Organic Chemistry Grade 10 PDF

1 O UNIT H2N — C — NH2 10...

1 O UNIT H2N — C — NH2 10 9. 5° — C— C— C C —C C— Introduction to Organic Chemistry Unit Outcomes After completing this unit, you will be able to: ! know the historical development of organic chemistry and the classification of organic compounds; ! know the general formulas of alkanes, alkenes, alkynes, alcohols, aldehydes, ketones, carboxylic acids and esters; ! develop skills in naming and writing the molecular and structural formulas of simple alkanes, branched-chain alkanes, simple alkenes, branched-chain alkenes, simple alkynes, alcohols, aldehydes, ketones, carboxylic acids and esters; ! understand isomerism and know possible isomers of alkanes, alkenes and alkynes; ! know the major natural sources of hydrocarbons; ! understand the physical and chemical properties and the general methods of preparation of alkanes, alkenes, alkynes, benzene and alcohols; ! know the uses of organic compounds in the manufacture of plastics, beverages, pharmaceuticals, soaps and detergents, dry cleaning chemicals, fuels, pesticides, and herbicides; and ! demonstrate scientific inquiry skills: observing, classifying, communicating, measuring, asking questions, interpreting data, drawing conclusions, applying concepts, predicting and problem solving. 1  CHEMISTRY GRADE 10 MAIN CONTENTS 1.1 Introduction 1.2 Saturated hydrocarbons (alkanes) 1.3 Unsaturated hydrocarbons (alkenes and alkynes) 1.4 Aromatic hydrocarbons 1.5 Natural sources of hydrocarbons 1.6 Alcohols 1.7 Industrial and agricultural applications of organic compounds – Unit Summary – Review Exercise 1.1 INTRODUCTION Competencies By the end of this section, you will be able to: narrate the historical development of organic compounds; define the term functional group; classify organic compounds based on their functional groups. Historical Notes Friedrich Wöhler In 1828 German chemist, Friedrich Wöhler converted ammonium cyanate, an inorganic compound, into the organic substance urea. Wöhler’s discovery revolutionized the study of chemistry by redefining the manner in which chemists distinguished between inorganic and organic compounds. He is also credited for his work in isolating the elements aluminium Friedrich Wöhler and beryllium. Activity 1.1 1. Do you agree with the notion that says: “carbon compounds can be synthesized only by animals and plants”? 2. Draw diagrams to show how carbon atoms can link to one another in different ways to form a variety of compounds by considering only four carbon atoms. Discuss with your group and present it to the class. 2 2  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) History of Organic Chemistry Before the beginning of the nineteenth century, chemists classified compounds into two classes; organic and inorganic. Those derived from living things (plants and animals) were classified as organic compounds, while those that came from mineral constituents of the earth or were found with non-living things are classified as inorganic compounds. What was the basis of this classification? Early chemists believed that organic compounds differed from inorganic compounds because living things had a special ‘life force’ within them, which was invisible and hard to detect. According to the theory of ‘life force’, the only source of organic compounds was nature itself (plants and animals). It was thought impossible to synthesize them in the laboratory. This was because man could not add or create the ‘life force’ within them. The belief in the ‘life force’ theory continued until Friedrich Wöhler synthesized urea for the first time in 1828. In attempting to prepare ammonium cyanate, NH4CNO, from the reaction of silver cyanate, AgCNO, and ammonium chloride, NH4Cl, he accidentally and surprisingly obtained urea, (NH2)2CO: AgCNO(aq) + NH4Cl(aq) Æ AgCl(s) + NH4CNO(aq) Urea was the first organic compound synthesized in the laboratory. The synthesis of urea by Friedrich Wöhler and subsequent synthesis of other organic compounds marked the downfall of the ‘life force’ theory. How do you explain organic compounds at present and define organic chemistry? The common feature of organic compounds is that they all contain the element carbon. Organic compounds are the compounds of carbon found in and derived from plants and animals. They also include those substances synthesized in laboratories except the oxides of carbon, carbonates, hydrogen carbonates, cyanides and cyanates. Besides carbon, these compounds contain a few other elements such as hydrogen, oxygen, nitrogen, sulphur, halogens and phosphorus. The branch of chemistry that studies carbon compounds is called organic chemistry. This branch of chemistry was developed, starting from the theory of ‘life force’ to the era in which synthetic organic compounds are used in our daily lives. The number of inorganic compounds discovered and prepared may be in the region of some hundred thousand. At present, millions of organic compounds have been discovered, synthesized, and used. 3 3  CHEMISTRY GRADE 10 The main reason for the presence of millions of carbon compounds is the unique property of carbon called catenation. Catenation is the ability of atoms of the same element to join together forming short or long chains and rings. A few other elements like sulphur, silicon and boron show this behaviour, but to a much lesser extent. Classification of Organic Compounds Activity 1.2 CH3CH2CHO and CH3COCH3 have the same chemical formula C3H6O. Write their detailed structures and observe their difference? Why do these compounds have different properties? Discuss with your group and present it to the class. Organic compounds are generally classified based on their functional group. What are functional groups and what groups of organic compounds are known on the basis of this classification? The functional group is the part of a molecule or a compound that determines the chemical properties of that molecule or compound. This group also determines some of the physical properties of a compound. Based on the functional groups they possess, the most common classification of organic compounds including alkanes, alkenes, alkynes, aromatics, alcohols, aldehydes, ketones, carboxylic acids and esters. The common functional groups of some organic compounds are given in Table 1.1. Table 1.1 Some classes of organic compounds, their functional groups, general structural formula with typical examples (R = alkyl or H). Organic Functional General Example Compound Group Structure Alkane C C R — CH2 — CH3 H3C — CH2 — CH3 Propane (single bond) C C H2C CH2 Alkene R — CH CH2 Ethene (double bond) C C H—C C—H Alkyne R—C C—R (triple bond) Ethyne Continued next page 4 4  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Organic Functional General Example Compound Group Structure H 3C Aromatic R (benzene ring) Toluene CH3CH2 OH Alcohols — OH R — OH Ethanol — CHO group R — CHO CH3 — CHO O O O Aldehydes C C C H H3C H R H Ethanal — CO — group CH3COCH3 O O Ketone C C H3C CH3 Propanone — COOH group R — COOH O Carboxylic O O Acid H3C C OH C OH R C OH Ethanoic acid — COOR group O Ester O H3C — C — O CH2CH3 C O R O Ethyl ethanoate Exercise 1.1 1. Which compounds were classified as organic compounds, according to the early chemists? 2. According to the belief of early chemists, which class of compounds has a special ‘life force’ within them? 3. What are the differences between organic and inorganic compounds, according to early chemists? 4. What is the main concept of the theory of ‘life force’? 5 5  CHEMISTRY GRADE 10 5. Who disproved the ‘life force’ theory; and how? 6. What was the first organic compound synthesized in the laboratory? 7. Identify the functional groups in each of the following compounds? a CH3CH2COCH2CH2CH3 c CH3CH2CH2COOH b CH3CH2COOCH2CH3 d CH3CH2CH2CH2OH 8. Define the following terms. a Organic chemistry b Catenation c Functional group 9. Why is it necessary to assign organic chemistry exclusively to the study of carbon compounds? 10. What is the basis for the classification of organic compounds? 11. Write the general structures of: a alkanes b alkenes c alkynes d aromatics e alcohols 1.2 SATURATED HYDROCARBONS Competencies At the end of this topic, you will be able to: define hydrocarbons; define saturated hydrocarbons; define homologous series; write the general formula of alkanes; write the first ten members of alkanes; write the molecular formulas of alkanes from the given number of carbon atoms; explain the physical properties of alkanes; write the structural formulas of the first ten alkanes; apply IUPAC rules to name straight and branched chain alkanes; define isomerism as the way compounds having the same formula differ in the way their atoms are arranged; define structural isomerism; write possible structural isomers for C4H10, C5H12 and C6H14; describe the general method for the preparation of alkanes in the laboratory; 6 6  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) prepare methane in a laboratory by the decarboxylation method; carry out a project to produce biogas from cow dung; and explain the chemical properties of alkanes. Activity 1.3 1. Draw the Lewis structure of carbon (Z = 6) and hydrogen (Z = 1) atoms. How many valence electrons are there in an atom of carbon and hydrogen respectively? 