp3 hybridization. All these four sp3 hybrid orbitals are degenerate (having equal energy) and are directed at an angle of 109.50 in space to give a tetrahedral geometry. When a carbon atom forms single bonds with other atoms, these hybrid orbitals overlap with the orbitals of these atoms to form four sigma bonds. This type of hybridization explains the bonding and shapes of all those compounds in which carbon atom is saturated. x y z Fig. 7.2 sp3 hybridization of carbon to give methane (CH4) 17 7. Fundamental Principles of Organic Chemistry eLearn.Punjab In the formation of methane, the four hybrid atomic orbitals of carbon overlap separately with four 1s atomic orbitals of hydrogen to form four equivalent C-H bonds. The shape of methane thus formed is very similar to the actual methane molecule. All the four hydrogen atoms do not lie in the same plane. 3 109.50 In ethane, CH3 - CH3, the two tetrahedrons of each carbon are joined together as shown in the above figure. Further addition of a carbon atom with ethane will mean the attachment of another tetrahedron. At this stage, it is necessary to answer an important question.From where does the energy come to excite the carbon atom? The answer to this question is simple. Before excitation the carbon should make two covalent bonds releasing an adequate amount of energy. After excitation, however, it will form four covalent bonds releasing almost double the amount of energy. This excess energy is more than that needed to excite the carbon atom. So a tetravalent carbon atom is expected to be more stable than a divalent carbon atom. sp2 Hybridization In order to explain the bonding in unsaturated compounds, two more modes of hybridization have been developed. 18 7. Fundamental Principles of Organic Chemistry eLearn.Punjab The structure of alkenes can be explained by sp2 mode of hybridizaton. In this type one 2s and two 2p orbitals of carbon are mixed together to give three equivalent and coplanar sp2 hybridized orbitals, Fig. 7.3. Each sp2 hybrid orbital is directed from the centre of an equilateral triangle to its three corners. The bond angle between any two sp2 hybrid orbitals is 120°.The unhybridized 2pz orbital will remain perpendicular to the triangle thus formed. Fig. 7.3 sp2-hybridization of carbon. In the formation of ethene molecule, three sp2 orbitals of each carbon atom overlap separately with sp2 orbital of another carbon and 1s orbitals of two hydrogen atoms to form three s bonds. This gives rise to what is called the s-frame work of ethene molecule. The unhybridized orbitals of each carbon atom will then overlap in a parallel fashion to form a π - bond, Fig. 7.4 19 7. Fundamental Principles of Organic Chemistry eLearn.Punjab π - bond Fig. 7.4 Formation of ethene. sp-Hybridization The structure of alkynes can be explained by yet another mode of hybridization called sp hybridization. In this type one 2s and one 2p orbitals of the carbon atom mix together to give rise to two degenerate sp hybridized atomic orbitals. These orbitals have a linera shape with a bond angle 180o. The two unhybridized atomic orbitals, 2py and 2pz are perpendicular to these sp hybridized orbitals. Ethyne molecule is formed when two sp hybridized carbon atoms join together to from a s-bond by sp-sp overlap. The other sp orbital is utilized to form a s- bond with 1s orbital of hydrogen atom. 20 7. Fundamental Principles of Organic Chemistry eLearn.Punjab Fig. 7.6 Formation of ethyne The two unhybridized p orbitals on a carbon atom will overlap separately with the p orbitals of the other carbon atom to give two π -bonds both perpendicular to the s -framework of ethyne. The presence of a s and two π bonds between two carbon atoms is responsible for shortening the bond distance. 7.10 ISOMERISM The concept of isomerism is an important feature of organic compounds. Two or more compounds having the same molecular formula but different structural formulas and properties are said to be isomers and the phenomenon is called isomerism. The structural formula of a compound shows the arrangement of atoms and bonds present in it. The simplest hydrocarbon to have structural isomers is butane (C4H10). The alkanes, methane, ethane and propane do not show the phenom- enon of isomerism because each exists in one structural form only. If we study the structural formula of butane or other higher hydrocar- bons of the alkane family, we will observe that it is possible to