Esters

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8]VeiZg &% ALKANOIC ACIDS AND ESTERS  Introduction Most people have used Aspirin to relieve headache pain or to reduce a fever. Aspirin (or 2-ethanoylhydroxybenzoic acid) is an example of a common group of organic chemicals called esters . Aspirin is made from a weak organic acid called salicylic acid , which can be extracted from the leaves of the willow tree. This extraction was performed by Hippocrates in 400 BC, and he used the extract to ease labour pains. It was soon discovered that the body cannot tolerate too large a dose of salicylic acid as it causes irritation to the stomach and mouth. The name Aspirin was coined as a trade name in 1898, when the Bayer Pharmaceutical Company patented an analgesic that was the acetyl ester of salicylic acid. It did not cause the same side effects as salicylic acid, but it did reduce pain and fever. In this chapter  10.1 Figure 10.1 Aspirin is an important analgesic. It reduces pain and fever. It is an example of a group of compounds called esters . 216 THE ACIDIC ENVIRONMENT Organic acids and esterification page 217 &%#& Remember  Before beginning this section, you should be able to: • identify factors that can affect the equilibrium in a reversible reaction • write equations to represent all chemical reactions. /2'!.) # !#)$3 ! .$ %34%2)&)#!4)/.   Alkanols and alkanoic acids  Alkanols and alkanoic acids are important families of organic compounds.  Alkanols are compounds that contain the alcohol functional group (—OH). The alkanoic acid family are weak organic acids. They contain the carboxylic acid functional group (—COOH). (a) (b) (c) Key content By the end of this section, you should be able to: • describe the differences between the alkanol and alkanoic acid functional groups in carbon compounds • identify the IUPAC nomenclature for describing the esters produced by reactions of straight-chained alkanoic acids from C1 to C8 and straight-chained primary alkanols from C1 to C8 • explain the difference in melting point and boiling point caused by straight-chained alkanoic acid and straightchained primary alkanol structures • identify esterification as the reaction between an acid and an alkanol, and describe, using equations, examples of esterification • describe the purpose of using acid in esterification for catalysis • explain the need for refluxing during esterification • outline some examples of the occurrence, production and uses of esters • identify data, plan, select equipment and perform a first-hand investigation to prepare an ester using reflux • process information from secondary sources to identify and describe the uses of esters as flavours and perfumes in processed foods and cosmetics. Figure 10.2 The ester called (a) ethyl acetate is manufactured from (b) ethanol and (c) acetic acid (ethanoic acid).  Alkanols In the Production of Materials module , we investigated the importance of a family of molecules called alkanols. Ethanol is an important member of this homologous series of compounds. The general formula of the alkanol homologous series is: Cn H2n  + 1OH It is worthwhile at this stage to look back to Module 1 and review the rules used to name these molecules. Sample problem 10.1 will help you to revise.  Alkanols are polar molecules due to the presence of the alcohol functional group. The melting and boiling points of the alkanols are higher than their corresponding alkanes because of dipole–dipole attractions (Table 10.1). Dispersion forces are the only forces that bind hydrocarbon molecules together. Methanol and ethanol have significantly higher melting points than would be normally expected for such small molecules. This is explained by the hydrogen bonding between the electropositive hydrogen atoms and the electronegative oxygen atoms of alcohol groups on neighbouring molecules. Hydrogen bonding also explains the high  water solubility of the first four members of the alkanol homologous series. As the chain length increases, however, they become increasingly insoluble.  Alkanols are essentially neutral molecules. The alcohol functional group is strongly bonded to the carbon chain and there is no tendency for the loss of hydroxide ions when alkanols dissolve in water. H Figure 10.3 The hydrogen bonding between short-chain alkanol molecules raises their melting points. The hydrogen bonding between short-chain alkanols and water increases their water solubility. (n  = 1, 2, 3. . .) H H C H C H O  . . H . .  .  . Hydrogen bonding H H C H C H O H H H  . . .H H . . .O O H Hydrogen bonding CHAPTER 10 ALKANOIC ACIDS AND ESTERS 217 0 Figure 10.4 Methanol and ethanol have melting points that are higher than would normally be expected for low molecular weight molecules. SAMPLE PROBLEM 10.1    )  –20    C    °    (    t  –40   n    i   o  –60   p   g   n  –80    i    t    l   e –100    M  –120  –140 0 2 4 6 8 Name the straight-chain alkanol with the condensed structural formula: CH3CH2CH2CH2CH2CH2CH2CH2OH SOLUTION Step 1. Count the number of carbon atoms in the chain. Step 2. Step 3. Step 4. Step 5. Answer = 8. Select the stem that corresponds to an alkane with 8 carbons. Answer = octane. Delete the ‘e’ and add the suffix ‘ol’. Answer = octanol. Assign a locant to the carbon to which the alcohol functional group is attached. Answer = 1. Name the alkanol using the preferred IUPAC nomenclature. Name = octan-1-ol (or 1-octanol, which is also accepted by IUPAC as a systematic name). Table 10.1 Melting and boiling points of alkanes and alkanols Compound 218 THE ACIDIC ENVIRONMENT M (g/mol) Melting point (ºC) Boiling point (ºC) methane 16.0 –182.5 –161.5 methanol 32.0 –97.7 64.7 ethane 30.1 –182.8 –88.6 ethanol 46.1 –114.1 78.3 propane 44.1 –187.7 –42.1 propan-1-ol (1-propanol) 60.1 –126.2 97.2 butane 58.1 –138.3 –0.5 butan-1-ol (1-butanol) 74.1 –89.8 117.7 pentane 72.1 –129.7 36.1 pentan-1-ol (1-pentanol) 88.2 –78.9 138.0 O H C O H Figure 10.5 Methanoic acid is commonly called formic acid  as is present in some ant stings ( formis = ‘ant’ in Latin).  Alkanoic acids  Another important family or homologous series derived from alkanes are the alkanoic acids. Alkanoic acids contain the carboxylic acid functional group (COOH). Their general formula is: Cn H2n  + 1COOH (n  = 0, 1, 2, 3. . .) The first member of this homologous series is called formic acid. Figure 10.5 shows the structure of formic acid (methanoic acid). IUPAC nomenclature rules for straight-chain alkanoic acids The straight-chain alkanoic acids are named after the corresponding parent alkane. The carboxylic acid functional group is always at the end of the chain, so this carbon is the locant ‘1’. The rule for naming is: Identify the number of carbons present in the str aight chain. Select the correct stem to name the parent alkane. Remove the ‘e’ and replace it  with the suffix ‘oic acid’. The IUPAC committee (2005) has recommended that the preferred name for the first two members of the alkanoic acid homologous series be their common name rather than their systematic name. Thus methanoic acid is called  formic acid  and ethanoic acid is called acetic acid. SAMPLE PROBLEM 10.2 Name the alkanoic acid whose structural formula is shown in Figure 10.6. Figure 10.6 Use the nomenclature rules to name this alkanoic acid. SOLUTION H H H H H H H C C C C C C H H H H H H O C O H Step 1. Count the number of carbons in the chain (including the carbon of the carboxylic acid group). Answer = 7 carbons. Step 2. Select the stem that corresponds to an alkane with 7 carbons. Answer = heptane. Step 3. Delete the ‘e’ and add the suffix ‘oic acid’. Name = heptanoic acid. Table 10.2 lists the IUPAC preferred names of the first eight members of the alkanoic acid homologous series. Table 10.2. Alkanoic acids IUPAC preferred name formic acid (methanoic acid) acetic acid (ethanoic acid) Condensed structural formula   HCOOH CH 3COOH propanoic acid CH3CH2COOH butanoic acid CH3CH2CH2COOH pentanoic acid CH3CH2CH2CH2COOH hexanoic acid CH3CH2CH2CH2CH2COOH heptanoic acid CH3CH2CH2CH2CH2CH2COOH octanoic acid CH3CH2CH2CH2CH2CH2CH2COOH CHAPTER 10 ALKANOIC ACIDS AND ESTERS 219 Comparing the physical properties of alkanols and alkanoic acids  Alkanols and alkanoic acids are polar molecules Table 10.3 lists the melting and boiling points of straight-chain alkanoic acids. Table 10.3 Melting and boiling points of alkanes, alkanols and alkanoic acids Compound H H C H Hydrogen bonding H O H O C C H O H O H Hydrogen bonding ... C ... Figure 10.7 The strong hydrogen bonding between acetic acid molecules leads to the formation of dimers. M  (g/mol) Melting point (ºC) Boiling point (ºC) formic (methanoic)acid 46.0 8.4 100.8 acetic (ethanoic) acid 60.1 16.6 117.9 propanoic acid 74.1 –20.7 140.8 butanoic acid 88.1 –5.4 163.3 pentanoic acid 102.1 –34 185.5 hexanoic acid 116.2 –3 205.7 Tables 10.2 and 10.3 show that alkanols and alkanoic acids have higher melting and boiling points compared with their parent alkanes. It is also apparent that the rise in melting and boiling points is greatest for the alkanoic acid molecules. Alkanoic acids have greater molar weights than their equivalent alkanols or alkanes and thus the dispersion forces between their molecules are also greater. Alkanoic acids are usually slightly more polar than alkanols and thus the dipole–dipole forces are greater. When an alkanol and an alkanoic acid of the same molar weight are compared, the alkanoic acid has the higher melting point or boiling point. This is mainly due to the more extensive hydrogen bonding between alkanoic acid molecules. As the carbon chain lengthens, the increasing dispersion forces dominate and there is a general trend of increasing boiling point  with increasing molar weight. The hydrogen bonding between acetic acid molecules is particularly strong. Experiments have shown that stable pairs of acetic acid molecules (dimers) are present in the liquid. These dimers tend to break down on heating.  