2. Consider three organic compounds containing two carbon atoms each. The two carbon atoms contributed one electron each in the first, two electrons each in the second and three electrons each in the third for the bond they form between them by sharing the electrons. The remaining valence electrons of each carbon atom in all the three compounds were used to form bonds with hydrogen atoms. a What type of covalent bond do the two-carbon atoms form between themselves in the first, the second and the third compound, respectively? b How many hydrogen atoms can form bonds with each carbon atom in the first, the second and the third compound, respectively? c Draw line or dot formula to show the formation of the bonds between the carbon atoms and carbon and the hydrogen atoms in the first, the second and the third compound, respectively. d If the three compounds mentioned above are hydrocarbons, define hydrocarbons? f If the first compound is a saturated hydrocarbon, the second and the third are unsaturated, what is the basis for such a classification of hydrocarbons? Discuss with your group and present it to the class. What are hydrocarbons and on what basis do we categorize them? Hydrocarbons are organic compounds composed of the elements carbon and hydrogen only. Hydrocarbons are subdivided into two groups, based on the type of bonding between carbon atoms, as saturated hydrocarbons and unsaturated hydrocarbons. Saturated Hydrocarbons are those compounds of carbon and hydrogen containing only carbon-carbon single bonds. Methane, ethane, and propane are typical examples of this group. Unsaturated Hydrocarbons are those compounds of carbon and hydrogen possessing one or more multiple bonds (double or triple bonds) between carbon atoms. Alkenes, alkynes and aromatic hydrocarbons are examples of unsaturated hydrocarbons. A formula that shows all the bonds and atoms is called a detailed structure. The shorter structure is called condensed structure. 7 7  CHEMISTRY GRADE 10 Example 1.2.1 Alkanes or Paraffins Activity 1.4 Draw a graph using the information in Table 1.2 by plotting the number of carbon atoms of each compound on one axis, and hydrogen atoms on the other. Predict the formulas of the saturated hydrocarbons containing 8, 9 and 10 carbon atoms and complete Table 1.2. Discuss with your group and present it to the class. Alkanes are saturated hydrocarbons. They contain chains of carbon atoms linked by single bonds only. Every carbon atom in the molecule forms four single covalent bonds with other atoms. Alkanes have the general formula CnH2n+2, where, n = 1, 2, 3... Using this general formula, we can write the molecular formula of any alkane containing a specific number of carbon atoms. For example, the chemical formulas of alkanes containing one, two and three carbon atoms are C1H2×1+2 = CH4, C2H2×2 + 2 = C2H6, and C3H2×3 + 2 = C3H8, respectively. When we compare the formulas of CH4 and C2H6 or C2H6 and C3H8, they differ by one carbon and two hydrogen atoms or – CH2 – group called the methylene group. A group of compounds in which each member of the group differs from the next member by a –CH2, is called a homologous series. Compounds in the same homologous series can be represented by the same general formula. The individual members of the group are called homologues. Alkanes contain carbon-carbon single bonds (— C — C —) as their functional group. 8 8  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) The first ten members of the homologous series of straight-chain alkanes are given in Table 1.2. Table 1.2 Homologous series and physical constants of the first ten straight chain alkanes Name Formula Physical M.P B.P Density No. of state (at (°C) (°C) at 20°C isomers room temp.) Methane CH4 gas –183 –162 0.717 g/L 1 Ethane C2H6 gas –172 –89 1.35 g/L 1 Propane C3H8 gas –187 –42 2.02 g/L 1 n-Butane C4H10 gas –135 –0.5 2.48 g/L 2 n-Pentane C5H12 liquid –130 36 0.63 g/mL 3 n-Hexane C6H14 liquid –94 68 0.66 g/mL 5 n-Heptane C7H16 liquid –91 98 0.68 g/mL 9 n-Octane ? liquid –57 126 0.70 g/mL 18 n-Nonane ? liquid –54 151 0.71 g/mL 35 n-Decane ? liquid –30 174 0.73 g/mL 75 Activity 1.5 1. What happens to the percentage by mass of hydrogen and the physical state of alkanes as the number of carbon atoms increase? 2. Categorize the petroleum related products you encounter everyday as solids, liquids and gases? Discuss in your group and present it to the class. 1.2.2 Physical Properties of Alkanes Do you know which alkanes exist in the liquid, solid or gaseous state at room temperature? What types of intermolecular forces of attraction exist between the molecules of alkanes? Explain why the boiling points and melting points of alkanes increase with increasing number of carbon atoms. At room temperature, the first four alkanes, methane to butane, are gases; whereas pentane (C5H12) to heptadecane (C17H36) are liquids, and the alkanes containing eighteen and more carbon atoms are solids. Alkanes are nonpolar organic compounds. Weak intermolecular forces called Van der Waal’s forces hold their molecules together. The strength of these forces increases with the 9 9  CHEMISTRY GRADE 10 increasing surface area (molecular mass) of the alkanes. Since alkanes are non-polar, they are almost insoluble in polar solvents like water, but, they are soluble in non-polar solvents like benzene, toluene, ether and carbon tetrachloride. The density, melting point, and boiling point of the homologous series of alkanes increase as the carbon number increases. For alkanes of the same carbon number, branched-chain isomers have lower boiling points than the straight-chain (normal) alkanes. The reason is, as branching increases, there is a decrease in surface area and the strength of the intermolecular force, which, in turn, results in a decrease in boiling point; e.g., the boiling points of n-pentane, iso-pentane and neo-pentane are 36°C, 28°C and 9.5°C, respectively. Alkanes containing all the carbon atoms in one continuous chain are called normal or straight-chain alkanes and those containing chains with branches are known as branched- chain alkanes. Example In alkanes (branched or straight-chain) each carbon atom is tetrahedrally bonded to four atoms with a bond angle of 109.5°. a Ball and Stick Model of Methane b Tetrahedral structure of methane Figure 1.1 Structure of methane. In alkanes containing a continuous chain of carbon atoms, the carbon atoms are not linked in straight line but in zigzag chains to keep the tetrahedral distribution of atoms. 10 10  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Zigzag chain in butane Ball and stick model of butane Figure 1.2 Zigzag chain and ball and stick model of butane. Activity 1.6 1. Why rules are important for naming organic compounds? Discuss in your group some of the rules you know for naming organic compounds and present it to your class. 2. In mathematics, the names pentagon, hexagon, heptagon and octagon are used to describe polygons. What does each of the prefixes- pent, -hex, -hept and -oct indicate? What is the significance of these prefixes in chemistry? Discuss in groups and present it to the class. 1.2.3 Nomenclature (naming) of Alkanes Organic chemistry uses a simplified and systematic way of naming organic compounds. The names of alkanes and most of the organic compounds are derived from: i) a prefix- indicating the number of carbon atoms (listed in Table 1.3)and ii) a -suffix indicating the type of the functional group present in the molecule. The following table introduces the prefixes that are used to indicate the presence of one to ten carbon atoms in the longest continuous carbon chain. Table 1.3 Prefixes commonly used to indicate one to ten carbon atoms. Prefix Number of carbon atoms Prefix Number of carbon atoms Meth- One Hex- Six Eth- Two Hept- Seven Prop- Three Oct- Eight But- Four Non- Nine Pent- Five Dec- Ten From Table 1.2, you can note that the names of all alkanes contain the suffix ‘-ane’, which indicates that their functional group is a carbon — carbon single bond. Thus, to name an 11 11  CHEMISTRY GRADE 10 alkane, you should use the appropriate prefix that indicates the number of carbon atoms and the suffix ‘-ane’. For example, to name the alkane C6H14, use the prefix ‘Hex’ that indicate the presence of six carbon atoms and add the suffix ‘-ane’ so that the name becomes hexane. Activity 1.7 Practice naming alkanes using first four prefixes mentioned in Table 1.3. In naming alkanes, why are number of carbon atoms used as prefixes? Discuss in groups. Common Names of Alkanes Lower members of the alkane homologous series have common names. The prefixes used in the common names are n- (normal), ‘iso-’ and ‘neo-’. The prefix n- is used when all the carbon atoms form a continuous chain. Example Iso- is used when all of the carbon atoms form a continuous chain, except for the one next to the last carbon. Example Neo- is used when the central carbon is bonded to four other carbon atoms. Example 12 12  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) IUPAC System of Nomenclature of Alkanes It is difficult to have common names for the more branched and complex alkanes. In naming such alkanes, the more systematic, convenient and appropriate one is the International Union of Pure and Applied Chemistry (IUPAC) system. In order to apply this system, it is very important to know the rules to be followed, what alkyl radicals are, and how they are named. Alkyl radicals are obtained by removing one hydrogen atom from the corresponding alkanes. Their general formula is CnH2n+1, where n = 1, 2, 3.... The names of alkyl radicals are derived from the names of the corresponding parent alkanes by changing the suffix -ane to -yl. Table 1.4 Formulas, names and structures of some alkyl groups. Formula of Condensed Common name Formula of alkyl radical structure of alkyl of alkyl radical alkane radical CH4 CH3– CH3– Methyl C2 H 6 C2H5– CH3CH2– Ethyl C3 H 8 C3H7– CH3CH2CH2– n-Propyl CH3 — CH — Isopropyl CH3 C4H10 C4H9– CH3CH2CH2CH2– n-Butyl CH3CH — CH2— Isobutyl CH3 CH3 sec-Butyl H3C— CH2 — C — (s-Butyl) H CH3 tert-Butyl H3C— C — (t-Butyl) CH3 The system of naming organic compounds is based on the recommendations of IUPAC. The following are the rules established by IUPAC to name branched-chain hydrocarbons. 