arrange the atoms present in the molecule in more than one way to satisfy all valencies. This means that it is possible to have two or more different arrangements for the same molecular formula.For example, butane molecule can have two dif- ferent arrangements as represented by the following structural formulas: 21 7. Fundamental Principles of Organic Chemistry eLearn.Punjab CH 3 -CH 2 -CH 2 -CH 3 n-Butane Isobutane This fact has been supported by an experimental evidence that there are two compounds with different physical properties but with the same molecular formula of C4H10. Isomerism is not only possible but common if the compound contains more than three carbon atoms. As the number of carbon atoms in a hydrocarbon increases, the number of possible isomers increase very rapidly. The five carbon compound, pentane, has three isomers. When the number of carbon atoms increases to thirty, the number of isomers amount to over four billions. 7.10.1 Types of Isomerism (1) Structural Isomerism The structural isomerism is not confined to hydrocarbons only. In fact, all classes of organic compounds and their derivatives show the phenomenon of structural isomerism. The structural isomerism arises due to the difference in the arrangement of atoms within the molecule. The structural isomerism can be exhibited in five different ways. These are : (i) The Chain Isomerism. This type of isomerism arises due to the difference in the nature of the carbon chain. For example, for pentane (C5H12), the following arrangements are possible. 22 7. Fundamental Principles of Organic Chemistry eLearn.Punjab CH 3 -CH 2 -CH 2 -CH 2 -CH 3 n-Pentane (ii) Position Isomerism. This type of isomerism arises due to the difference in the position of the same functional group on the carbon chain. The arrangement of carbon atoms remains the same. For example, (a) Chloropropane can have two positional isomers given below. CH 3 -CH 2 -CH 2 -Cl 1-Chloropropane (b) Butene (C4H8) can have two positional isomers. CH 3 -CH 2 -CH=CH 2 CH 3 -CH=CH-CH 3 I-Butene 2-Butene (iii) Functional Group Isomerism The compounds having the same molecular formula but different functional groups are said to exhibit functional group isomerism. For example, there are two compounds having the same molecular formula C2H6O , but different arrangement of atoms. CH 3 -O-CH 3 CH 3 -CH 2 -OH Diethyl Dimethyl etherether Eethyl alchohal Ethyl alcohol 23 7. Fundamental Principles of Organic Chemistry eLearn.Punjab (iv) Metamerism This type of isomerism arises due to the unequal distribution of carbon atoms on either side of the functional group. Such compounds belong to the same homologous series. For example, diethyl ether and methyl n-propyl ether are metamers. CH 3 − CH 2 − O − CH 2 − CH 3 CH 3 − O − CH 2 − CH 2 − CH 3 Diethyl ether Methyl n-propyl ether For a ketonic compound having the molecular formula C5H10O, the following two metamers are possible. (v) Tautomerism This type of isomerism arises due to shifting of proton from one atom to other in the same molecule. (2) Cis-trans Isomerism or Geometric Isomersim Two carbon atoms joined by a single bond are capable of free rotation about it. However, when two carbon atoms are joined by a double bond, they cannot rotate freely. As a result, the relative positions of the various groups attached to these carbon atoms get fixed and gives rise to cis- trans isomers. 24 7. Fundamental Principles of Organic Chemistry eLearn.Punjab Such compounds which possess the same structural formula, but differ with respect to the positions of the identical groups in space are called cis- trans isomers and the phenomenon is known as the cis-trans or geometric isomerism. The necessary and sufficient condition for a compound to exhibit geomet- ric isomerism is that the two groups attached to the same carbon must be different. 2-Butene can exist in the form of cis and trans isomers. Similarly 2-pentene and l-bromo-2-chloropropene also show cis-trans isomerism. In the cis-form, the similar groups lie on the same side of the double bond whereas in the trans-form, the similar groups lie on the opposite sides of the double bond. The rotation of two carbon atoms joined by a double bond could happen only if the π bond breaks.This ordinarily costs too much energy, making geometric isomers possible. 