Alkanoic acids are weak acids. They can be neutralised by strong bases to form salts and water. In the following example, butanoic acid is neutralised by sodium hydroxide to form sodium butanoate and water. CH3CH2CH2COOH + NaOH NaCH3CH2CH2COO + H2O(l) Esters The fragrances of flowers and the smell and taste of their fruits are produced by a variety of volatile molecules, including a group of organic compounds called esters. Not all esters have pleasant smells, but the low molecular weight esters are usually volatile and aromatic. Chemists have developed a range of synthetic esters that can be used as food flavourings as well as in perfumes. Esters are produced by the acid-catalysed reaction between an alcohol and a carboxylic acid. Water is a by-product of the reaction. If the alcohol is an alkanol and the carboxylic acid is an alkanoic acid, then the ester that is formed is called an alkyl alkanoate. 220 THE ACIDIC ENVIRONMENT Alkyl group O R C  ALKANOL + ALKANOIC ACID p ALKYL ALKANOATE + WATER  R O Alkyl group derived from original alkanol Figure 10.8 The general formula of esters is often written as RCOOR , where R is a hydrogen atom or an alkyl group and R  is an alkyl group. ` This reaction is not an acid–base reaction. It is classified as a condensation reaction. Isotopic studies have shown that the water that is formed comes from the ‘OH’ group of the alkanoic acid and the ‘H’ of the alkanol functional group. Figure 10.8 shows the general structure of alkyl alkanoate esters. All esters contain the ester linkage or ester functional group (—COO—). ` Nomenclature of esters condensation reaction: a reaction in which two molecules combine together with the elimination of a smaller molecule such as water O R C + O H H O R Water splits out as the alkanoic acid and alkanol condense Figure 10.9 When the alkanol and alkanoic acid condense together, the water that is formed comes from the OH group of the alkanoic acid and the hydrogen from the alcohol functional group of the alkanol.  SAMPLE PROBLEM 10.3  Alkyl alkanoate esters are named after their parent alkanols and alkanoic acids. The rules for naming these esters are: 1. Count the number of carbon atoms in the alkyl group that is derived from the original O alkanol. Name this alkyl group by deleting + H O C R the ‘anol’ suffix from the alkanol and R O replacing it with the suffix ‘yl’ (e.g. butanol H Ester becomes butyl). 2. Identify the number of carbon atoms in the alkanoate chain that is derived from the alkanoic acid. Name this alkanoate chain by deleting the ‘oic acid’ suffix and replacing it with the suffix ‘oate’ (e.g. propanoic acid becomes propanoate). 3. The name of the ester is two separate words. The first name comes from the alkanol and the second from the alkanoic acid.  Use the nomenclature rules for esters to name the ester whose structural formula is shown in Figure 10.10. H SOLUTION H H H H C C C C H H H H O H H H O C C C H H H C H Figure 10.10 Use the nomenclature rules to name this ester. Step 1. Identify the number of carbon atoms in the alkyl group derived from the original alkanol. This is the hydrocarbon chain that is attached to the single bonded oxygen atom of the ester linkage. Number of carbon atoms = 3. Step 2. Name the alkyl group. Name = propyl (or, 1-propyl). Step 3. Identify the number of carbon atoms in the alkanoate chain derived from the original alkanoic acid. This is the chain that includes the carbon attached to the double bonded oxygen atom (or carbonyl oxygen). Number of carbon atoms = 5. Step 4. Name the alkanoate group. Name = pentanoate. Step 5. Name the ester. Name = propyl pentanoate (or 1-propyl pentanoate). CHAPTER 10 ALKANOIC ACIDS AND ESTERS 221 SYLLABUS FOCUS  21. USING INSTRUCTION TERMS CORRECTLY   When answering questions, it is important to know what the instruction terms (‘verbs’) require you to do. Here are some examples: ‘Recall’ This instruction word requires you to present remembered facts, ideas or experiences. Example:  Recall the common name for the compounds that react with alcohols to produce esters.  Answer:  Carboxylic acids refluxing: the process of heating a mixture of liquids in a flask with an attached condenser in order to prevent the loss of volatile reactants or products Esterification Esterification is the process of making an ester. Esterification reactions are usually quite slow and the reaction does not proceed to completion. The rate of the reaction can be increased by: adding a suitable catalyst (e.g. 1–3 mL of concentrated sulfuric acid) Apparatus open to air heating the reaction mixture to increase the kinetic energy of the molecules. The concentrated sulfuric acid that is used as Water condenser a catalyst also acts as a dehydrating agent. When  water is removed from the reaction equilibrium, the equilibrium shifts to the right and increases the yield of ester. This shift is small as only about 1–2 mole% of the reaction mixture is catalyst. It is important to remember that catalysts do not increase the yield Hose Vapours condense of products; they only decrease the time to reach equilibrium. Water out The reactants and products of the esterification to sink  Cold water reaction are volatile, and readily vaporise on heating. from tap To avoid loss of material from the reaction flask, the Hose mixture is heated using a reflux apparatus as shown in Figure 10.11. A water condenser is mounted above the reaction flask (normally a round-bottom Pyrex