13 13  CHEMISTRY GRADE 10 1. Select the longest continuous chain of carbon atoms in the molecule as the parent structure. The name of the straight-chain alkane possessing the same number of carbon atoms is used as the name of this longest chain. The groups attached to the parent structure are called side chains or substituents. Example 1 There are seven carbon atoms in the longest chain (the parent structure). Therefore, it gets the name “heptane”. 2. Assign numbers to the carbon atoms of the longest chain starting from one end to the other so that the carbon atoms to which side chains are attached get the lowest possible number. To check which direction of numbering is correct; add the locants (numbers assigned to the carbon atoms to which substituents are attached). The sum of locants in the first case is 3 + 4 = 7 and in the second case, 4 + 5 = 9. So numbering should be done from left to right in this case. Thus, the first option is correct. 3. Indicate the position of the side chain by using the number assigned to the carbon atom to which it is attached. If the same substituents appear more than once, use the 14 14  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) prefix di-, tri-, tetra- etc. before the name of the substituent to show two, three, four, etc. substituents, respectively. 4. Numbers are separated from each other by commas while they are separated from substituent names by hyphens. 5. Arrange the names of the side chains before the parent name in alphabetical order. The complete name of the alkane given in the above example will then be 4-ethyl-3- methylheptane (substituents in alphabetical order). Example 2 CH3 Cl CH3 1 2 3 4 H3C — C — CH — C — CH3 5 6 CH3 H2C — CH3 The longest chain has six carbon atoms and the parent name is hexane. There are two methyl groups at carbon-2 and another two methyl groups on carbon-4 and one chlorine atom at carbon-3. Therefore, the correct name of the compound is 3-chloro-2,2,4,4- tetramethylhexane. Example 3 The longest chain contains eight carbon atoms. Hence, the parent name of the given molecule is octane. There are three methyl groups attached to carbon number 3, 4 and 6; one bromine atom is attached to carbon-4 and two chlorine atoms are attached to carbon-3 and carbon-5. Thus, the correct name of the compound is 4-bromo-3,5-dichloro-3,4,6- trimethyloctane. 15 15  CHEMISTRY GRADE 10 Note: When F, Cl, Br and I appear as substituents, change the names fluorine, chlorine, bromine and iodine to fluoro, chloro, bromo and iodo, respectively. Example 4 CH3 CH3 1 2 3 4 5 6 7 H3C — C — CH — CH — CH — CH2 — CH3 CH3 F CH3 There are seven carbon atoms in the parent structure, so it takes the name heptane. There are two methyl groups at carbon-2 and two methyl groups at carbon-4 and carbon-5 and one fluorine at carbon-3. So the name of the compound is 3-fluoro-2,2,4,5-tetramethyl heptane. Activity 1.8 Is there any scientific background for the IUPAC rules or is it just a convention? Why IUPAC recommendations are followed throughout the world. Present your opinion to the class. Exercise 1.2 1. Write any three possible structures for octane, C8H18, and give their IUPAC names. 2. Give the IUPAC name of: 3. Write the structure of 2,2-dimethyl butane. 4. Write the structure of 2,3-dimethylpentane. 5. When F, Cl, Br and I appear as substituents in an organic compound, how are they named? 16 16  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) 1.2.4 Isomerism Activity 1.9 1. Write the molecular formula of the following compounds? Do they have the same or different molecular formula? 2. Do they have the same or different molecular formula? Do they have the same or different physical and chemical properties? Why? What do we call compounds of this type? Discuss the results in your group and present it to the class. There is only one possible structure for each of the first three alkanes, namely, methane (CH4), ethane (C2H6) and propane (C3H8). Those alkanes containing four or more carbon atoms have more than one structure. The existence of two or more chemical compounds with the same molecular formula but different structures is called isomerism. The compounds that have the same molecular formula but different structures are called isomers. Alkanes exhibit a type of structural isomerism called chain or skeletal isomerism. These structures differ in the arrangement of the carbon atoms and hydrogen atoms. To understand what isomers are, consider the isomers of butane (C4H10) and pentane (C5H12). 1. Isomers of butane (C4H10) Butane has two isomers: 2. Isomers of pentane (C5H12) Pentane has three isomers. 17 17  CHEMISTRY GRADE 10 Exercise 1.3 1. Which of the following statements are true about isomers: a They have the same molecular formula. b They are different compounds. c They have different boiling and melting points. d They have similar chemical properties. 2. Draw all the possible structural isomers for hexane and name them. 1.2.5 Preparation of Alkanes Alkanes are the major constituents of petroleum and natural gas. They are mainly obtained by fractional distillation of petroleum. Alkanes can also be prepared in the laboratory. Some methods of their preparation are as follows: 1. Hydrogenation of alkenes with a metal catalyst. General Reaction: Example 2. Wûrtz Synthesis (reaction). This method involves the reaction of a halogenated alkane with sodium and the reaction is somewhat exothermic. 18 18  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) General Reaction: Example 2CH3I + 2Na CH3 — CH3 + 2NaI Methyl iodide Ethane Sodium iodide This reaction is named as Wûrtz reaction after the French chemist Charles-Adolphe Wûrtz (1817–1884). 3. Heating sodium salt of an organic acid with soda lime (mixture of sodium hydroxide and calcium oxide); the reaction brings about the removal of the carboxylate group from the sodium salt of the carboxylic acid. This type of reaction is called decarboxylation. General Reaction: Example Methane Methane is the main constituent (about 90%) of natural gas. It is obtained during fractional distillation of petroleum. Methane is also formed by the decay and decomposition of animal and plant remains in swampy or marshy lands. 19 19  CHEMISTRY GRADE 10 Activity 1.10 Discuss the following points in groups, and present your ideas to the class. 1. Have you ever walked near a marshy or swampy area? Which characteristic smell do you observe? 2. In most parts of Ethiopia, the walls of houses are constructed from wood and mud. The mud for this purpose is usually prepared by mixing soil, water and hay or straw. After two or three weeks of decay and decomposition, an unusual smell is perceived. What do you think is the cause of this? 3. If Ethiopia introduces the technology of producing biogas to its residents, what benefits will be obtained by the country? What materials can be used to generate biogas? Which compound is the main constituent of biogas? Experiment 1.1 Laboratory Preparation of Methane Objective: To prepare methane and study its properties. Materials required: Sodium acetate (CH3COONa) and soda lime (CaO, NaOH), test tubes, delivery tube, stopper, gas jar and gas jar lid, pneumatic trough, stand, clamp, beehive shelf, Bunsen burner and balance. Procedure: Arrange the assembly as shown in Figure 1.3. Mix thoroughly 5 g powdered sodium acetate with 10 g of soda lime. Place the mixture in the test tube and heat it. Collect the gas by the downward displacement of water. Collect several jars of the gas. Rubber Delivery tube stopper Gas jar Sodium acetate + Methane Soda lime Boiling tube Pneumatic trough Beehive Bunsen Water shelf burner Figure 1.3 Preparation of methane. 