25 7. Fundamental Principles of Organic Chemistry eLearn.Punjab KEY POINTS 1. Chemical compounds were classified as organic and inorganic compounds based upon their origin. Organic compounds are obtained from living things whereas inorganic compounds are obtained from mineral sources. 2. It was thought that organic compounds could not be synthesized in the laboratory from inorganic sources. 3. Organic chemistry is now-a-days defined as the chemistry of carbon compounds. 4. Most of the commercially important compounds we use everyday are organic in nature. 5. Coal, petroleum and natural gas are important sources of organic compounds. 6. The process of cracking is developed to increase the yield of lower hydrocarbons which serve as important fuels commercially. 7. Organic compounds are classified into acyclic and cyclic compounds. 8. The study of organic chemistry is organized around functional groups. Each functional group defines an organic family. 9. The type of bonding and the shapes of different type of compounds formed by carbon can be explained by sp3, sp2 and sp modes of hybridization. 10.Compounds having the same molecular formula but different structural formulas are called isomers. There are four different type of structural isomers. 11.Isomerism arises due to restricted rotation around a carbon- carbon double bond is called cis-trans isomerism. EXERCISE Q l. Fill in the blanks i) Organic compounds having same molecular formula but different ______are called isomers. ii) The state of hybridization of carbon atom in ______ is sp2. iii) Alkenes show______ due to restricted rotation around a carbon-carbon double bond. iv) Heating an organic compound in the absence of oxygen and in the presence of________as a catalyst is called cracking. 26 7. Fundamental Principles of Organic Chemistry eLearn.Punjab v) A group of atoms which confers characteristic properties to an organic compound is called _________. vi) 2-Butene is________of 1-butene. vii) Carbonyl functional group is present in both_________and ketones. viii) A heterocyclic compound contains an atom other than______ in its ring. ix) The quality of gasoline can be checked by finding out its_____. x) A carboxylic acid contains___________ as a functional group. Q.2 Indicate true or false. (i) There are three possible isomers forpentane. (ii) Alkynes do not show the phenomenon of cis-trans isomerism. (iii) Organic compounds can not be synthesized from inorganic compounds. (iv) All close chain compounds are aromatic in nature. (v) The functional group present in amides is called an amino group. (vi) Government of Pakistan is trying to use coal for power generation. (vii) Crude petroleum is subjected to fractional sublimation in order to separate it into different fractions, (viii) A bond between carbon and hydrogen serves as a functional group for alkanes. (ix) o-Nitrotoluene and p-nitrotoluene are the examples of functional group isomerism. (x) Almost all the chemical reactions taking place in our body are inorganic in nature. Q 3. Multiple choice questions. Encircle the correct answer. (i) The state of hybridization of carbon atom in methane is: (a) sp3 (b) sp2 (c) sp (d) dsp2 (ii) In t-butyl alcohol, the tertiary carbon is bonded to: (a) two hydrogen atoms (b) three hydrogen atoms (c) one hydrogen atom (d) no hydrogen atom (iii) Which set of hybrid orbitals has planar triangular shape. (a) sp3 (b) sp (c) sp2 (d) dsp2 (iv) The chemist who synthesized urea from ammonium cyanate was: (a) Berzelius (b)Kolbe (c) Wholer (d) Lavoisier 27 7. Fundamental Principles of Organic Chemistry eLearn.Punjab (v) Linear shape is associated with which set of hybrid orbitals? (a) sp (b) sp2 (c) sp3 (d) dsp2 (vi) A double bond consists of: (a) two sigma bonds (b) one sigma and one pi bond (c) one sigma and two pi bonds (d) two pi bond (vii) Ethers show the phenonenom of: (a) position isomerism (b) functional group isomerism (c) metamerism (d) cis-trans isomerism (viii) Select From the following the one which is alcohol: (a) CH3-CH2-OH (b) CH3-O-CH3 (c) CH3COOH (d) CH3-CH2-Br Q 4. How organic compounds are classified? Give suitable example of each type. Q 5. What are homocyclic and heterocyclic compounds? Give one example of each. Q 6. Write the structural formulas of the two possible isomers of C4H10. Q 7. Why is ethene an important industrial chemical? Q 8. What is meant by a functional group? Name typical functional groups containing oxygen. Q 9. What is an organic compound? Explain the importance of Wohler’s work in the developrnent of organic chemistry. Q 10. Write a short note on cracking of hydrocarbons. Q 11. Explain reforming of petroleum with the help of suitable example. Q 12. Describe important sources of organic compounds. Q13. What is orbital hybridization? Explain sp3 sp2 and sp modes of hybridization of carbon. Q14. Explain the type of bonds and shapes of the following molecules using hybridization approach. CH3 - CH3, CH2 = CH2, CH = CH, HCHO, CH3CI Q 15. Why there is no free rotation around a double bond and a free rotation around a single bond ? Discuss cis-trans isomerism. 