chapter 1 structure and bonding organic chemistry.pdf

FGdLTenido Department of Chemistry , College of Science and Mathematics 2  CHAPTER 1 Structure and Bonding ; Acids and Bases Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematic...

FGdLTenido Department of Chemistry , College of Science and Mathematics 2  CHAPTER 1 Structure and Bonding ; Acids and Bases Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 3 Structure and Bonding ; Acids and Bases Catalyzes a crucial step in the body's synthesis of cholesterol. Understanding how this enzyme functions has led to the development of drugs credited with saving millions of lives. Enzyme HMG – CoA reductase ribbon model FGdLTenido Department of Chemistry , College of Science and Mathematics 4 Structure and Bonding ; Acids and Bases Introduction A scientific revolution is now taking place - a revolution that will give us safer and more effective medicines cure our genetic diseases increase our life spans improve the quality of our lives. FGdLTenido Department of Chemistry , College of Science and Mathematics 5 Structure and Bonding ; Acids and Bases Introduction The revolution is based in understanding the structure and function of the approximately 21,000 genes in the human body, but it relies on organic chemistry as the enabling science. FGdLTenido Department of Chemistry , College of Science and Mathematics 6 Structure and Bonding ; Acids and Bases Introduction Organic Chemistry is all around us… the reactions and interactions of organic molecules allow us to see, smell, fight, and feel provide the molecules that feed us, treat our illnesses, protect our crops, clean our clothes FGdLTenido Department of Chemistry , College of Science and Mathematics 7 Structure and Bonding ; Acids and Bases History In the late 1700s, the term organic chemistry, was used to mean the chemistry of compounds found in living organisms. Organic compounds were generally low-melting solids and were usually more difficult to isolate, purify, and work with than high-melting inorganic compounds. By the mid-1800s, it was clear that there was no fundamental difference between organic and inorganic compounds. The only distinguishing characteristic of organic chemicals is that all contain the element carbon. Department of Chemistry , College of Science and Mathematics 8 Structure and Bonding ; Acids and Bases What is organic chemistry? Organic chemistry is the study of the compounds of carbon. includes biological molecules, drugs, solvents, dyes does not include metal salts and materials (inorganic) does not include materials of large repeating molecules without sequences Department of Chemistry , College of Science and Mathematics 9 Structure and Bonding ; Acids and Bases More than 99% of the presently known chemical compounds contain carbon.  what makes carbon special? the electronic structure of carbon and its consequent position in the Periodic Table as a group 4A element, carbon can share four valence electrons and form four strong covalent bonds Department of Chemistry , College of Science and Mathematics 10 Structure and Bonding ; Acids and Bases  what makes carbon special? carbon atoms can bond to one another, forming long chains and rings carbon is able to form an immense diversity of compounds from a simple methane to the staggeringly complex DNA, which can have more than 100 million carbons Department of Chemistry , College of Science and Mathematics 11 Structure and Bonding ; Acids and Bases the position of carbon in the periodic table Department of Chemistry , College of Science and Mathematics 12 Structure and Bonding ; Acids and Bases Other elements commonly found in organic compounds are shown in the colors typically used to represent them. Department of Chemistry , College of Science and Mathematics 13  1.1 Atomic Structure Atom: (2 x 10-10 m) Nucleus (~ 10-14 to 10-15 m) protons (+) neutrons Electrons (-) 10-10 m Department of Chemistry , College of Science and Mathematics 14  1.1 Atomic Structure A specific atom is described by its atomic number (Z), which gives the number of protons (or electrons) it contains, and its mass number (A), which gives the total number of protons plus neutrons in its nucleus. All the atoms of a given element have the same atomic number but they can have different mass numbers depending on how many neutrons they contain. Department of Chemistry , College of Science and Mathematics 15  1.1 Atomic Structure Isotopes - atoms with the same atomic number but different mass numbers Atomic mass (atomic weight) – weighted average mass units (amu) of an element’s naturally occurring isotopes e.g. 1.001 amu for hydrogen 12.011 amu for carbon Department of Chemistry , College of Science and Mathematics 16 1.1 Atomic Structure : The Electrons the behavior of a specific electron in an atom can be described by a mathematical expression called a wave equation (Quantum Mechanical Model of Atomic Structure) the same sort of expression used to describe the motion of waves in a fluid The solution to a wave equation is a wave function or orbital. Department of Chemistry , College of Science and Mathematics 17 1.1 Atomic Structure : The Orbital An orbital, Ψ, can be thought of as defining a region of space around the nucleus where the electron can most likely be found. Department of Chemistry , College of Science and Mathematics 18 1.1 Atomic Structure : Atomic Orbitals Orbitals () : s, p, d, and f s and p = most common among organic and biological chemistry Department of Chemistry , College of Science and Mathematics 19 1.1 Atomic Structure : Atomic Orbitals an s orbital is spherical, with the nucleus at its center a p orbital is dumbbell-shaped and can be oriented in space along any of three mutually perpendicular