Condensed liquid drops back  flask) and cold water circulates to cool the hot, rising  vapours. The vapours condense back to the liquid Round bottom flask  state and drip back into the reaction flask so that the reaction can continue without loss of reactants or products. This process is called refluxing . Water bath Because the system is open to the atmosphere Boiling chips Reaction mixture there is no build up of pressure due to the producHotplate tion of vapours. The organic liquids and vapours are also flammable, and care must be taken to avoid Electrical heater fires and explosions. The reaction vessel is normally heated by a hot-water bath (supported on an electric hotplate) or by using a special electrical heating Figure 10.11 mantle in which the round-bottom flask is clamped. The reaction mixture, including a catalyst and boiling Naked flames from a Bunsen burner are avoided to chips, is heated under reflux. Vapours condense back to reduce the risk of fire. the liquid state and drip back into the reaction vessel. u u 222 THE ACIDIC ENVIRONMENT 10.1   PRACTICAL ACTIVITIES Preparation of an ester ESTERIFICATION If aspirin is stored in a hot, humid environment for a long time it starts to smell like vinegar. This change is due to the hydrolysis of the aspirin. SAMPLE PROBLEM 10.4  Another feature of the reflux procedure is the use of small boiling chips that are normally pieces of crushed ceramic. These boiling chips prevent a process called ‘bumping’ as they provide a large surface area on which  vaporisation can occur without the risks of sudden superheating and the explosive ejection of vapours. Refluxing may occur for hours or days until the system reaches equilibrium. Table 10.4 shows some typical data for the preparation of simple esters. Instead of the 1 : 1 mole ratio of alkanol to alkanoic acid, it is common to use an excess of one reactant (usually the alkanoic acid) in order to shift the equilibrium in the direction of the formation of the ester. The percentage yields obtained are variable, and this is often the result of ester being lost during the separation and purification process. Table 10.3 Experimental data for the preparation of esters  Alkanol Alkanoic acid methanol (1 mol) acetic acid (3 mol) methyl acetate (67%) ethanol (1 mol) acetic acid (3 mol) ethyl acetate (29%) 12 ethanol (1 mol) butanoic acid (2 mol) ethyl butanoate (69%) 14 butan-1-ol (1 mol) formic acid (2 mol) butyl formate (75%) 24 5 Complete the following table and draw structural formulae for each ester.  Alkanol Alkanoic acid methanol Ester pentanoic acid propan-2-ol (2-propanol) (c) SOLUTION Reflux time (h) Ester (% yield) (a) (b) 2-propyl hexanoate propanoic acid 2-pentyl propanoate (a) The alkyl group of this ester will be called ‘methyl’ and the alkanoate group will be ‘pentanoate’. The name of the ester is methyl pentanoate. H H H H O H C C C C H C O H H H C H methyl pentanoate H H (b) The hexanoate group is derived from hexanoic acid. H H H H H H O C C C C C C H H H H H H O 2-propyl hexanoate C H C H C H H H H CHAPTER 10 ALKANOIC ACIDS AND ESTERS 223 (c) The alkyl group is derived from an alkanol with the alcohol functional group on C-2. This alkyl group is called ‘2-pentyl’. H H O H C H Figure 10.12 C H  H   H C C C O C   H H C H C H  H H H 2-pentyl propanoate H (a)  Alkanol Alkanoic acid Ester methanol pentanoic acid methyl pentanoate propan-2-ol (2-propanol) hexanoic acid 2-propyl hexanoate pentan-2-ol (2-pentanol) propanoic acid 2-pentyl propanaoate Extraction of the ester from the reaction mixture The ester is usually separated from the reaction mixture in a number of steps. The physical properties of the reactants and products must be known in order to design the most appropriate procedure.  Example 1:  Figure 10.13 (a) The ester may be able to be removed from the reaction mixture by distillation. (b) In some cases however, the mixture must be separated in a separating funnel using appropriate solvents before distillation is performed. (b) Methyl acetate — Following reflux, the reaction mixture is distilled and the crude ester vaporises at 57ºC and forms the distillate. The excess acetic acid (b.p. = 118ºC) and most of the methanol (b.p. = 65ºC) stays in the distillation flask. The crude ester is washed in a separating funnel  with a little saturated salty water containing sodium hydrogen carbonate to neutralise any acetic acid. The ester layer is removed, dried and then re-distilled to obtain the pure ester.  