28 CHAPTER 8 ALIPHATIC HYDROCARBONS Animation 8.1 : Cycloalkanes Source and credit : Stackexchange 8. ALIPHATIC HYDROCARBONS eLearn.Punjab In This Chapter You Will Learn: 1. How to name the aliphatic hydrocarbons according to IUPAC rules. 2. The synthesis of alkanes, alkenes and alkynes and their important reactions. 3. The comparison of reactivity of s bond and p bond. 4. About the free radical nature of reactions of alkanes and electrophilic addition of alkenes and alkynes. 5. The comparison of reactivities of alkanes, alkenes and alkynes. 8.1 INTRODUCTION Hydrocarbons are organic compounds which contain carbon and hydro- gen only. The number of such compounds is very large because of the prop- erty of catenation. Hydrocarbons have been divided into various class- es on the basis of structure of the chain or size and nature of the ring. 2 8. ALIPHATIC HYDROCARBONS eLearn.Punjab If all the valencies of the carbon atoms in a molecule are fully satisfied and these cannot further take up any more hydrogen atoms, then the hydrocarbons are named as saturated hydrocarbons or alkanes. The compounds of carbon and hydrogen in which all the four valen- cies of carbon are not fully utilized and they contain either a double or a triple bond, such compounds are called unsaturated hydrocarbons. Those unsaturated hydrocarbons which contain a double bond are called alkenes while those containing a triple bond are called alkynes. Classification of hydrocarbons has been’ shown at page 136. 8.2 NOMENCLATURE 8.2.1 Common or Trivial Names: In the early days, the compounds were named on the basis of their history, the method of preparation or name of the person working on it, e.g., the name marsh gas was given to methane because it was found in marshy places. Acetic acid derives its name from vinegar (Latin, acetum means vinegar). Organic compounds were named after a person, like barbituric acid after Barbara. Such a system may have a certain charm but is never manageable. For alkanes with five or more carbon atoms, the root word is derived from the Greek or Latin numerals indicating the number of carbon atoms in a molecule, and the name is completed by adding ‘ane’ as a suffix, e.g. pentane (C5H12), hexane (C6H14), heptane (C7H16), etc. The common or trivial names are applicable to all isomers of a given molecular formula. The prefixes n, iso, neo are, however, to differentiate between isomers. 3 8. ALIPHATIC HYDROCARBONS eLearn.Punjab CH3 CH2 CH2 CH3 H3C CH CH3 | n-butane CH3 Isobutane CH3 CH2 CH2 CH2 CH3 n-pentane CH3 | H3C CH CH2 CH3 H3C C CH3 | | CH3 CH3 Isopentane Neophentane These prefixes have only limited use, as they are not workable with complex molecules. Moreover, common names give only minimum information about the structure of the compounds.Alkenes are similarly named by replacing the ending -ane of the name of alkane with ylene. e.g. CH3 | H3C C=CH2 H2C=CH2 H3C CH =CH2 Ethylene Propylene Isobutylene 4 8. ALIPHATIC HYDROCARBONS eLearn.Punjab 8.2.2 IUPAC Names In 1889 the solution for naming the organic compounds systematically was sought by International Chemical Congress. A report was accepted in 1892 in Geneva but it was found incomplete. In 1930, International Union of Chemistry (IUC) gave a modified report which is also referred as Liege Rules. This report was further modified by International union of Pure and Applied Chemists (IUPAC) in the year 1947. Since that date the union has issued periodic reports on rules for the systematic nomenclature of organic compounds, the most recent of which was published in the year 1979. IUPAC system of nomenclature is based on the following principle. ‘Each different compound should have a different name’. Thus through a systematic set of rules, the IUPAC system provides different names for more than 7 million known organic compounds. Nomenclature of Alkyl Groups: If we remove one hydrogen atom from an alkane, we obtain what is called an alkyl group. These alkyl groups have names that end in — yl.When the alkane is unbranched and the hydrogen atom that is removed is a terminal hydrogen atom, the names are straight forward: Alkane Alkyl Group Abbreviation CH3 H CH3 Me- Methane Methyl Et- CH3 CH2 H CH3CH2 Ethane Ethyl Pr- CH3 CH2 CH2 H CH3CH2CH2 Propane n-propyl n-Bu- CH3 CH2 CH2 CH2 H CH3CH2CH2CH2 n-Butane n-Butyl 5 8. ALIPHATIC HYDROCARBONS eLearn.Punjab 8.2.3 Nomenclature of Alkanes Branched-chain alkanes are named according to the following rules. 