directions, arbitrarily denoted px , py , and pz Department of Chemistry , College of Science and Mathematics 20 1.1 Atomic Structure : Atomic Orbitals The two parts, or lobes, of a p orbital have different algebraic signs (+ and -) in the wave function and are separated by a region of zero electron density called a node. Department of Chemistry , College of Science and Mathematics 21 1.1 Atomic Structure : Atomic Orbitals s and p orbitals Department of Chemistry , College of Science and Mathematics 22 1.1 Atomic Structure : Atomic Orbitals s and p orbitals Department of Chemistry , College of Science and Mathematics 23 1.1 Atomic Structure : Atomic Orbitals Orbitals are organized into layers (or electron shells) around the nucleus of successively larger size and energy. Electron shells contain different numbers and kinds of orbitals. Each orbital can accommodate a maximum of 2 electrons. Department of Chemistry , College of Science and Mathematics 24 1.1 Atomic Structure : Atomic Orbitals the energy levels of electrons in an atom Department of Chemistry , College of Science and Mathematics 25 1.2 Atomic Structure : Electron Configurations The lowest-energy arrangement, or ground-state electron configuration, of an atom is a listing of the orbitals that the atom’s electrons occupy. Department of Chemistry , College of Science and Mathematics 26 1.2 Atomic Structure : Electron Configurations RULE 1 : Aufbau Principle Electrons will fill the lower energy levels before moving to higher energy orbitals. Department of Chemistry , College of Science and Mathematics 27 1.2 Atomic Structure : Electron Configurations RULE 1 : Aufbau Principle The orbitals of lowest energy are filled first, according to the order 1s 2s 2p 3s 3p 4s 3d Department of Chemistry , College of Science and Mathematics 28 1.2 Atomic Structure : Electron Configurations RULE 2 : Pauli – Exclusion Principle Only two electrons can occupy an orbital, and they must be of opposite spin. This spin can have two orientations, denoted as up  and down . Department of Chemistry , College of Science and Mathematics 29 1.2 Atomic Structure : Electron Configurations RULE 3 : Hund’s Rule If two or more empty orbitals of equal energy are available, one electron occupies each with the spins parallel until all orbitals are half-full. Department of Chemistry , College of Science and Mathematics 30 1.2 Atomic Structure : Electron Configurations Department of Chemistry , College of Science and Mathematics 31 1.3 Development of Chemical Bonding Theory 1858 : August Kekulé and Archibald Couper independently proposed that, in all organic compounds, carbon is tetravalent; i.e., it always forms four bonds when it joins other elements to form chemical compounds. Kekulé further stated that carbon atoms can bond to one another to form extended chains of linked atoms and chains can double back on themselves to form rings. Department of Chemistry , College of Science and Mathematics 32 1.3 Development of Chemical Bonding Theory 1874 : Jacobus van’t Hoff and Joseph Le Bel added a third dimension to our ideas about organic compounds. They proposed that the four bonds of carbon are not oriented randomly but have specific spatial directions. Van’t Hoff went even further and suggested that the four atoms to which carbon is bonded sit at the corners of a regular tetrahedron, with carbon in the center. Department of Chemistry , College of Science and Mathematics 33 1.3 Development of Chemical Bonding Theory a representation of van’t Hoff’s tetrahedral carbon atom Department of Chemistry , College of Science and Mathematics 34 1.3 Development of Chemical Bonding Theory a representation of van’t Hoff’s conventions : tetrahedral carbon atom solid lines represent bonds in the plane of the page heavy wedged line represents a bond coming out of the page toward the viewer dashed line represents a bond receding back behind the page away from the viewer Department of Chemistry , College of Science and Mathematics 35  1.4 The Nature of Chemical Bonds Why do atoms bond together? results to more stable compound lower in energy than the separate atoms energy is released and flows out of the chemical system when a bond forms Department of Chemistry , College of Science and Mathematics 36  1.4 The Nature of Chemical Bonds How are bonds formed? bonds are formed through the valence electrons of the atoms involved valence electrons – electrons in the outermost shell when bonds are formed, the valence shell would contain a total of 8 electrons (octet rule) after the bond formation, the resulting electron configuration of the atoms involved is isoelectronic with any of the noble gases Department of Chemistry , College of Science and Mathematics 37 1.4 The Nature of Chemical Bonds types of chemical bonds 1. Ionic Bonds when metals bond to nonmetals, some electrons from the metal atoms are transferred to the nonmetal atoms metals have low ionization energy, relatively easy to lose an electron nonmetals have high electron affinities, relatively good to accept electrons Department of Chemistry , College of Science and Mathematics 38 1.4 The Nature of Chemical Bonds types of chemical bonds : ionic bonds Examples: Na + Cl  Na+ Cl– Mg + 2 Br  Mg2+ 2Br– Department of Chemistry , College of Science and Mathematics 39  1.4 The Nature of Chemical Bonds types of chemical bonds 2. Covalent Bonds nonmetals have relatively high ionization energies, so it is difficult to remove electrons from them when nonmetals bond together, it is better in terms of potential energy for the atoms to share valence electrons potential energy is lowest when the electrons are between the nuclei shared electrons hold the atoms together by attracting nuclei of both atoms Department of Chemistry , College of Science and