20 20  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Carry out the following activities with the collected gas: Insert a burning splint into the jar full of the gas. Observe what happens in the jar. Add Ca(OH)2 solution to the jar and observe. Add a few drops of bromine water to any one of the gas jars filled with methane. Cover the gas jar and shake well. Observations and analysis: a What is the colour of the gas? Is it soluble in water? b Is the gas combustible? Write a balanced equation for the change. c What change did you observe upon addition of Ca(OH)2? d Is there any change when bromine water is added? Write a laboratory report on your observation and submit to your teacher. Methane can be produced in a biogas plant. The biogas plant generates biogas, mainly consisting of methane by anaerobic fermentation of organic materials such as human excreta, animal dung and agricultural residue. The conversion of organic materials into biogas involves a sequence of reactions and is graphically depicted in Figure 1.4. Figure 1.4 Reactions in the production of biogas. The materials that can be used to produce biogas include dung from cattle, pigs, chickens, chopped green plants and plant wastes. The advantages of biogas technology include saving on fuel such as kerosene, wood and charcoal, and decrease in local deforestation.The residue obtained after preparing the biogas can be used as fertilizer. 21 21  CHEMISTRY GRADE 10 Experiment 1.2 Production of Methane from Cow Dung Objective: To prepare biogas from locally available materials. Materials required: Conical flask, cow dung, water, delivery tube with tap and stopper with one hole. Procedure: 1. Mix some cow dung with water and pour it in a conical flask. 2. Fit the conical flask with a stopper in which a delivery tube with a tap is inserted. 3. Cover the conical flask with a cotton wool and place it near a window and leave it there for 3 to 4 days. 4. Check the formation of methane after 4 days. (Bring a lighted splint closer to the outlet of the delivery tube and open the tap). See what happens. Cotton wool Cow dung + water Figure 1.5 Preparation of methane gas from cow dung. Observations and analysis: a What is the importance of covering the conical flask with a cotton wool and placing it near the window? b What is your observation when you bring a burning splint close to the outlet of the tube? c What change do you think has occurred in the conical flask that leads to the formation of methane? Write a laboratory report in group and present to the class. PROJECT 1.1 Organize a trip to visit a biogas plant in your Kebele or neighbouring Kebeles if any and write a report on how it operates. Construct a small scale biogas plant in groups. 22 22  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) 1.2.6 Chemical Properties of Alkanes Activity 1.11 Discuss the following points in groups and present your opinions to the class. 1. Which chemical property of alkanes is responsible for generating electricity in diesel power stations or for moving motor vehicles? Name the major product formed when the fuel has been consumed in the diesel power station. 2. The reaction of methane with oxygen produces two different oxides of carbon, CO2 and CO. 1 What condition determines whether the product of the methane reaction is CO2 or CO? b If a domestic heating system is fuelled by methane, what difference does it make if the combustion produces CO2 or CO? Alkanes are generally not considered to be very reactive organic compounds. The name paraffin for alkanes arose from two Latin words ‘parum’ meaning little and ‘affinis’ means affinity. Thus, paraffin means little affinity. This name was suggested because alkanes are inert towards many reagents like acids, bases, oxidizing and reducing agents. However, they undergo several reactions under suitable conditions. The major reactions of alkanes are the following: 1. Combustion Reactions: If alkanes are burned with limited supply of oxygen, they will form water and carbon monoxide and will liberate lesser amounts of heat energy. Alkanes burn in excess oxygen to form carbon dioxide and water, liberating a greater amounts of heat. The general equation for the reaction is: Ê 3n + 1 ˆ CnH2n+2 + Ë 2 ¯ O2 Æ nCO2 + (n + 1) H2O + heat where n is the number of carbon atoms in the alkane molecule. Example C3H8 + 5O2 Æ 3CO2 + 4H2O + 2299 kJ The heat energy liberated is used to generate electricity, to move motor vehicles or to cook our food. 23 23  CHEMISTRY GRADE 10 Activity 1.12 1. Write the general equation for the combustion of alkanes in a limited supply of oxygen that forms carbon monoxide, water and energy. 2. Why is a petroleum refinery is built in the open air? Present your opinion to the class. 2. Substitution Reaction: This is a reaction that involves the replacement of one atom or a group of atoms by another atom or group of atoms. Halogenation of alkanes is a very good example of substitution reaction. This reaction involves reacting alkanes with chlorine and bromine. The reaction of chlorine and bromine with alkanes proceeds in presence of heat or sunlight. This type of reaction is called photochemical reaction. The reaction of alkanes with chlorine and bromine proceeds in a sequence of steps. For example, let us consider the photochemical reaction of methane with chlorine (chlorination): i) Chain Initiating Step: This step involves absorption of energy to generate reactive particles known as free radicals. A free radical is an atom or a group of atoms possessing unpaired electron. A free radical is electrically neutral. A chlorine molecule absorbs light and decomposes into two chlorine atoms: ii) Chain Propagating Step: This is a step which consumes a reactive particle (free radical) produced in the chain initiation step and generates another free radical. iii) Chain Terminating Step: In the chain terminating step reactive particles (free radicals) are consumed but not generated. In this step, ‘side reactions’ that do not lead to the formation of the desired products often take place. Examples Such a detailed step by step description of a chemical reaction is called a reaction mechanism. 24 24  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Chlorination of methane may produce different products depending on the relative amounts of methane and chlorine. CH4 (g) + Cl2 (g) Æ CH3Cl (g) + HCl (g) Methyl chloride CH3Cl (g) + Cl2 (g) Æ CH2Cl2 (g) + HCl (g) Methylene chloride (dichloromethane) CH2Cl2 (g) + Cl2 (g) Æ CHCl3 (g) + HCl Chloroform (trichloromethane) CHCl3 (g) + Cl2 (g) Æ CCl4 (g) + HCl (g) Carbontetrachloride (tetrachloromethane) The chlorination of methane is an example of a chain reaction that involves a series of steps. Activity 1.13 Alkanes do not react with chlorine and bromine in the dark but they do so in the presence of sunlight. Discuss the role of sunlight for the reaction in your group and present to the class. 3. Elimination reaction: This type of reaction involves the removal of smaller molecules from a compound and leads to the formation of a compound containing a multiple bond (unsaturated compound). It can also involve the removal of hydrogen atoms from adjacent carbon atoms at relatively high temperatures. Cycloalkanes Cycloalkanes are saturated hydrocarbons in which the carbon atoms are linked in such a manner as to form closed chains or ring structures. They are represented by the general formula, CnH2n, where n ≥ 3. Cycloalkanes are isomeric with open-chain alkenes. Their names are obtained by prefixing “cyclo” to the name of an alkane containing the same number of carbon atoms. The following structures represent some examples of cycloalkanes. 25 25  CHEMISTRY GRADE 10 Uses of Alkanes Alkanes are primarily used as fuels. They are also used as solvents, raw materials for making alkenes, alcohols, soaps, detergents and plastics. Exercise 1.4 1. Write the molecular formulas of alkanes containing the following number of carbon atoms. a 5 b 8 c 15 2. Define the terms a Homologous series d Substitution reaction b Alkyl radical e Combustion reaction c Isomerism f Functional groups 3. Write the balanced equation for the complete combustion of octane. 4. How many chain isomers are there for an alkane that contains 7 carbon atoms? 5. Give IUPAC names to each of the following. 26 26  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) 6. Write the structural formulas of the following compounds. a 2,3-dimethylbutane b 2-bromo-3,3,4,4-tetramethylhexane. c 2-chloro-4-ethyl-2,3-dimethylhexane. 7. Complete and balance the following chemical reaction: Complete C5H12 + O2 ææææææÆ Combustion 1.3 UNSATURATED HYDROCARBONS Competencies By the end of this section, you will be able to: 27 27  CHEMISTRY GRADE 10 define unsaturated hydrocarbons; define alkenes; write the general formula of alkenes; write the molecular formula for the first nine members in the homologous series of alkenes; define alkynes; write the general formula of alkynes; write the molecular formula for the first nine members in the homologous series of alkynes; write the molecular formulas of alkenes and alkynes from the given number of carbon atoms; describe the physical properties of alkenes and alkynes; apply IUPAC rules to name straight-and branched-chain alkenes and alkynes; write the structural formulas of alkenes and alkynes up to ten carbon atoms; write the possible structural isomers for C4H8 and C5H10; define geometric (cis-trans) isomerism; give examples of molecules that show geometric isomerism; construct models that show cis-trans isomerism; describe the general methods for the preparation of alkenes in a laboratory; prepare ethylene in the laboratory by the dehydration of ethanol; describe the general method for preparation of alkynes in a laboratory; prepare acetylene in the laboratory by the reaction of CaC2 with water; test for unsaturation of ethylene and ethyne; explain chemical properties of alkenes; explain chemical properties of alkynes; explain the uses of ethylene and acetylene; compare and contrast the properties of ethane, ethene and ethyne. 