1. Locate the longest continuous chain of carbon atoms; this chain determines the parent name for the alkane. We designate the following compound as a hexane because the longest continuous chain contains six carbon atoms. H3C CH2 CH2 CH2 CH CH3 | CH3 The longest continuous chain may not always be obvious from the way the formula is written. Notice, for example, that the following alkane is designated as a heptane because the longest chain contains seven carbon atoms. 2.Number the longest chain beginning from the end of the chain nearer the substituent. Applying this rule, we number the two alkanes shown above in the following way. 6 8. ALIPHATIC HYDROCARBONS eLearn.Punjab 3.Use the numbers obtained by the application of rule 2 to designate the location of the substituent group. The parent name is placed last, and the substituent group, preceded by the number designating its location on the chain, is placed first. Numbers are separated from words by a hyphen. The systematic names ot the two compounds shown above will then be: 4. When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain. For example, we designate the following compound as 4 -ethyl-2 -methylhexane. The substituent groups should be listed alphabetically (i.e. ethyl before methyl). In deciding on alphabetical order disregard multiplying prefixes such as “di” and “tri”. 5. When two substituents are present on the same carbon atom, use that number twice. 7 8. ALIPHATIC HYDROCARBONS eLearn.Punjab 6.When two or more substituents are identical, indicate this by the use of the prefixes di, tri , tetra , and so on. Then make certain that each and every substituent has a number. Commas are used to separate numbers from each other. Application of these six rules allows us to name most of the alkanes that we shall encounter. Two other rules, however, may be required occasionally. 7.When two chains of equal length compete for selection as the parent chain, choose the chain with the greater number of substituents. 8 8. ALIPHATIC HYDROCARBONS eLearn.Punjab 8. When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference. 8.2.4 Nomenclature of Alkenes: The IUPAC rules for naming alkenes are similar in many respects to those for naming alkanes. 1.Select the longest continuous chain that contains the C = C as the parent chain. Change the ending of the name of the alkane of identical length from — ane to — ene, e.g., 2.Number the chain so as to include both carbon atoms of the double bond. Numbering begins from the end nearer to the double bond. 9 8. ALIPHATIC HYDROCARBONS eLearn.Punjab 3.Designate the location of the double bond by using the number of the first atom of the double bond as a prefix. 1 2 3 4 5 1 2 3 4 H2 C = CH CH2 CH3 H2 C = CH CH2 CH2 CH3 1-Pentene 1-Butene 4.Indicate the locations of the substituent groups by the numbers of the carbon atoms to which they are attached. 5.If the parent chain contains more than one double bonds, they are alkadienes for two, alkatrienes for three and so on. 1 2 3 4 = = CH2 CH CH CH2 1,3-Butadiene 8.2.5 Nomenclature of Alkynes: 1.The largest continuous carbon chain containing triple bond is selected. The name of the identical alkane is changed from ane to — yne. e.g. 2 1 3 2 1 CH ≡ CH H3 C C ≡ CH Ethyne Propyne 10 8. ALIPHATIC HYDROCARBONS eLearn.Punjab 2.The position of triple bond is shown by numbering the alkyne, so that minimum number is assigned to the triple bond. 4 3 2 1 H3 C CH2 C ≡ CH 1-Butyne 3.If a hydrocarbon contains more than one triple bonds, it is named as alkadiyne and triyne, etc. depending on the number of triple bonds. 6 5 4 3 2 1 HC ≡ C CH2 CH2 C ≡ CH 1,5 - Hexadiyne 4.If both double and triple bonds are present in the compound then ending enyne is given to the root. a.Lowest possible number is assigned to a double or a triple bond irrespective of whether ene or yne gets the lower number. 