Mathematics 40  1.4 The Nature of Chemical Bonds types of chemical bonds : covalent bonds 1916 G. N. Lewis proposed the shared-electron bond shared-electron bond is called a covalent bond The neutral group of atoms held together by covalent bonds is called a molecule. A simple way of indicating the covalent bonds in molecules is to use the Lewis structures or the electron-dot structures. electron-dots indicate the number of valence electrons in an atom nonbonding electrons - lone-pair electrons Department of Chemistry , College of Science and Mathematics 41 1.4 The Nature of Chemical Bonds covalent bonds : Lewis structures Department of Chemistry , College of Science and Mathematics 42 1.4 The Nature of Chemical Bonds Lewis structures and Kekulé structures Lewis structures : Electron-dot structures Kekulé structures : Line-bond structures Department of Chemistry , College of Science and Mathematics 43  1.4 The Nature of Chemical Bonds Number of Covalent Bonds The number of covalent bonds an atom forms depends on how many additional valence electrons it needs to reach a noble-gas configuration. Department of Chemistry , College of Science and Mathematics 44  1.4 The Nature of Chemical Bonds Lone-pair Electrons Valence electrons not used for bonding are called lone-pair electrons, or nonbonding electrons. Example: the nitrogen atom in ammonia (NH3), shares six valence electrons in three covalent bonds and has its remaining two valence electrons in a nonbonding lone pair Department of Chemistry , College of Science and Mathematics 45 Exercises: 1. What are likely formulas for the following molecules? 2. Write both electron-dot and line-bond structures for the following molecules, showing all nonbonded electrons: 3. Why can’t an organic molecule have the formula C2H7? Department of Chemistry , College of Science and Mathematics 46 Exercises: 1. What are likely formulas for the following molecules? Department of Chemistry , College of Science and Mathematics 47 Exercises: 2. Write both electron-dot and line-bond structures for the following molecules, showing all nonbonded electrons: Department of Chemistry , College of Science and Mathematics 48 Exercises: 3. Why can’t an organic molecule have the formula C2H7? C2H7 has too many hydrogens for a compound with two carbons. Department of Chemistry , College of Science and Mathematics 49 1.5 Forming Covalent Bonds : Valence Bond Theory How does electron sharing lead to bonding between atoms? Valence bond theory A covalent bond forms when two atoms approach each other closely and a singly occupied orbital on one atom overlaps a singly occupied orbital on the other atom. Example: In the H2 molecule, the H - H bond results from the overlap of two singly occupied hydrogen 1s orbitals. Department of Chemistry , College of Science and Mathematics 50 1.5 Forming Covalent Bonds : Valence Bond Theory the electrons are now paired in the overlapping orbitals the electrons are attracted to the nuclei of both atoms, bonding the atoms together Department of Chemistry , College of Science and Mathematics 51 1.5 Forming Covalent Bonds : Valence Bond Theory During the bond-forming reaction, 436 kJ/mol (104 kcal/mol) of energy is released. 2 H  H2 + 436 kJ/mol  the H2 molecule formed has 436 kJ/mol less energy than the starting 2H atoms thus H2 is more stable than the 2H atoms new H-H bond has a bond strength of 436 kJ/mol Department of Chemistry , College of Science and Mathematics 52 1.5 Forming Covalent Bonds : Valence Bond Theory How close are the two nuclei in the H2 molecule? The 2 nuclei in the H2 molecule must not be too close nor too far apart. if they are too close, they will repel each other because both are positively charged if they are too far apart, they won’t be able to share the bonding electrons Department of Chemistry , College of Science and Mathematics 53 1.5 Forming Covalent Bonds : Valence Bond Theory How close are the two nuclei in the H2 molecule? There is an optimum distance between nuclei that leads to maximum stability called the bond length. In the H2 molecule, the bond length is 74 pm. Department of Chemistry , College of Science and Mathematics 54 1.5 Forming Covalent Bonds : Valence Bond Theory A plot of energy versus internuclear distance for two hydrogen atoms. The distance at the minimum energy point is the bond length. Every covalent bond has both a characteristic bond strength and bond length. For the H2 molecule bond strength of H – H 436 kJ/mol bond length of H – H 74 pm Department of Chemistry , College of Science and Mathematics 55 1.6 sp3 Hybrid Orbitals and the Structure of Methane Carbon has four valence electrons (2s2 2p2) and forms four bonds. Because carbon uses two kinds of orbitals for bonding, 2s and 2p, we might expect methane to have two kinds of C-H bonds. In fact, though, all four C-H bonds in methane are identical and are spatially oriented toward the corners of a regular tetrahedron. Department of Chemistry , College of Science and Mathematics 56 1.6 sp3 Hybrid Orbitals and the Structure of Methane in 1931 Linus Pauling proposed that an s orbital and three p orbitals can combine, or hybridize, to form four equivalent atomic orbitals with tetrahedral orientation these tetrahedrally oriented orbitals are called sp3 hybrids Note the superscript 3 in the name sp3 tells how many of each type of atomic orbital combine to form the hybrid, not how many electrons occupy it Department of Chemistry , College of Science and Mathematics 57 1.6 sp3 Hybrid Orbitals and the Structure of Methane 1s + 3p = 4 sp3 Department of Chemistry , College of Science and Mathematics 58 1.6 sp3 Hybrid Orbitals and the Structure of Methane Four sp3 hybrid orbitals (green), oriented to the corners of a regular tetrahedron, are