28 28  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Unsaturated hydrocarbons are those compounds of carbon and hydrogen containing either double or triple bonds. These groups of hydrocarbons include alkenes and alkynes. 1.3.1 Alkenes or Olefins Activity 1.14 1. The molecular formulas for the first three members of an alkene series are C2H4, C3H6 and C4H8. Derive the general formula for alkenes and compare it with that of alkanes. Is there a difference in the number of hydrogen atoms? 2. By referring to Table 1.5, what relationship do you observe between the number of carbon atoms and melting point, boiling point and density? Discuss with your group and present it to the class. Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond as their functional group: They are also known as olefins. They form a homologous series represented by the general formula CnH2n, where n = 2, 3, 4…. Table 1.5 The homologous series of alkenes and their physical constants. Molecular Condensed IUPAC Melting Boiling Density formula structure name point (°C) point (°C) C2H4 CH2 = CH2 Ethene –169 –102 0.61 g/L C3H6 CH2 = CH – CH3 Propene –185 –48 0.61 g/L C4H8 CH2 = CH – CH2 – CH3 1-butene –130 –6.5 0.63 g/L C5H10 CH2 = CH – (CH2)2 – CH3 1-pentene –130.5 30 0.64 g/L C6 H12 CH2 = CH – (CH2)3 – CH3 1-hexene –138 63 0.67 g/L C7 H14 CH2 = CH – (CH2)4 – CH3 1-heptene –119 93 0.69 g/L C8 H16 CH2 = CH – (CH2)5 – CH3 1-octene –104 122 0.72 g/L C9 H18 CH2 = CH – (CH2)6 – CH3 1-nonene –95 146 0.73 g/L C10 H20 CH2 = CH – (CH2)7 – CH3 1-decene –87 171 0.74 g/L 29 29  CHEMISTRY GRADE 10 Physical Properties of Alkenes Activity 1.15 1. Why some alkenes exist in gaseous and liquid states at room temperature? 2. The melting points, boiling points and densities of alkenes increase with increasing number of carbon atoms; why? 3. Are alkenes coloured compounds? Do they have smell? Discuss with your group and present it to the class. At room temperature, alkenes containing two to four carbon atoms are gases. Those containing five to seventeen carbon atoms are liquids, and those containing eighteen or more carbon atoms are solids. Alkenes are non-polar. Therefore, their molecules are held together by weak inter-molecular forces. Since they are non-polar, they are almost insoluble in polar solvents like water, but soluble in non-polar solvents like ether, benzene, toluene and carbon tetrachloride. Nomenclature of Alkenes Alkenes can have common names as well as IUPAC names. The common names of alkenes are obtained by using the prefixes in Table 1.3 and adding the suffix ‘-ylene’: Table 1.6 Common names of few alkenes. Formula of Condensed Common Alkene Structure Name C2H4 CH2 = CH2 Ethylene C3H6 CH2 = CH – CH3 Propylene C4H8 CH2 = CH –CH2–CH3 Butylene......... IUPAC System Activity 1.16 1. What difference do you expect in the IUPAC names of alkanes and alkenes? 2. If you are asked to name the following hydrocarbons: 30 30  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) i) ii) a How many carbon atoms are there in the longest chain of each hydrocarbon? b From which end do you start numbering the carbon atoms of the longest chain of the hydrocarbon shown in structure i? c From which end do you start numbering the carbon atoms of the longest chain of the hydrocarbon shown in structure ii? d Is there any difference in how you assign number to the carbon atoms of the longest chain represented by structures i and ii? If so, why? e Which name do you think is more appropriate for the longest chain of carbon atoms in structure ii? 5-hexene or 1-hexene or hexane? Discuss these questions in your group and present to the class. The IUPAC names of alkenes are obtained by using the prefixes listed in Table 1.3 to indicate the number of carbon atoms in the molecule and adding the suffix ‘-ene’. The suffix ‘-ene’ indicates the presence of a double bond. For example, the alkene having the formula C8H16 is named as octene. The prefix oct- indicates that there are eight carbon atoms in the molecule and ‘-ene’ signifies the presence of a double bond. In the IUPAC system of naming alkenes, the rules we follow are similar to those rules we follow for naming alkanes with slight modifications. These are: 1. Select the longest continuous chain of carbon atoms in the molecule that includes the double bond as a parent structure. Its name will be the same as the alkene containing the same number of carbon atoms. Activity 1.17 Why should the double bond(s) in alkenes be included in the longest continuous chain? Express your opinion to the class. 31 31  CHEMISTRY GRADE 10 2. Number the carbon atoms starting from one end to the other in such a way that the carbon atom preceding the double bond takes the lowest possible number and indicate the position of the double bond by this number in the name. Example 1 Example 2 The longest chain contains 7 carbon atoms and a double bond between the 2nd and 3rd carbon atoms. Therefore, it takes the name 2-heptene or hept-2-ene. There are three methyl groups at carbon number 3, 4 and 5. So the complete name of the compound is: 3, 4, 5-trimethyl-2-heptene or 3, 4, 5-trimethyl hept-2-ene Example 3 The parent structure contains 8 carbon atoms and a double bond between 3rd and 4th carbon atoms. Thus, it is named as 3-octene or oct-3-ene. The side chains attached are chlorine at the 6th, two methyl groups at 5th and 6th and an ethyl group at carbon number 3. Therefore, the complete name of the compound is: 6-chloro-3-ethyl-5,6-dimethyl-3-octene, or 6-chloro-3-ethyl-5,6-dimethyloct-3-ene. 32 32  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Exercise 1.5 1. Write the names of the following compounds, using IUPAC rules: a b 2. Write the structure of each of the following compounds: a 3-methyl-3-heptene b 3,4-diethyl-1-hexene Isomerism in Alkenes Alkenes containing two and three carbon atoms have only one possible structure. Alkenes containing four or more carbon atoms exhibit isomerism. Alkenes show the following three types of isomerism: 1. Chain isomerism: This is due to the difference in the arrangement of the carbon atoms in the longest continuous carbon chain. 2. Position isomerism: This is due to the difference in the position of the double bond in the carbon chain. Example Consider the following compounds with the same molecular formula (C5H12): 33 33  CHEMISTRY GRADE 10 Compounds (a) and (b) represent the position isomers of pentene, (c) and (d) are position isomers of methylbutene, (a), (c) and (e) are the chain isomers, while all these compounds are the isomers of C5H12. 3. Geometrical isomerism. This results from the difference in the relative spatial arrangement of atoms or groups about the double bond. This isomerism exists because free rotation about the double bond is not possible. To differentiate geometrical isomers, we use the prefix ‘cis’ if two similar groups are on the same side of the double bond and ‘trans’ when the two similar groups are on opposite sides of it; ‘cis’ means the same and ‘trans’ means across. The geometrical isomers of 2-pentene are: Alkenes have more isomers than the corresponding alkanes. Exercise 1.6 Write three position isomers of hexene. PROJECT 1.2 Prepare a model from locally available materials, to show the cis-trans isomers of 2-butene. Preparation of Alkenes Do you know the major source of alkenes and the process that leads to their formation? Alkenes are mainly obtained during fractional distillation of petroleum when the process called cracking is carried out. In the laboratory, alkenes can be prepared by: 1. Dehydration of alcohols with concentrated sulphuric acid or alumina (Al2O3). Dehydration means removal of water. General Reaction: 34 34  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Example 2. Dehydrohalogenation of alkyl halides with a base (KOH). Dehydrohalogenation means removal of hydrogen and a halogen atom. Alkyl halides are compounds of an alkyl group and a halogen such as CH3Cl and CH3 – CH2 – Br. General reaction: Example Activity 1.18 In the above two methods of preparations of alkenes, (a) what difference do you observe between the structures of the major reactant and the product? (b) What is the type of reaction? Share your findings with your group. Exercise 1.7 Write the complete and balanced chemical equations for each of the following reactions: a b 35 35  CHEMISTRY GRADE 10 Experiment 1.3 Laboratory preparation of Ethene Objective: To prepare ethene and study some of its properties Materials required: Ethanol, concentrated H2SO4, water, bromine water, basic KMnO4. Round-bottomed flask, stopper, delivery tube, gas jar, pneumatic trough, thermometer, tripod, Bunsen burner, stand and clamp, beehive shelf and wire gauze. Procedure: 1. Arrange the set-up as shown in Figure 1.6 2. Take ethanol in the flask and add concentrated H2SO4 through the thistle funnel; heat carefully until the temperature reaches about 170°C. Discard the initially formed gas as it might contain air; collect the ethene gas in three gas jars by downward displacement of water; perform the following activities. a Insert a lighted splint into the jar containing the gas and see what happens. b Add two drops of bromine water to the other gas jar, cover the jar and shake well. Thistle funnel Ethanol Inverted Ethene Flask + gas jar Stand concentrated sulphuric acid Burner Beehive shelf Water Figure 1.6 Preparation of Ethene. 