Example 2:  Ethyl acetate — The reaction mixture cannot be distilled as the boiling points of the ethanol (78ºC) and the ester (77ºC) are too close. Therefore, following reflux, the reaction mixture is added to an equal volume of water and saturated with salt to reduce the solubility of the ethyl acetate and produce an ester layer. The ester layer is treated with sodium hydrogen carbonate to neutralise any acetic acid and then saturated with salt again. The ester layer is removed and dried before distillation to obtain the pure ester. Much ester is lost due to its solubility in water. Fractional distillation can be used to improve the yield. Comparison of the boiling points of alkanols, alkanoic acids and esters Table 10.4 compares the boiling points of three molecules with the same molecular weight. As they have the same molecular weights the dispersion forces between the molecules are very similar. Table 10.4 shows that the alkanoic acid and the alkanol have significantly higher boiling points compared with that of the ester. All three molecules are polar. The alkanol and the alkanoic acid, however, exhibit hydrogen bonding between their 224 THE ACIDIC ENVIRONMENT molecules, and this leads to an elevation in the boiling point. The ester cannot form hydrogen-bonds as there are no OH functional groups in the molecule. Consequently its boiling point is much lower. Table 10.4 Comparison of the boiling points of an alkanol, alkanoic acid and ester with the same molecular weight. Compound propan-1-ol (1-propanol) MF   M  (g/mol) b.p. (ºC) C3H7OH 60.1 97.2 acetic acid CH3COOH 60.1 117.9 methyl formate HCOOCH3 60.1 31.5 Occurrence, production and uses of esters Esters occur widely in nature. They are also produced by various industries. H H O H C O C (CH2)14 C H H O H C O C (CH2)16 C H H H H O H C O C H Derived from glycerol H H C O H H C O H H C O H H (CH2)12 C Esters in nature Esters occur in all natural systems. Esters can be formed not only from alkanoic acids, but also from other organic and inorganic acids. Phosphate esters are esters formed from phosphoric acid and some of these (e.g. ATP) are vitally important in energy transfer reactions in cells. Fats and oils are triesters of glycerol and various long-chain carboxylic acids. They are commonly known as triglycerides. Natural waxes consist of mixed esters of long-chain alkanols and long-chain alkanoic acids (fatty acids). H Esters as lubricants H Esters are also manufactured for a wide variety of applications. Esters have been used as jet engine lubricants for many decades due to their low viscosity at low temperatures, as well as their Derived from long-chain clean high-temperature operation. Esters are excellent lubricants fatty acids of variable in refrigeration systems. chain length The use of esters as lubricants stems from their molecular polarity. Polar ester molecules have stronger intermolecular attractions than other nonpolar oils, so they have lower rates of evaporation and higher flash points. Polar ester molecules are attracted to metal surfaces, forming a film that requires more energy to penetrate. Esters are good solvents and can solubilise or disperse degradation products; this maintains the lubrication properties. Esters are also highly biodegradable compared with other oils. Figure 10.14 Glycerol is a alkantriol. Its has three alcohol functional groups. Each of these can bond with a long-chain carboxylic acid (or fatty acid) to form a triester or fat. Esters as solvents and coatings Many natural and synthetic esters are used in wood lacquers, thinners and a variety of coatings. Some low molecular weight esters have a high volatility and so are useful components of some solvents used in surface finishes. Those esters of higher molecular weight are less volatile and find use in latex paints. Esters of acetic acid, for example, are used in aerosol sprays and printing inks, and as solvents for personal care products and cosmetics. Esters of propanoic acid improve solvent diffusion from coating films. They are used in automotive refinishing and in cosmetics. CHAPTER 10 ALKANOIC ACIDS AND ESTERS 225 Table 10.5 shows some of the uses of various low molecular weight esters. Table 10.5 Uses of some important esters of acetic acid and propanoic acid (g/mol) b.p. (ºC) Relative evaporation rate 1-propyl acetate, CH3COOC3H7 102.1 102 2.75 volatile solvent, wood lacquers, aerosol sprays, cosmetics and personal care solvent  1-butyl acetate, CH3COOC4H9 116.2 126 1(standard) fragrance solvent, coatings for plastics, cosmetics and personal care solvent  1-pentyl acetate (amyl acetate), CH3COOC5H11 130.2 146 0.5 coatings, cleaning fluids, extraction solvent for pharmaceuticals, printing, finishing fabrics 1-butyl propanoate, CH3CH2COOC4H9 130.2 145 0.45 appliance coatings, automotive refinishing, enamels, lacquers 1-pentyl propanoate, CH3CH2COOC5H11 144.2 165 0.2 slow evaporating solvent, appliance coatings, automotive refinishing, enamels, lacquers, personal care products M  Ester O C H C    — H C    — C H H O C—   C O C —   H Uses C O R     R = alkyl or other group Figure 10.15 Phthalate esters make excellent plasticisers. They convert stiff plastic into flexible plastic. Esters as plasticisers Phthalate esters are oily liquids that are added to hard plastics (such as PVC) in order to soften them and increase their flexibility. These esters are examples of plasticisers. Many products, such as medical devices (including tubing, gloves and medical containers), shower curtains, toys and synthetic floor coverings are made from this plasticised PVC. There has been considerable controversy in recent years about the longterm health consequences of these phthalate esters as they are present in so many household products. Animal studies have implicated these esters in