formed by combination of an atomic s orbital (red) and three atomic p orbitals (red/blue). The sp3 hybrids have two lobes and are unsymmetrical about the nucleus, giving them a directionality and allowing them to form strong bonds when they overlap an orbital from another atom. Department of Chemistry , College of Science and Mathematics 59 1.6 sp3 Hybrid Orbitals and the Structure of Methane why does carbon forms four equivalent tetrahedral bonds? When an s orbital hybridizes with three p orbitals, the resultant sp3 hybrid orbitals are unsymmetrical about the nucleus. One of the two lobes is much larger than the other and can therefore overlap better with another orbital when it forms a bond. As a result, sp3 hybrid orbitals form stronger bonds than the unhybridized s or p orbitals. Department of Chemistry , College of Science and Mathematics 60 1.6 sp3 Hybrid Orbitals and the Structure of Methane The asymmetry of sp3 orbitals arises because the two lobes of a p orbital have different algebraic signs, + and -. Thus, when a p orbital hybridizes with an s orbital, the positive p lobe adds to the s orbital but the negative p lobe subtracts from the s orbital. The resultant hybrid orbital is therefore unsymmetrical about the nucleus and is strongly oriented in one direction. Department of Chemistry , College of Science and Mathematics 61 1.6 sp3 Hybrid Orbitals and the Structure of Methane formation of sp3 hybrid orbitals Department of Chemistry , College of Science and Mathematics 62 1.6 sp3 Hybrid Orbitals and the Structure of Methane When each of the four identical sp3 hybrid orbitals of a carbon atom overlaps with the 1s orbital of a hydrogen atom, four identical C - H bonds are formed and methane results. Department of Chemistry , College of Science and Mathematics 63 1.6 sp3 Hybrid Orbitals and the Structure of Methane Each C - H bond in methane has a strength of 439 kJ/mol (105 kcal/mol) and a length of 109 pm. Because the four C – H bonds have a specific geometry, we also can define a property called the bond angle. The angle formed by each H-C-H is 109.5°, the so- called tetrahedral angle. Department of Chemistry , College of Science and Mathematics 64 1.6 sp3 Hybrid Orbitals and the Structure of Methane Department of Chemistry , College of Science and Mathematics 65 1.7 sp3 Hybrid Orbitals and the Structure of Ethane The same kind of orbital hybridization that accounts for the methane structure also accounts for the bonding together of carbon atoms into chains and rings to make possible many millions of organic compounds. Ethane, C2H6, is the simplest molecule containing a carbon–carbon bond. Department of Chemistry , College of Science and Mathematics 66 1.7 sp3 Hybrid Orbitals and the Structure of Ethane Ethane, C2H6, is the simplest molecule containing a carbon–carbon bond. Department of Chemistry , College of Science and Mathematics 67 1.7 sp3 Hybrid Orbitals and the Structure of Ethane For the ethane molecule, we can imagine that the two carbon atoms bond to each other by overlap of an sp3 hybrid orbital from each carbon. The remaining three sp3 hybrid orbitals of each carbon overlap with the 1s orbitals of three hydrogens to form the six C - H bonds. Department of Chemistry , College of Science and Mathematics 68 1.7 sp3 Hybrid Orbitals and the Structure of Ethane Department of Chemistry , College of Science and Mathematics 69 1.7 sp3 Hybrid Orbitals and the Structure of Ethane The C - H bonds in ethane are similar to those in methane, although a bit weaker — 421 kJ/mol (101 kcal/mol) for ethane versus 439 kJ/mol for methane. The C - C bond is 154 pm long and has a strength of 377 kJ/mol (90 kcal/mol). Department of Chemistry , College of Science and Mathematics 70 1.7 sp3 Hybrid Orbitals and the Structure of Ethane All the bond angles of ethane are near, although not exactly at, the tetrahedral value of 109.5°. Department of Chemistry , College of Science and Mathematics 71 1.7 sp3 Hybrid Orbitals and the Structure of Ethane Department of Chemistry , College of Science and Mathematics 72 1.8 Other Kinds of Hybrid Orbitals : sp2 and sp The bonds we’ve seen in methane and ethane are called single bonds because they result from the sharing of one electron pair between bonded atoms. In some molecules carbon atoms can also form a double bond by sharing two electron pairs between atoms or a triple bond by sharing three electron pairs. Ethylene, for instance, has the structure H2CCH2 and contains a carbon– carbon double bond, while acetylene has the structure HCCH and contains a carbon–carbon triple bond. Department of Chemistry , College of Science and Mathematics 73 1.8 Other Kinds of Hybrid Orbitals : sp2 and sp Department of Chemistry , College of Science and Mathematics 74 1.8 Other Kinds of Hybrid Orbitals : sp2 and sp formation of sp2 hybrid orbitals Department of Chemistry , College of Science and Mathematics 75 1.8 Other Kinds of Hybrid Orbitals : sp2 and sp The three equivalent sp2 hybrid orbitals (green) lie in a plane at angles of 120° to one another, and a single unhybridized p orbital (red/blue) is perpendicular to the sp2 plane. Department of Chemistry , College of Science and Mathematics 76  1.8 Other Kinds of Hybrid Orbitals : sp2 and sp The structure of ethylene. Orbital overlap of two sp2-hybridized carbons forms a carbon–carbon double bond. One part of the double bond results from σ (head-on) overlap of sp2 orbitals (green), and the other part results from π (sideways) overlap of unhybridized p orbitals (red/blue). The π bond has regions of electron density above and below a line drawn between nuclei. Department of Chemistry , College of Science and Mathematics 77 1.8 Other Kinds of Hybrid Orbitals : sp2 and sp Ethylene : C2H4 : 1 σ bond and 1 π bond