36 36  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) c Add a few drops of alkaline KMnO4 solution to another jar containing the gas and observe what happens. Observations and analysis: a What is the colour and odour of the gas? b What is the substance left after combustion of the gas in the jar? c What change is observed when the gas is shaken with bromine water or alkaline potassium permanganate? Write equations for the reactions? d Name the dehydrating agent used in the experiment? e Which method of preparation of an alkene is used in this experiment? Write a laboratory report about the experiment and submit to your teacher. Chemical properties of Alkenes Alkenes are unsaturated hydrocarbons containing double bond (a strong bond and a weak bond) between the two carbon atoms. They are more reactive than alkanes, because the weak bond can be used for further reaction. They undergo several types of reactions. 1. Combustion reaction. Alkenes burn in oxygen with a luminous flame to form carbon dioxide and water.  3n  General reaction: CnH2n +   O2 Æ nCO2 + nH2O + heat  2 Example C4H8(g) + 6O2 (g) Æ 4CO2 (g) + 4 H2O (g) + heat 2. Addition reaction. Alkenes undergo mainly addition reactions. The addition occurs at the carbon-carbon double bond. a Addition of halogens (halogenation): When a halogen molecule, X2, (where, X2 = Cl2 or Br2) is added to alkenes, the product is a dihaloalkane. General reaction: 37 37  CHEMISTRY GRADE 10 Example If bromine water (bromine in CCl4) is added to alkenes, the reddish brown colour of Br2 in CCl4 will disappear. This is due to the addition of bromine (Br2) across the double bond. So, Br2 in CCl4 is used to detect unsaturation (presence of multiple bond) in a compound. b Addition of hydrogen (Hydrogenation): Addition of hydrogen molecule to alkenes in the presence of a nickel or platinum catalyst yields alkanes. General reaction: R – CH == CH 2 + H 2  Pt → R – CH 2 – CH 3 Alkene Alkane Example c Addition of hydrogen halides, HX (hydrohalogenation): Addition of hydrogen halide (HCl, HBr or HI) to alkenes leads to the formation of alkyl halides. The product of the reaction can be predicted by Markovnikov’s rule; which states that “when an alkene reacts with a hydrogen halide to give an alkyl halide, the hydrogen adds to the carbon atom of the double bond that has the greater number of hydrogen atoms, and the halogen to the carbon that has the fewer number of hydrogen atoms”. The same is true when an alkene reacts with water in an addition reaction to form an alcohol. General reaction: 38 38  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Example Have you noticed the location where H and Br have formed bonds? d Addition of water (hydration): When alkenes are hydrated in the presence of an acid catalyst, they produce alcohols, in accordance with Markovnikov’s rule. General reaction: Example e Oxidation of alkenes: Oxidation of alkenes with cold alkaline potassium permanganate solution (KMnO4) forms diols (compounds containing two hydroxyl groups). General reaction: Example 39 39  CHEMISTRY GRADE 10 Alkaline KMnO4 solution is also used as a qualitative test for the identification of unsaturation in a compound. The solution is called Baeyer’s reagent. In the presence of unsaturated hydrocarbons, the purple colour of alkaline KMnO4 solution fades and a brown precipitate is formed. f Polymerization (self-addition) of alkenes: Polymerization is the union of small molecules called monomers to form a large molecule called a polymer. Example Uses of Ethene (Ethylene) Ethene is used in the production of ethanol, and polymers such as polyethene, polyvinylchloride (PVC), polystyrene and teflon. It is also used in the production of 1,2-ethanediol, which is used as an antifreeze material; and for the preservation as well as artificial ripening of fruits. Activity 1.19 Take some green tomatoes in first basket and put a ripe banana in between them. In a second basket put only green tomatoes (but not a banana). Compare the tomatoes in the two baskets. Do the tomatoes ripen at the same rate in both baskets; if not, why? Share your opinion with your classmates. Cycloalkenes Cycloalkenes are unsaturated cyclic hydrocarbons. The carbon atoms are linked in such a manner as to form a closed chain or a ring structure. They contain a double bond between carbon atoms and are represented by the general formula CnH2n–2, where n should be equal to or greater than 3. They are isomeric with alkynes. They are named by prefixing “cyclo” to the name of alkenes containing the same number of carbon atoms. The given structures show some examples of cycloalkenes. The IUPAC names for alkynes are obtained by using the prefixes listed in Table 1.3. The suffix ‘-yne’ is added to the prefixes, which indicates the presence of a triple bond between a pair of adjacent carbon atoms. Alkynes are named in the same way as alkenes. 40 40  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Examples Exercise 1.8 Complete each of the following equations and name the products: a CH3CH2CH == CH2 + HCl Æ b CH3CH2CH == CH2 + Br2 Æ H+ c CH3CH2CH == CH2 + H2O  → Alkaline d CH3CH2CH == CH2  KMnO → 4 1.3.2 Alkynes Alkynes are another group of unsaturated hydrocarbons possessing a triple bond as their functional group. The homologous series of alkynes is represented by the general formula CnH2n–2, where n ≥ 2. The first member of the group is commonly known as acetylene. The homologous series of alkynes is also called acetylene series. Table 1.7 Homologous series of alkynes and their physical constants. Formula of Condensed IUPAC Melting Boiling Alkyne Structure Name Point (°C) Point (°C) C2H2 CH ∫ CH Ethyne – 82 –75 C3H4 CH ∫ C – CH3 Propyne –101.5 –23 C4H6 CH ∫ C – CH2 1 - butyne –122 9 Continued next page 41 41  CHEMISTRY GRADE 10 Formula of Condensed IUPAC Melting Boiling Alkyne Structure Name Point (°C) Point (°C) C5H8 CH ∫ C – (CH2)2 – CH3 1 - pentyne –98 40 C6H10 CH ∫ C – (CH2)3 – CH3 1 - hexyne –124 72 C7H12 CH ∫ C – (CH2)4 – CH3 1 - heptyne –80 100 C8H14 CH ∫ C – (CH2)5 – CH3 1 - octyne –70 126 1- nonyne –65 151 1- decyne –36 182 Physical Properties of Alkynes Alkynes are non-polar compounds whose molecules are held together by weak intermolecular forces. The strength of the intermolecular forces increases with increasing molecular size. The physical properties of alkynes are almost similar to those of alkenes. Activity 1.20 1. By looking at the trends for alkynes in Table 1.7, write the molecular formula and and the structures of the alkynes containing 9 and 10 carbon atoms. 2. Compare the physical state, melting points and boiling points of alkynes with alkenes and alkanes, as the number of carbon atoms increases. Discuss with your group and present to the class. Nomenclature of Alkynes Alkynes are commonly named as a derivative of acetylene. Example IUPAC System: The IUPAC names for alkynes are obtained by using the prefixes listed in Table 1.3. The suffix ‘-yne’ is added to the prefixes. ‘-yne’ indicate the presence of a triple bond between a pair of adjacent carbon atoms. Alkynes are named in the same way as alkenes. 42 42  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Example Isomerism in Alkynes Alkynes show both chain and position isomerism, but not geometrical isomerism. Example Isomers of butyne, C4H6 1-Butyne and 2-butyne are position isomers. Activity 1.21 Write the structural formulas for pentane, pentene, and pentyne. Why are not these three hydrocarbons considered as isomers? Tell your idea to the class. Exercise 1.9 1. Write the possible isomers of 1-pentyne and identify which isomers are: a chain isomers? b position isomers? 2. Write the structures of all isomers of 1-pentyne (C5H8) and name them. 3. How many isomers are possible for 1-hexyne? 