1 2 3 4 5 1 2 3 4 5 HC ≡ C CH=CH CH3 = H2 C CH C= ≡ C CH3 3 - Penten - 1- yne 1- Penten - 3 - yne b.In case a double and a triple bond are present at identical positions, the double bond is given the lower number. 11 8. ALIPHATIC HYDROCARBONS eLearn.Punjab 8.3 ALKANES OR PARAFFINS Alkanes are the simplest organic compounds made up of carbon and hydrogen only. They have a general formula of CnH2n+2. In these compounds the four valencies of carbon atoms are satisfied by single bonds to either other carbon atoms or hydrogen atom. They are, therefore known as Saturated Hydrocarbons. Methane (CH4) is the simplest member of this family. Each carbon atom in alkane is sp3 hybridized and has a tetrahedral geometry. 8.3.1 General Methods of Preparations (1) Hydrogenation of Unsaturated Hydrocarbons (Sabatier-Sendem’s Reaction) Hydrogenation of alkenes or alkynes in the presence of nickel at 200-300OC yields alkanes. R CH = CH2 + H2  Ni 200−300o C → R CH2 CH3 Alkene e.g CH2 = CH2 + H2  Ni 200−300o C → CH3 CH3 Ethane The hydrogenation can also be carried out with platinum or palladium at room temperature but they are expensive than nickel. The method is of industrial importance. Production of vegetable ghee by the catalytic hydrogenation of vegetable oil (unsaturated fatty acids) is an example of the application of this method on industrial scale. (2) From Alkyl Halides: An alkane is produced when an alkyl halide reacts with zinc in the presence of an aqueous acid. 12 8. ALIPHATIC HYDROCARBONS eLearn.Punjab R X + Zn + H+ + X −  → R H + ZnX 2 Alkyl halide Alkane CH3 I + Zn + H+ + I−  → CH4 + ZnI2 Methyl iodide Methane CH3 CH2 CH CH3 | Br + Zn + H+ + Br −  → CH3 CH2 CH2 CH3 + ZnBr 2-Bromo-butane n-Butane Alkanes can also be prepared from alkyl halides using palladium-charcoal as acatalyst. The method is known as Hydrogenolysis (hydrogenation accompanied by bond cleavage) R X + H2  Pd/C ∆ →R H+H X (3) Decarboxylation of Monocarboxylic Acids i) When sodium salts of fatty acids are heated with soda-lime (prepared by soaking quick lime (CaO) with caustic soda solution and drying the product). They eliminate a molecule of CO2 to form alkanes. e.g 13 8. ALIPHATIC HYDROCARBONS eLearn.Punjab ii) Kolbe's Electrolytic Method When a concentrated solution of sodium or potassium salt of a mono carboxylic acid is electrolysed, an alkane is produced. This method is only suitable for the preparation of symmetrical alkanes i.e. those of the type R—R. Methane cannot be prepared by this method. 2RCOO−Na + + 2 H2O  Electrolysis →R R+2CO2 + 2NaOH+H2 It is known to involve the following mechanism. When potassium salt of acetic acid is electrolysed, acetate ion migrates. towards the anode gives up one electron to produce acetate free. radical (CH3COO), which decomposes to give a methyl free radical (CH3) and CO2.Two such methyl radicals combine to give ethane. At Anode O || 2H3C C O   + 2CO → 2CH3 2  + CH CH  → H3C CH3 3 3 At Cathode 2H2O + 2e−  → 2OH + H2 2K + + 2OH  → 2KOH This reaction has limited synthetic applications as it forms a number of side products. 14 8. ALIPHATIC HYDROCARBONS eLearn.Punjab (4) From Carbonyl Compounds (Aldehydes or Ketones) The carbonyl groups of aldehydes or ketones are reduced to methyl or methylene group respectively by either Clemmensen or Wolf-Kishner’s reduction. In the former reaction a ketone is reduced to an alkane using zinc amalgam and hydrochloric acid whereas in the later an aldehyde is reduced to alkane with hydrazine in the presence of KOH. 3 Acetone Propane (5) From Grignard Reagents Alkyl halides react in anhydrous ether with magnesium to form alkyl magnesium halides, known as Grignard Reagent. They decompose on treatment with water or dilute acid to give alkanes. ether 15 8. ALIPHATIC HYDROCARBONS eLearn.Punjab 8.3.2. Physical Properties 1. Alkanes containing upto four carbon atoms are colourless, odourless gases while pentane to heptadecane (C5 to C17) are colourless, odourless liquids. The higher members from C18 onwards are waxy solids which are also colourless and odourless. 2. Alkanes are non-polar or very weakly polar and are insoluble in polar solvents like water, but soluble in non-polar solvents like benzene, ether, carbon tetra chloride,etc. 3. Their physical constants like boiling.points, melting points, density, etc increase with the increase in number of carbon atoms, whereas solubility decreases with increase in molecular mass. The boiling point increases by 20 to 30°C for addition of each CH2 group to the molecule. The boiling points of alkanes having branched chain structures are lower than their isomeric normal chain alkanes, e.g. n-butane has a higher boiling point-0.50 C than isobutane (-1 1.7°C). 