Department of Chemistry , College of Science and Mathematics 78 1.8 Other Kinds of Hybrid Orbitals : sp2 and sp formation of sp hybrid orbitals Department of Chemistry , College of Science and Mathematics 79 1.8 Other Kinds of Hybrid Orbitals : sp2 and sp An sp-hybridized carbon atom. The two sp hybrid orbitals (green) are oriented 180° away from each other, perpendicular to the two remaining p orbitals (red/blue). Department of Chemistry , College of Science and Mathematics 80 1.8 Other Kinds of Hybrid Orbitals : sp2 and sp The structure of acetylene. The two sp- hybridized carbon atoms are joined by one sp–sp σ bond and two p–p π bonds. Department of Chemistry , College of Science and Mathematics 81 1.8 Other Kinds of Hybrid Orbitals : sp2 and sp Acetylene : C2H2 : 1 σ bond and 2 π bonds Department of Chemistry , College of Science and Mathematics 82 Department of Chemistry , College of Science and Mathematics 83  Hybridization and Geometry The shape of organic molecules is determined by the hybridization of the atoms. Department of Chemistry , College of Science and Mathematics 84  Strength of pi () and sigma () Bonds Functional groups which contain  bonds are generally more reactive as the  bond is weaker than the σ bond. The  bond in an alkene or alkyne is around 210 to 230 kJ/mol, while the σ bond is around 350 kJ/mol. Department of Chemistry , College of Science and Mathematics 85  Bond Strength the greater the ‘s’ character of the carbon orbitals, the shorter the bond length, because the electrons are held closer to the nucleus the shorter the bond length, the stronger the bond Department of Chemistry , College of Science and Mathematics 86  1.9 Polar Covalent Bonds : Electronegativity chemical bonds : ionic or covalent the bond in sodium chloride is ionic sodium transfers an electron to chlorine to give Na+ and Cl ions which are held together in the solid by electrostatic attractions between the unlike charges the C - C bond in ethane is covalent the two bonding electrons are shared equally by the two equivalent carbon atoms, resulting in a symmetrical electron distribution in the bond Department of Chemistry , College of Science and Mathematics 87  1.9 Polar Covalent Bonds : Electronegativity Most bonds are neither fully ionic nor fully covalent but are somewhere between the two extremes. Such bonds are called polar covalent bonds, meaning that the bonding electrons are attracted more strongly by one atom than the other so that the electron distribution between atoms is not symmetrical. Department of Chemistry , College of Science and Mathematics 88  1.9 Polar Covalent Bonds : Electronegativity Covalent bond to Ionic bond The continuum in bonding from covalent to ionic is a result of an unequal distribution of bonding electrons between atoms. The symbol  (lowercase Greek delta) means partial charge, either partial positive (+) for the electron-poor atom or partial negative (–) for the electron-rich atom. Department of Chemistry , College of Science and Mathematics 89  1.9 Polar Covalent Bonds : Electronegativity Types of Covalent Bond nonpolar covalent bond results from the combination of two nonmetallic atoms of the same electronegativity equal sharing of valence electrons polar covalent bond results from the combination of two nonmetallic atoms of different electronegativity unequal sharing of valence electrons separation of charges bonding electrons are attracted towards the more electronegative atom Department of Chemistry , College of Science and Mathematics 90 1.9 Polar Covalent Bonds : Electronegativity Bond Polarity bond polarity is due to differences in electronegativity (EN) electronegativity (EN) is the intrinsic ability of an atom to attract the shared electrons in a covalent bond Department of Chemistry , College of Science and Mathematics 91  Electronegativity Values of the Elements Elements in orange are the most electronegative, those in peach are medium, and those in green are the least electronegative. Department of Chemistry , College of Science and Mathematics 92  1.9 Polar Covalent Bonds : Electronegativity a bond between atoms with similar electronegativities is nonpolar covalent a bond between atoms whose electronegativities differ by less than 2 units is polar covalent a bond between atoms whose electronegativities differ by 2 units or more is largely ionic Department of Chemistry , College of Science and Mathematics 93 1.9 Polar Covalent Bonds : Electronegativity EN = 0 0 < EN < 2.0 EN ≥ 2.0 Department of Chemistry , College of Science and Mathematics 94  1.9 Polar Covalent Bonds : Electronegativity A carbon–hydrogen bond is relatively nonpolar because carbon and hydrogen have similar electronegativities. ENcarbon = 2.5 ENhydrogen = 2.1 Department of Chemistry , College of Science and Mathematics 95  1.9 Polar Covalent Bonds : Electronegativity A bond between carbon and a more electronegative element such as oxygen or chlorine is polar covalent. The electrons in such a bond are drawn away from carbon toward the more electronegative atom, leaving the carbon with a partial positive charge, +, and leaving the more electronegative atom with a partial negative charge, -. Department of Chemistry , College of Science and Mathematics 96 1.9 Polar Covalent Bonds : Electronegativity Department of Chemistry , College of Science and Mathematics 97 1.9 Polar Covalent Bonds : Electronegativity A bond between carbon and a less electronegative element is polarized so that carbon bears a partial negative charge (-) and the other atom bears a partial positive charge (+). Department of Chemistry , College of Science and Mathematics 98 1.9 Polar Covalent Bonds : Electronegativity Department of Chemistry , College of Science and Mathematics 99 1.9 Polar Covalent