43 43  CHEMISTRY GRADE 10 Preparation of Alkynes Alkynes can be prepared by several methods. Some of the general methods of preparation of alkynes are: 1. Dehydrohalogenation of vicinal (adjacent) dihalides with a base NaOH or KOH or NaNH2. General reaction: Example 2. Alkylation of sodium acetylide (dicarbide) with a primary alkyl halide. General reaction: Example Exercise 1.10 Write the major product of the following reactions: 44 44  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Experiment 1.4 Laboratory Preparation of Ethyne Objective: To prepare ethyne from calcium carbide and water. Materials required: CaC2, H2O, bromine water, round-bottomed flask, separating funnel, delivery tube, trough, beehive shelf and gas jar. Procedure: 1. Arrange the set up as shown in Figure 1.7. Put a layer of sand in a conical flask and place calcium carbide on the sand. 2. Add water drop by drop from the separating funnel onto the calcium carbide. Watch carefully and record your observation. Touch the flask with the tip of your finger. 3. Collect several jars of the gas over water and carry out the following tests: a Burn the gas as you did with methane and ethene. b Add a few drops of bromine water to another jar filled with ethyne and note the changes. Figure 1.7 Laboratory Preparation of Ethyne. Observations and analysis: a What do you feel when you touch the flask? Is the reaction exothermic or endothermic? b How do you compare the colour of the flame produced with that of methane and ethene? Write a complete laboratory report and submit to your teacher. 45 45  CHEMISTRY GRADE 10 Chemical Properties of Alkynes Alkynes are more reactive than alkanes and alkenes. Why? Alkynes are more unsaturated than alkanes and alkenes due to the presence of a carbon- carbon triple bond. They can undergo combustion and addition reactions. Some of the common reactions of alkynes are: 1. Combustion reaction: Alkynes burn with a smoky luminous flame, forming CO2 and water. Smoky luminous flames result from the combustion of alkynes due to their high carbon content. 3n –1 General reaction: CnH2n–2 + O2 Æ nCO2 + (n – 1) H2O 2 Example C3H4 + 4O2 Æ 3CO2 + 2H2O + heat 2. Addtion reaction: Alkynes undergo addition reaction at the carbon-carbon triple bond. Some of the addition reactions of alkynes are the following: a Addition of hydrogen (hydrogenation): In the presence of nickel or palladium catalyst, alkynes produce alkanes. General relation: Example Partial hydrogenation of alkynes in the presence of Lindlar’s catalyst gives alkenes. Lindlar’s catalyst is powdered palladium partially deactivated with lead acetate. 46 46  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) b Addition of halogens: When molecule of an alkyne reacts with one and two molecules of halogens, it gives a dihaloalkene and a tetrahaloalkane, respectively. General Reaction: Example Activity 1.22 1. Do you expect alkynes to decolorize bromine water; if so why? 2. Write the balanced chemical equation for the reaction between C2H2 and 2Br2. 47 47  CHEMISTRY GRADE 10 c Addition of Hydrogen Halides: Alkynes react with hydrogen halides to form a monohaloalkene and a dihaloalkane. The addition reaction occurs according to Markovnikov’s rule. General Reaction: Example Br CH3 — C CH HBr CH3 — C CH2 HBr CH3 — C — CH3 Br Br Propyne 2-Bromopropene 2, 2-Dibromopropane Activity 1.23 1. Is Markonikov’s rule applied in the reaction between propyne and hydrobromic acid? How? 2. The addition reaction of alkynes proceeds in two steps. Explain what changes occur during the first and the second steps. 3. Compare the reaction products when 1-butyne and 2-butyne react with HBr. 3. Trimerization of Acetylene: Acetylene, on prolonged heating at 600 – 700oC, yields benzene. 1.3.3 Properties and uses of Acetylene or Ethyne Acetylene is a colourless, sweet-smelling gas in pure form and is insoluble in water. It is usually stored as a solution of acetone in steel cylinders. Combustion of acetylene with oxygen produces an intensely hot flame of about 3000oC. Thus, a large quantity of acetylene is used as a fuel in oxy-acetylene torches for cutting and welding metals. Acetylene is also used to prepare acrylonitrile which is a starting material for producing polyacrylonitrile, a raw material for textile fibre. It is also used for making vinyl chloride which polymerizes to 48 48  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) give polyvinyl chloride (PVC) commonly used for making floor tiles, electrical insulators, shoe soles, water pipes etc. Acetylene is used to produce 1,1,2,2-tetrachloroethane that serves as a solvent for wax, grease, rubber etc. Table 1.8 Comparison of some properties of Ethane, Ethene and Ethyne Property Ethane Ethene Ethyne Nature of flame Non-luminous Luminous Smoky luminous Effect on colour of Br2 in CCl4 No effect Decolorizes Decolorizes Effect on colour of alkaline KMnO4 No effect Decolorizes Decolorizes Exercise 1.11 1. What reagents can be used to test for unsaturation of hydrocarbons? 2. Name the following hydrocarbons. 3. Which of the following hydrocarbons exhibit geometrical isomerism? a 1-butene c 1-pentene b 2-butene d 3-hexene 4. Complete the following reactions and name the main products: heat a CH3CH2CH2Cl + KOH æææÆ b CH3 – CH = CH – CH3 + Cl2 Æ 49 49  CHEMISTRY GRADE 10 c CH3 – CH2 – CH = CH2 + HCl Æ H+ d CH3– CH = CH2 + H2O æææ Æ e CaC2 + 2H2O Æ f CH3 – C ∫ CNa + CH3CH2Br Æ Ni g CH3 – C ∫ CH + 2H2 æææÆ heat h CH3 – CH2 – C ∫ CH + 2HBr Æ i H – C ∫ C – H + Br2 Æ j H – C ∫ C – H + 2Br2 Æ 1.4 AROMATIC HYDROCARBONS Competencies At the end of this section you will be able to: define the term aromatic hydrocarbons; draw the structure of benzene; describe the main physical properties of benzene; explain the chemical reactions of benzene; carry out test-tube reactions with Br2 in CCl4, KMnO4, concentrated H2SO4. Activity 1.24 Discuss the following in your group and present it to the class: 1. What comes to your mind when you hear the word “benzene”? 2. What is the word “aroma” mean? Which substances have an aroma smell? 3. Are aromatic hydrocarbons unsaturated as alkenes? Why? The term aromatic is derived from the word ‘aroma’ meaning pleasant smell, which was originally used to describe naturally occurring compounds with pleasant smells. At present, the term is used without its original significance. Aromatic hydrocarbons are generally obtained from petroleum and coal tar. They are a group of hydrocarbons characterized by the presence of a benzene ring or related structures. Some examples of aromatic hydrocarbon structures are given below: 50 50  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Figure 1.9 Structures of some Aromatic Hydrocarbons. 1.4.1 Benzene Benzene is the simplest aromatic hydrocarbon. Its molecular formula is C6H6. Friedrich A. Kekule, in 1865, suggested that the six carbon atoms of benzene are arranged in the shape of a hexagon. He also suggested a resonance hybrid structure. He represented benzene by the resonance structures (Figure 1.10) in which the two structures shown make equal contribution for the resonance hybrid or actual structure. Figure 1.10 Resonance structures of benzene. The bonds in benzene are neither single nor double but have an intermediate character between those of single and double bonds. All the carbon-carbon bonds in the molecule are same in length and nature. Since structure I or II given above are not the true structures of the benzene molecule, the benzene ring is written, in most cases, in its resonance hybrid form as follows: 51 51  CHEMISTRY GRADE 10 The six electrons in the hexagonal ring are delocalized. From structures I and II in Figure 1.10, one may think that benzene has three double bonds and has the same chemistry as that of alkenes. But, this is not true. Benzene and other aromatic hydrocarbons are not as unsaturated as alkenes because the three double bonds in benzene are delocalized due to resonance. They are more stable than alkenes. They undergo substitution reactions to a far greater extent than addition reactions, which is a different characteristic compared to alkenes. Physical Properties of Benzene Benzene is a flammable, colourless, and volatile liquid with a characteristic smell. It is nonpolar and immiscible with water but miscible with non-polar solvents like ether and carbon tetrachloride. It freezes at 5.4oC and boils at 80.4oC. It is a carcinogenic (cancer causing) substance. Benzene is a good solvent for fats, gums, rubber etc., and is used in the manufacture of dyes, drugs and explosives. Chemical Properties of Benzene Benzene and other aromatic hydrocarbons are more stable than alkenes and alkynes due to the stability of the aromatic ring. It neither decolorizes bromine water (Br2 in CCl4) nor is reactive towards cold potassium permanganate solution. However, benzene undergoes the following reactions: 1. Combustion reaction: Benzene is highly inflammable. It burns with a smoky luminous flame to form CO2 and H2O. 2C6H6 + 15O2 Æ 12CO2 + 6H2O + Heat 2. Substitution reaction: The reactions of benzene are chiefly substitution but not addition reactions. In this reaction, hydrogen atom from the benzene ring is replaced by another atom or group. a Halogenation: Benzene reacts with bromine and chlorine in the presence of iron (III) chloride or aluminium chloride catalyst to form substitution products. 