4.The melting points of alkanes also increase with the increase in molecular mass but this increase is not so regular. 8.3.3. Reactivity of Alkanes The alkanes or paraffins (Latin: parum = little, affins = affinity) under ordinary condition are inert towards acids, alkalis, oxidizing and reducing agents. However, under suitable conditions, alkanes do undergo two types of reactions. 1. Substitution Reactions 2. Thermal and Catalytic Reactions These reactions take place at high temperature or on absorption of light energy through the formation of highly reactive free radicals. The unreactivity of alkanes under normal conditions may be explained on the basis of the non-polarity of the bonds forming them. The eletronegativity values of carbon (2.5) and hydrogen (2.1) do not differ appreciably and the bonding electrons between C-H and C-C are equally shared making them almost non- polar. In view of this, the ionic reagents such as acids, alkalies, oxidizing agents, etc find no reaction site in the alkane molecules to which they could be attached. 16 8. ALIPHATIC HYDROCARBONS eLearn.Punjab Inertness of s-bond The unreactivity of alkanes can also be explained on the basis of inertness of a s-bond. In a s -bond the electrons are very tightly held between the nuclei which makes it a very stable bond. A lot of energy is required to break it. Moreover the electrons present in a s-bond can neither attack on any electrophile nor a nucleophile can attack on them. Both these facts make alkanes less reactive. 8.3.4 Reactions 1. Combustion Burning of an alkane in the presence of oxygen is known as Combustion. Complete combustion of an alkane yields CO2, H2O and heat. The amount of heat evolved when one mole of a hydrocarbon is burnt to CO2 and H2O is called heat of combustion, e.g; CH4 ( g ) + 2O2 ( g )  Flame → CO2 ( g ) + 2H2O ( g ) + 891kJmol-1 Although the reaction is highly exothermic, it requires very high temperature to initiate it, e.g. by a flame or a spark. Combustion is the major reaction occurring in the internal combustion engines of automobiles. A compressed mixture of alkanes and air burns smoothly in the internal combustion engine and increases its efficiency. 2. Oxidation Oxidation of methane under different conditions gives different products. i) Incomplete oxidation occurs in a limited supply of oxygen or air and results in the formation of CO and carbon black. 3CH4 ( g )+ 4O2 ( g )  Flame → 2CO ( g ) + 6H2O ( g ) + C ( s ) 17 8. ALIPHATIC HYDROCARBONS eLearn.Punjab ii) Catalytic Oxidation: Lower alkanes when burnt in the presence of metallic catalysts, at high temperature and pressure, result in the formation of useful products. CH4 + [O]  → H3C Cu 400o C/200atm OH Methyl alcohol H3C OH + O  Cu 400o C/200atm → HCHO+H2O Formaldehyde HCHO + [O]  Cu 400o C/200atm → HCOOH Formic acid HCOOH + [O]  → CO2 + H2O Cu 400o C/200atm Catalytic oxidation of alkanes is used industrially to prepare higher fatty acids used in soap and vegetable oil industries. 3. Nitration: It is a substitution reaction of alkanes in which a hydrogen atom of an alkane is replaced by nitro group (-NO2). Alkanes undergo vapour-phase nitration under drastic condition (at 400-500°C) to give nitroalkanes, e.g. 450o C CH4 + HONO2  → CH3NO2 + H2O Nitromethane Nitroalkanes generally find use as fuels, solvents, and in organic synthesis. 18 8. ALIPHATIC HYDROCARBONS eLearn.Punjab 4. Halogenation Alkanes react with chlorine and bromine in the presence of sunlight or UV light or at high temperature resulting in the successive replacement of hydrogen atoms with halogens called halogenation. Extent of halogenation depends upon the amount of halogen used. Reaction of alkanes with fluorine is highly violent and results in a mixture of carbon, fluorinated alkanes and hydrofluoriq acid. Iodine does not substitute directly because the reaction is too slow and reversible. The order of reactivity of halogens is F2>Cl2>Br2>I2. Halogenation is believed to proceed through free radical mechanism. It involves the following three steps. hυ )Initiation( Step I Cl Cl  → Cl − + Cl − ] hυ Step 2 H3C H + Cl  → CH 3 + HCl )Propagation( hυ CH 3 + Cl Cl  → CH3 Cl + Cl )Termination( Step 3 CH 3 + Cl  → CH3 Cl By repetition of step II, a mi