Bonds : Electronegativity Electrostatic potential maps Electron rich (red) Electron poor (blue) Department of Chemistry , College of Science and Mathematics 100 1.9 Polar Covalent Bonds : Electronegativity methanol, CH3OH, has a polar covalent C  O bond Department of Chemistry , College of Science and Mathematics 101 1.9 Polar Covalent Bonds : Electronegativity methyllithium, CH3Li, has a polar covalent C  Li bond Department of Chemistry , College of Science and Mathematics 102  1.9 Polar Covalent Bonds : Electronegativity crossed arrow is used to indicate the direction of bond polarity electrons are displaced in the direction of the arrow The tail of the arrow (which looks like a plus sign) is electron-poor (+), and the head of the arrow is electron-rich (–). Department of Chemistry , College of Science and Mathematics 103  1.9 Polar Covalent Bonds : Electronegativity Electronegativity and Inductive Effect An inductive effect is simply the shifting of electrons in a σ bond in response to the electronegativity of nearby atoms. Metals, such as lithium and magnesium, inductively donate electrons, whereas reactive nonmetals, such as oxygen and nitrogen, inductively withdraw electrons. Department of Chemistry , College of Science and Mathematics 104 Electronegativity and Inductive Effect Department of Chemistry , College of Science and Mathematics 105  Predicting the Polarity of Bonds Predict the extent and direction of polarization of the O‒H bonds in H2O. Which element in each of the following pairs is more electronegative? (a) Li or H (b) Be or Br (c) Cl or I Department of Chemistry , College of Science and Mathematics 106  Predicting the Polarity of Bonds Predict the extent and direction of polarization of the O‒H bonds in H2O. Which element in each of the following pairs is more electronegative? (a) Li or H (b) Be or Br (c) Cl or I Department of Chemistry , College of Science and Mathematics 107  Predicting the Polarity of Bonds Use the +/‒ convention to indicate the direction of expected polarity for each of the bonds shown: (a) H3C―Br (b) H3C―NH2 (c) H2N―H (d) H3C―SH (e) H3C―MgBr (f) H3C―F Order the bonds in the following compounds according to their increasing ionic character: CCl4 , MgCl2 , TiCl3 , Cl2O Department of Chemistry , College of Science and Mathematics 108  Predicting the Polarity of Bonds Use the +/‒ convention to indicate the direction of expected polarity for each of the bonds shown: (a) H3C―Br (b) H3C―NH2 (c) H2N―H C + , Br ‒ C + , N ‒ H + , N ‒ (d) H3C―SH (e) H3C―MgBr (f) H3C―F C + , S ‒ Mg + , C ‒ C + , F ‒ Order the bonds in the following compounds according to their increasing ionic character: CCl4 , MgCl2 , TiCl3 , Cl2O CCl4 and Cl2O < TiCl3 < MgCl2 Department of Chemistry , College of Science and Mathematics 109 1.10 Acids and Bases : The Bronsted-Lowry Definition a Brønsted–Lowry acid is a substance that donates a hydrogen ion (H+) a Brønsted–Lowry base is a substance that accepts a hydrogen ion (H+) the conjugate base of the acid is the product that results when the acid loses a proton the conjugate acid of the base is the product that results when the base gains a proton Department of Chemistry , College of Science and Mathematics 110 1.10 Acids and Bases : The Bronsted-Lowry Definition Donates/accepts a hydrogen ion (H+) or a proton Conjugate A/B Department of Chemistry , College of Science and Mathematics 111 1.10 Acids and Bases : The Bronsted-Lowry Definition Department of Chemistry , College of Science and Mathematics 112  Acid – Base Reactions A strong acid yields a weak conjugate base, and a weak acid yields a strong conjugate base. A strong acid is one that loses H+ easily, meaning that its conjugate base holds the H+ weakly and is therefore a weak base. A weak acid is one that loses H+ with difficulty, meaning that its conjugate base does hold the proton tightly and is therefore a strong base. Department of Chemistry , College of Science and Mathematics 113  Acid – Base Reactions example : HCl is a strong acid means that Cl‒ does not hold H+ tightly and is a weak base water is a weak acid means that OH‒ holds H+ tightly and is a strong base Department of Chemistry , College of Science and Mathematics 114  Acid – Base Reactions In an acid–base reaction, a proton always goes from the stronger acid to the stronger base. an acid donates a proton to the conjugate base of any acid with a larger pKa , and the conjugate base of an acid removes a proton from any acid with a smaller pKa Department of Chemistry , College of Science and Mathematics 115 Acid – Base Reactions Department of Chemistry , College of Science and Mathematics 116  Acid – Base Reactions in an acid–base reaction, the product conjugate acid must be weaker and less reactive than the starting acid and the product conjugate base must be weaker and less reactive than the starting base Department of Chemistry , College of Science and Mathematics 117  Predicting Acid – Base Reactions Water has pKa = 15.74, and acetylene has pKa = 25. Which of the two is more acidic? Will hydroxide ion react with acetylene? Department of Chemistry , College of Science and Mathematics 118  Predicting Acid – Base Reactions Water has pKa = 15.74, and acetylene has pKa = 25. Which of the two is more acidic? Will hydroxide ion react with acetylene? water is a stronger acid than acetylene ; water loses a proton more easily than acetylene, thus the HO‒ ion has less affinity for a proton than the HC  C: ‒ ion. In other words, the anion of acetylene is a stronger base than hydroxide ion, and the reaction will not proceed as written. Department of Chemistry , College of Science and Mathematics 119  Predicting Acid – Base Reactions Formic acid, HCO2H, has pKa = 3.75, and picric acid, C6H3N3O7, has pKa = 0.38. Which is