52 52  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) b Nitration: Concentrated nitric acid mixed with some concentrated sulphuric acid reacts with benzene at moderate temperatures to form nitrobenzene. c Sulphonation: Benzene reacts with concentrated sulphuric acid, H2SO4, at room temperature to form benzenesulphonic acid. 3. Addition reaction: Benzene undergoes addition reactions under special conditions. Example When a mixture of benzene vapour and hydrogen is passed over finely divided nickel catalyst at 200°C, cyclohexane is formed. Experiment 1.5 Chemical Reaction of Toluene Objective: To distinguish aromatic hydrocarbons from other unsaturated hydrocarbons Materials required: Toluene, Br2 in CCl4, KMnO4, concentrated H2SO4, test tubes, test tube rack, test tube holder, measuring cylinder, Bunsen burner and dropper. 53 53  CHEMISTRY GRADE 10 Procedure: 1. Take three test tubes and place them in the test tube rack. 2. Add 5 mL of toluene (methylbenzene) to each of the three test tubes. 3. Add one or two drops of Br2 in CCl4 in the first test tube and add the same amount of cold KMnO4 solution in the second test tube and observe the changes. 4. Add a few drops of concentrated sulphuric acid to the third test tube and, if necessary, heat it gently, holding it with a test tube holder. Observations and analysis: In which test tube does a reaction occur? Write a complete report of your observations and submit to your teacher. Exercise 1.12 1. What is an aromatic hydrocarbon? 2. Are aromatic hydrocarbons saturated or unsaturated? 3. What is the name of the simplest aromatic hydrocarbon? 4. Describe the main reactions of benzene? 5. Benzene does not change the colour of Br2 in CCl4 or that of KMnO4 solution; why? 6. How do you prepare the following compounds from benzene? a Chlorobenzene b Nitrobenzene c Benzene sulphonic acid. 1.5 NATURAL SOURCES OF HYDROCARBONS Competencies After completing this subunit, you will be able to: list the major natural sources of hydrocarbons; describe natural gas; define crude oil; explain the fractional distillation of crude oil; mention the products of the fractional distillation of crude oil; tell the uses of petroleum products; describe the composition of coal; explain the destructive distillation of coal. 54 54  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Activity 1.25 Discuss the following in your group and present it to the class: 1. What do you think is the major component of natural gas? 2. How can petroleum be separated into different fractions? 3. How are natural gas, petroleum and coal formed in nature? Can they be recycled? A. Natural Gas The principal sources of hydrocarbons are natural gas, crude oil and coal. Natural gas is found in association with petroleum or alone. It is found in underground deposits several hundreds or thousands of metres below the earth’s surface, where it originated from the decay and decomposition of animal and plant remains millions of years ago. Natural gas mainly contains methane (more than 90%), ethane, propane, butane and small quantities of higher alkanes. Other gases such as, CO2, N2, O2 and H2S, may also be present in natural gas. The composition of natural gas varies, depending on its place of origin. Natural gas is widely used as a fuel. It is advantageous over liquid and solid fuels and also other gases, except H2, due to its very high heat of combustion. B. Petroleum The word petroleum is derived from two Latin words, ‘petra’ meaning rock, and ‘oleum’ meaning oil. Thus, petroleum means rock oil. The term petroleum refers to a broad range of fossil hydrocarbons that are found as gases, liquids or solids beneath the surface of the earth. The crude oil is generally found along with natural gas in the form of a dark- coloured viscous liquid, which is a complex mixture of hydrocarbons. Its composition varies according to its place of origin. However, crude oil mainly contains alkanes, cycloalkanes and aromatic hydrocarbons. It also contains organic compounds consisting of nitrogen, sulphur and oxygen in small amounts. Refining of Crude Oil Crude oil or petroleum is obtained by drilling wells until the oil-bearing region is reached. After the crude oil is brought to the surface of the earth, it must be refined before it can be used. Crude oil is refined or separated into several components by fractional distillation based on the differences in boiling ranges of its fractions. (Figure 1.11). 55 55  CHEMISTRY GRADE 10 Figure 1.11 Fractional Distillation of Petroleum. The major products (fractions) obtained from fractional distillation of petroleum and their uses are given in Table 1.9. Table 1.9 Common petroleum products. Fractions Approximate Boiling Uses composition point range (carbon number) (°C) Gases C1 – C 4 below 20 – As fuel in the form of bottled gas. Petroleum ether C5 – C 7 20 – 60 – As solvent, in dry cleaning. Gasoline (petrol) C5 – C10 40 – 200 – Motor fuel for internal combustion engines. Kerosene C10 – C16 175 – 275 – Jet engine fuel, – household fuel. Continued next page 56 56  INTRODUCTION TO ORGANIC CHEMISTRY (UNIT 1) Fuel oil (diesel oil) C15 – C18 250 – 350 – Furnace fuel, diesel engine fuel. Lubricating oils ¸ Non-volatile liquids – Lubrication. Greases ˝ > C19 Petroleum jelly ˛ Paraffin wax Non-volatile – Candles, polishes. ¸ Asphalt Ô solids – Roofing and road (bitumen) ˝ > C20 construction. Coke Ô – Electrodes, fuel for ˛ power stations. Most hydrocarbons present in petroleum are long-chain hydrocarbons. However, there is a very high demand for petrol or gasoline, which contains hydrocarbons, composed of five to ten carbon atoms. Therefore, the amount of petrol obtained by fractional distillation of crude oil does not satisfy the demand for it. To satisfy the demand and maximize the output of petrol or gasoline, the process called cracking is carried out during fractional distillation. What does cracking mean? Cracking is the decomposition of large hydrocarbon molecules into smaller ones by the application of heat (thermal cracking or pyrolysis) or in the presence of catalysts (catalytic cracking). Example heat C16 H34 → C8 H18 + C8 H16 Hexadecane Octane Octene Hydrogen gas can be added during cracking to saturate the alkenes formed in the process. C. Coal Coal is formed naturally by the decomposition of plant matter over several millions of years. It is not a pure form of carbon. It is an important source of aromatic hydrocarbons. Heating mineral coal in the absence of air, or oxygen is called destructive distillation of coal or coking of coal. When coal is heated in the absence of air it gives volatile products and coke. The volatile products separate into coal gas and a liquid, called coal tar when cooled. Aromatic hydrocarbons and many other substances are isolated from coal tar by fractional distillation. Coke, which is a solid and relatively pure form of carbon, is used as a fuel in the blast furnace during the extraction of iron. It is also used to produce gaseous fuels, such as water gas (mixture of H2 and CO) and producer gas (mixture of N2 and CO). 57 57  Introduction Start-up Activity Perform the following activity in groups and present your response to the rest of the class. Take turns to name an item that contains organic molecules that we use in daily life, and identify what class of organic compound (s) it contains. 6.1  At the end of this section, you will be able to: ) classify organic compounds. Classification of organic compounds Why is classification of organic compounds needed? What is the basis of the classification? In Grade 10, you have learned about some general organic reactions of hydrocarbons. In this unit, you will continue to learn about more functional groups: oxygen- containing functional groups. The oxygenated hydrocarbons include alcohols, ethers, aldehydes, ketones, carboxylic acids and esters. Alcohols occur widely in nature and have many industrial and pharmaceutical applications. Methanol, for example, is one of the most important of all industrial chemicals. Ethers are relatively resistant to chemical transformation and are often used as solvents in chemical reactions. Can you name the common functional groups and give an example for each functional group? Aldehydes and ketones contain a carbonyl group that consists of a carbon–oxygen double bond (C=O). Many compounds found in nature have aldehyde or ketone functional groups. Aldehydes have pungent odors, whereas ketones tend to smell sweet. Carboxylic acids and esters are present in many fruits and flowers. Many carboxylic acids are used as food additives in jams, jellies, candies and pickles, etc. In this unit, you will also learn about fats and oils which are esters. UNIT 6 265 78  CHEMISTRY GRADE 11 6.2  At the end of this section, you will be able to: ) define alcohols ) tell the functional group of alcohols ) classify alcohols based on the number of hydroxyl groups ) write the general formulas of monohydric alcohols ) write the molecular formulas and the names of the first six members of the monohydric alcohols ) give the IUPAC names for the given alcohols ) classify monohydric alcohols based on the number of alkyl groups attached to the carbon atom carrying the hydroxyl group ) give some examples for primary, secondary and tertiary alcohols ) describe the physical properties of alcohols ) explain general m