reproductive diseases including cancer. Esters as alternative fuels Esters of natural fatty acids are used as biofuels. They are commonly called biodiesel. They are manufactured from natural oils such as soybean and canola using a process called transesterification. In this process, the  vegetable oil is reacted with sodium hydroxide dissolved in methanol. Esters such as methyl soyate are formed. After processing they can be used as a biofuel in diesel engines. Bioester fuels are biodegradable. Their higher viscosity makes them suitable engine lubricants. Esters as flavours, fragrances and emulsifiers The characteristic tastes and smells of fresh fruits and flowers are due to a complex mixture of chemicals. Esters are one example of these types of molecules. Table 10.6 lists some of the common esters that are used as flavours in foods. 226 THE ACIDIC ENVIRONMENT Table 10.6 Esters used as flavourings Ester emulsifier: a substance that can produce a stable homogeneous dispersion of one insoluble liquid in another; for example, soap emulsifies oil and water to produce a stable dispersion or emulsion emollient: a substance that softens and soothes the skin H H C   — C  — C H O — C O C H C C       — C H O ethyl formate, HCOOC 2H5 rum-like taste and odour; used in artificial rum 1-butyl formate, HCOOC4H9 plum flavour 2-butyl formate, HCOOCH(CH 3)(C2H5) raspberry flavour 1-pentyl acetate, CH 3COOC5H11 fruity flavour (pears and bananas) 1-octyl acetate, CH3COOC8H17 orange flavour methyl butanoate, CH 3CH2CH2COOCH3 apple flavour ethyl butanoate, CH 3CH2CH2COOC2H5 pineapple flavour butyl butanoate, CH 3CH2CH2COOC4H9 pineapple flavouring 1-pentyl butanoate, CH 3CH2CH2COOC5H11 hydrophilic: a substance that has a high affinity for water. H Flavour H Esters can also be used as fragrances in body care products and cosmetics. Some of the fruity esters listed in Table 10.6 are used with  various other aromatic compounds including aldehydes. Phenylmethyl acetate is an ester with a jasmine fragrance that is used in various cosmetics. Methyl salicylate or oil of wintergreen  is used in lotions and creams to soothe sore muscles. Esters that have a high molecular weight are also used in cosmetics and the food industry as emulsifying agents. These emulsifiers prevent oils and water separating into layers in cosmetics or in the food product. Citric acid esters of monoglycerides are excellent hydrophilic emulsifiers. They are used to stabilise mayonnaise, margarine and coffee  whitener. Some esters are used as emollients in face and body creams and lotions. Myristyl myristate is a solid emollient   ester that liquefies on contact with the body, enhances spreadability of the cream and delivers a  velvety feel to the skin. H methyl salicylate Figure 10.16 Oil of wintergreen has a characteristic odour associated with liniments and heat creams for sore muscles. H (a) Glycerol forms an ester with a fatty acid and citric acid. O H C O C H C O H R C Diacetyl esters of tartaric acid are used in the food industry in many ways.They strengthen doughs and enhance gas retention in the dough. This leads to improved crust crispness in baked goods. O C H H R fatty acid O H Figure 10.17 (a) Citric acid esters of monoglycerides are used to stabilise cream and mayonnaise products. (b) Myristyl myristate is an ester that is used in skin emollients. apricot flavouring H C H C O C H O O C H O H O C H (b) H H C H H O (CH2)12 C O (CH2)13 C H H CHAPTER 10 ALKANOIC ACIDS AND ESTERS 227 10.1  Questions 1. Examine the following molecular structure. CH3CH2CH2CH2CH2CH2CHOHCH3 The preferred IUPAC name for the molecule is  A octan-1-ol. B octan-2-ol. C octanoic acid. D octan-7-ol. 2. Examine the following molecular structure. CH3CH2CH2CH2COOCH2CH2CH2CH3 The preferred IUPAC name for the molecule is  A nonan-4-ol. B nonanoic acid. C 1-butyl pentanoate. D 1-pentyl butanoate. 3. Select the statement that is true concerning alkanols, alkanoic acids and esters.  A Alkanols and alkanoic acids are acidic in  water solution. B The boiling points of alkanols are higher than the boiling points of alkanoic acids  with the same number of carbon atoms per molecule. C The hydrogen bonding between ester molecules is stronger than the hydrogen bonding between alkanoic acid molecules. D Acetic acid forms dimers due to the extensive hydrogen bonding between their molecules. 4. Select the statement that is true about esterification.  A Concentrated sulfuric acid is added to the reaction mixture to act as a catalyst. B Esterification reactions are rapid and require little heating. C Esterification is an example of a neutralisation reaction. D Catalysts are added to increase the equilibrium yield of ester. 5. Methyl acetate is a simple ester. Name the alkanol and the alkanoic acid from which it is formed.  A methanoic acid and ethanol B methanol and acetic acid C methanol and sulfuric acid D formic acid and ethanol 228 THE ACIDIC ENVIRONMENT 6. Each row of the following table shows the names of molecules used to make esters. Copy and complete the table.  