stronger, formic acid or picric acid? Amide ion, H2N―, is a stronger base than hydroxide ion, HO―. Which is the stronger acid, H2N―H (ammonia) or HO―H (water)? Department of Chemistry , College of Science and Mathematics 120  Predicting Acid – Base Reactions Formic acid, HCO2H, has pKa = 3.75, and picric acid, C6H3N3O7, has pKa = 0.38. Which is stronger, formic acid or picric acid? picric acid Amide ion, H2N―, is a stronger base than hydroxide ion, HO―. Which is the stronger acid, H2N―H (ammonia) or HO―H (water)? water Department of Chemistry , College of Science and Mathematics 121  Predicting Acid – Base Reactions Given the following pKa values, which of the following reactions is likely to take place? HCN pKa = 9.31 CH3CO2H pKa = 4.76 CH3CH2OH pKa = 16.00 Department of Chemistry , College of Science and Mathematics 122 Predicting Acid – Base Reactions Department of Chemistry , College of Science and Mathematics 123  Predicting Acid – Base Reactions Will the following reaction take place given their pKa values ? yes Department of Chemistry , College of Science and Mathematics 124  1.11 Organic Acids and Organic Bases Organic Acids characterized by the presence of a positively polarized H atom two kinds : 1. those that lose a proton from O–H e.g. methanol and acetic acid Department of Chemistry , College of Science and Mathematics 125  1.11 Organic Acids and Organic Bases Organic Acids those acids that contain a hydrogen atom bonded to an electronegative oxygen atom (O-H) Department of Chemistry , College of Science and Mathematics 126  1.11 Organic Acids and Organic Bases Organic Acids 2. those that lose a proton from C–H, a carbon atom next to a C=O double bond (O=C–C–H) e.g. acetone Department of Chemistry , College of Science and Mathematics 127 1.11 Organic Acids and Organic Bases Organic Acids those that contain a hydrogen atom bonded to a carbon atom next to a C=O double bond (O=C-C-H) Department of Chemistry , College of Science and Mathematics 128  1.11 Organic Acids and Organic Bases Organic Acids Compounds called carboxylic acids, which contain the ‒CO2H grouping, are particularly common. They occur abundantly in all living organisms and are involved in almost all metabolic pathways. examples : acetic acid, pyruvic acid, and citric acid Department of Chemistry , College of Science and Mathematics 129 1.11 Organic Acids and Organic Bases Organic Bases characterized by the presence of an atom with a lone pair of electrons that can bond to H+ N-containing compounds are the most common organic bases e.g. methylamine Department of Chemistry , College of Science and Mathematics 130  1.11 Organic Acids and Organic Bases Organic Bases oxygen-containing compounds can also act as bases when reacting with a sufficiently strong acid some oxygen-containing compounds can act as both acids and bases depending on the circumstances Department of Chemistry , College of Science and Mathematics 131  1.11 Organic Acids and Organic Bases Organic Bases Methanol and acetone act as acids when they donate a proton but act as bases when their oxygen atom accepts a proton. Department of Chemistry , College of Science and Mathematics 132  1.11 Organic Acids and Organic Bases amino acids, so named because they are both amines (‒NH2) and carboxylic acids (‒CO2H), are the building blocks from which the proteins present in all living organisms are made Twenty different amino acids go into making up proteins ; example - alanine Interestingly, alanine and other amino acids exist primarily in a doubly charged form called a zwitterion rather than in the uncharged form. Department of Chemistry , College of Science and Mathematics 133  1.11 Organic Acids and Organic Bases The zwitterion form arises because amino acids have both acidic and basic sites within the same molecule and therefore undergo an internal acid–base reaction. Department of Chemistry , College of Science and Mathematics 134  1.12 Acids and Bases : The Lewis Definition a Lewis acid is a substance that accepts an electron pair a Lewis base is a substance that donates an electron pair the donated electron pair is shared between the acid and the base in a covalent bond Department of Chemistry , College of Science and Mathematics 135  1.12 Acids and Bases : The Lewis Definition The fact that a Lewis acid is able to accept an electron pair means that it must have either a vacant, low-energy orbital or a polar bond to hydrogen so that it can donate H+ (which has an empty 1s orbital). Thus, the Lewis definition of acidity includes many species in addition to H+. e.g. various metal cations, such as Mg2+, and metal compounds, such as AlCl3 , are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases Department of Chemistry , College of Science and Mathematics 136 1.12 Acids and Bases : The Lewis Definition Department of Chemistry , College of Science and Mathematics 137  1.12 Acids and Bases : The Lewis Definition The Lewis definition of a base - a compound with a pair of nonbonding electrons that it can use in bonding to a Lewis acid - is similar to the Brønsted–Lowry definition. e.g. H2O and trimethylamine Department of Chemistry , College of Science and Mathematics 138  1.12 Acids and Bases : The Lewis Definition H2O, with its two pairs of nonbonding electrons on oxygen, acts as a Lewis base by donating an electron pair to an H+ in forming the hydronium ion, H3O+ Department of Chemistry , College of Science and Mathematics 139  1.12 Acids and Bases : The Lewis Definition trimethylamine acts as a Lewis base by donating an electron pair on its nitrogen atom to aluminum chloride Department of Chemistry , College of Science and Mathematics 140  1.12 Acids and Bases : The Lewis Definition The Lewis definition of acidity includes metal cations, such