Alkanol Alkanoic acid Ester 1-propyl butanoate methanol formic acid propanoic acid butan-2-ol 1-pentyl propanoate hexanoic acid 7. Write condensed structural formulae equations for the esterification reaction in  which the following alkanols and alkanoic acids are reactants. (a) ethanol and propanoic acid (b) propan-1-ol and pentanoic acid 8. Explain the difference between the boiling points of the following compounds. Compound Molar weight (g/mol) Boiling point (ºC) hexan-1-ol 102.2 157.5 pentanoic acid 102.1 185.5 1-propyl acetate 102.1 101.6 9. The following table provides data on the boiling points of some alkanoic acids. The boiling points for formic acid and acetic acid are not listed.  Alkanoic acid Molar weight (g/mol) Boiling point (ºC) formic acid 46 acetic acid 60 propanoic acid 74 141 butanoic acid 88 163 pentanoic acid 102 186 hexanoic acid 116 206 heptanoic acid 130 223 (a) Plot a graph of the tabulated data. (b) Extrapolate the graph (using the data points for the C3, C4 and C5 acids) to estimate the boiling points of formic acid and acetic acid. (c) The experimental values for the boiling points of formic acid and acetic acid are 101ºC and 118ºC. Account for any differences between these values and  your answer in (b). 10. State one use of each of the following esters. (a) 1-octyl acetate (b) myristyl myristate (c) 1-pentyl propanoate 11. (a) Define the term esterification. (b) Explain why the reaction mixture is refluxed during the esterification process. 12. In 1895, Emil Fischer discovered that the reaction between acetic acid and butan-1-ol took several days to reach equilibrium under reflux. Fischer found that if hydrogen chloride gas was passed into the reaction mixture, equilibrium was achieved in a few hours. (a) Suggest an hypothesis to account for the behaviour of the hydrogen chloride. (b) Explain whether or not Fischer obtained a greater yield of product when hydrogen chloride was used. (c) Write condensed structural formulae to produce an equation for this esterification reaction. (d) Name the ester formed. 13. In esterification reactions involving diprotic acids such as oxalic acid, a larger amount of concentrated sulfuric acid is added to the reaction mixture. This action produces an increase in yield. Account for this observation. 14. Figure 10.18 shows the structure of Aspirin (acetyl salicylic acid). H H C      — H C      — C H C—   C O C —   O C O H H C C O H H Figure 10.18 Aspirin is a useful analgesic. Its systematic name is acetyl salicylic acid. (a) Aspirin is manufactured by the reaction between 2-hydroxybenzoic acid and acetic acid. Explain why Aspirin is classified as an ester. (b) Use condensed structural formulae to  write an equation for this esterification reaction. 15. Explain the purpose of each of the following procedures in the production of esters. (a) The reaction flask contains a few pieces of unglazed ceramic. (b) The reaction flask is heated with a hot-water bath or electric heating mantle rather than a Bunsen burner. (c) A water-cooled condenser is mounted above the reaction flask. (d) The reaction mixture is refluxed for many hours (and sometimes days). 16. Identify the statements listed below that are not part of the procedure used to prepare an ester. (a) Add 20 mL of 2-propanol. (b) Add 3 drops of universal indicator. (c) Distil the reaction mixture and recover the fractions in separate beakers. (d) Add 30 mL of glacial acetic acid. (e) Add 2 mL of concentrated sulfuric acid. (f) Add 5 mL of 2 mol/L NaOH. (g) Use a hot-water bath to reflux the reaction mixture. (h) Add boiling chips. 17. (a) Ethyl butanoate is a useful ester. Name one application of this ester in the food industry. (b) Justify the procedure used by a student in a school laboratory to prepare a sample of ethyl butanoate ester. Include relevant equations in your answer. 18. Use the data in Table 10.5 to explain why 1-pentyl propanoate , rather than 1-propyl acetate, is used as a solvent in automotive paints. 19. Look at Figure 10.16. (a) Name the acid and alkanol that react to form the ester illustrated. (b) State one use of this ester. 20. Propanoic acid has a molar weight of 74.1 g/mol and a boiling point of 140.8°C. (a) Name an alkanol that has the same molar weight as propanoic acid. (b) Predict whether this alkanol would have a higher or lower boiling point than that of propanoic acid. CHAPTER 10 ALKANOIC ACIDS AND ESTERS 229 SUMMARY www.jaconline.com.au/chemnsw/chemistry2 • Alkanols are alkane derivatives that contain the alcohol (OH) functional group. • Alkanoic acids are alkane derivatives that contain the carboxylic acid (COOH) functional group. MODULE 2 REVISION • Alkyl alkanoates are esters that form when alkanols condense with alkanoic acids with the loss of water. • The carboxylic acid group makes alkanoic acids highly polar. • Hydrogen bonding is an important intermolecular force between alkanoic acid molecules. This leads to an increase in their melting and boiling points compared with alkanes with the same number of carbon atoms per molecule. • Esters have no hydrogen atoms that are involved in hydrogen bonding. Their melting and boiling points are lower than those of the equivalent alkanoic acid. • The reaction