as Mg2+ : they accept a pair of electrons when they form a bond to a base Group 3A elements, such as BF3 and AlCl3, are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases Transition-metal compounds, such as TiCl4, FeCl3, ZnCl2, and SnCl4, are Lewis acids Organic compounds that undergo addition reactions with Lewis bases are called electrophiles and therefore Lewis Acids Department of Chemistry , College of Science and Mathematics 141  1.12 Acids and Bases : The Lewis Definition Most oxygen- and nitrogen-containing organic compounds are Lewis bases because they have lone pairs of electrons Some compounds can act as both acids and bases, depending on the reaction Department of Chemistry , College of Science and Mathematics 142 1.12 Acids and Bases : The Lewis Definition Department of Chemistry , College of Science and Mathematics 143 1.12 Acids and Bases : The Lewis Definition the direction of electron-pair flow from the electron-rich Lewis base to the electron- poor Lewis acid is shown using curved arrows. Department of Chemistry , College of Science and Mathematics 144  1.12 Acids and Bases : The Lewis Definition A curved arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. Department of Chemistry , College of Science and Mathematics 145  Using Curved Arrows to Show Electron Flow Using curved arrows, show how acetaldehyde, CH3CHO, can act as a Lewis base in a reaction with a strong acid, HA. Department of Chemistry , College of Science and Mathematics 146  Using Curved Arrows to Show Electron Flow Using curved arrows, show how acetaldehyde, CH3CHO, can act as a Lewis base in a reaction with a strong acid, HA. Department of Chemistry , College of Science and Mathematics 147 exercise Department of Chemistry , College of Science and Mathematics 148 exercise Answer (a) Lewis acid and Lewis base (b) Lewis base (c) Lewis acid (d) Lewis acid (e) Lewis acid (f) Lewis base Department of Chemistry , College of Science and Mathematics 149  Summary and Key Words Organic chemistry is the study of carbon compounds. Although a division into inorganic and organic chemistry occurred historically, there is no scientific reason for the division. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 150  Summary and Key Words An atom is composed of a positively charged nucleus surrounded by negatively charged electrons that occupy specific regions of space called orbitals. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 151  Summary and Key Words Different orbitals have different energy levels and shapes. s orbitals are spherical p orbitals are dumbbell-shaped Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 152  Summary and Key Words There are two fundamental kinds of chemical bonds: ionic bonds covalent bonds. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 153  Summary and Key Words The ionic bonds commonly found in inorganic salts result from the electrical attraction of unlike charges. The covalent bonds found in organic molecules result from the sharing of one or more electron pairs between atoms. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 154  Summary and Key Words Electron sharing occurs when two atoms approach and their atomic orbitals overlap. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 155  Summary and Key Words Bonds formed by head-on overlap of atomic orbitals are called sigma (σ) bonds, and bonds formed by sideways overlap of p orbitals are called pi (π) bonds. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 156  Summary and Key Words In the valence bond description, carbon uses hybrid orbitals to form bonds in organic molecules. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 157  Summary and Key Words When forming only single bonds with tetrahedral geometry, carbon uses four equivalent sp3 hybrid orbitals. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 158  Summary and Key Words When forming double bonds, carbon has three equivalent sp2 orbitals with planar geometry and one unhybridized p orbital. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 159  Summary and Key Words When forming triple bonds, carbon has two equivalent sp orbitals with linear geometry and two unhybridized p orbitals. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 160  Summary and Key Words Organic molecules often have polar covalent bonds because of unsymmetrical electron sharing caused by the electronegativity of atoms. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 161  Summary and Key Words A carbon–oxygen bond is polar because oxygen attracts the bonding electrons more strongly than carbon does. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 162  Summary and Key Words A carbon–metal bond is polarized in the opposite sense because carbon attracts electrons more strongly than metals do. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 163  Summary and Key Words A Brønsted–Lowry acid is a substance that can donate a proton (hydrogen ion, H+) A Brønsted–Lowry base is a substance that can accept a proton. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 164  Summary and Key Words The strength of an acid is given by its acidity constant, Ka. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 165  Summary and Key Words A Lewis acid is a substance that can accept an electron pair. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 166  Summary and Key Words A Lewis base is a substance that can donate an unshared electron pair. Most organic molecules that contain oxygen and nitrogen are Lewis bases. Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 167 Summary and Key Words E N D Fundamentals of Organic Chemistry John McMurry , 7th ed FGdLTenido Department of Chemistry , College of Science and Mathematics 168 Thank You