between alkanols and alkanoic acids is slow, so the mixture must be heated to raise the kinetic energies of the reactants. • The heating of the alkanol–alkanoic-acid mixture is performed under reflux to ensure no vapours escape. They are condensed and return to the reaction mixture. • A small amount of concentrated sulfuric acid is added as a catalyst during esterification. The acid also acts as a dehydrating agent, which helps to shift the equilibrium to the right as water is removed. Care must be taken when handling concentrated sulfuric acid. • Boiling chips are added to the reaction flask during esterification in order to prevent ‘bumping’ which can lead to dangerous situations where hot liquids may be ejected from the apparatus. • Esters are used in the food industry as flavours and emulsifiers. They are also used in body-care products as fragrances in perfumes. Some esters are used as solvents. 230 THE ACIDIC ENVIRONMENT PRACTICAL ACTIVITIES 10.1 PRACTICAL ACTIVITIES PREPARATION OF AN ESTER  Aim To prepare an ester using the reflux technique 1. Select the alkanol and acid from the following tabulated information. Measure out the required  volumes using clean measuring cylinders. Experiment Alkanoic acid Alkanol 1 acetic acid (24 mL) ethanol (8 mL) 2 acetic acid (24 mL) propan-1-ol (8 mL) 3 acetic acid (24 mL) butan-1-ol (18 mL) 4 acetic acid (24 mL) pentan-1-ol (10 mL) 5 butanoic acid (24 mL) ethanol (8 mL) Materials • alkanoic acid (choose either glacial acetic acid or butanoic acid) • alkanol (choose one from ethanol, propan-1-ol, butan-1-ol or pentan1-1-ol) • concentrated sulfuric acid (dropper bottle) (teacher use only) • Quickfit round-bottom boiling flask (100 mL or 250 mL) • reflux condenser with hoses • water bath and Bunsen burner or electric heating mantle • retort stand, boss heads and clamps • boiling chips • stirring rod • beaker • measuring cylinder • Pasteur pipette Safety  • Wear safety glasses throughout this experiment. • Sulfuric acid is corrosive. Clean up spills immediately. If the acid is spilled on the skin then wash the area with large quantities of water. Seek assistance. • Organic chemicals are flammable. Do not allow liquids or vapours to come into contact with sparks or flames. • Identify other safety precautions relevant to this experiment by reading the method. Method Preparation of the ester Refer to Figure 10.11 in the text to help you set up the apparatus. 2. Pour the acid and the alkanol into the dry round-bottom flask. Your teacher will then add 2 mL (40 drops) of concentrated sulfuric acid to the flask. Now add 2 or 3 boiling chips. 3. Set up the flask and condenser for reflux as shown. Use a heating mantle, hotplate, Bunsen burner (with gauze) or hot-water bath to supply the heat once the apparatus is set up. A Bunsen burner can be used, but care should be taken that the reflux rate is controlled. A hot-water bath is not very suitable if the boiling points of the reactants and product are too high. 4. Before you begin to heat the flask, ensure that cold water is flowing from the tap to the base of the condenser jacket and then out the top and back into the sink. 5. Heat the flask and maintain reflux so that no  vapours are lost, and that the condensate drips back into the reaction flask at about 2 drops per second. 6. Reflux the mixture for at least 30 minutes (or longer if time allows). 7. Turn off the heat and allow the apparatus to cool. (Remove the heating mantle, hotplate or  water bath). Isolation   You may wish to isolate the ester from the reaction mixture. If so, then omit this section and proceed  with the optional extension. 8. Remove the boiling flask and carefully pour the reaction mixture into a beaker containing CHAPTER 10 ALKANOIC ACIDS AND ESTERS 231 PRACTICAL ACTIVITIES 70 mL of water. Stir with a glass rod. The mixture contains the ester plus unreacted alkanol and alkanoic acid. Compare the odour of this solution with the odours of the original reactants (see Results and analysis). 9. Clean up the equipment and workbench as directed by your teacher. Keep the aqueous mixture of the ester for further examination. Results and analysis 1. Depending on the solubility of the ester you have made, the aqueous mixture will either remain as one phase, or separate into two layers. (e.g. 1-butyl acetate is insoluble in water). If separation occurs, the upper layer will be the crude ester. It can be removed carefully using a Pasteur pipette or by using a separation funnel. 2. Compare the odour of the ester (or the final aqueous solution) with that of the original alkanol and alkanoic acid. To do this you can  waft vapours towards your nose with your hand. Describe what you smell in each case. Did the ester have a distinctly different odour from that of the reactants? 3. Explain how the ester could be extracted and separated from the unreacted alkanol and alkanoic acid. 4. Name the ester you have prepared and draw its structural formula. 232 THE ACIDIC ENVIRONMENT Conclusion  Write an appropriate conclusion for your experiment. Optional extension:  Isolation of the ester 1. Add the reaction mixture to 30 mL of saturated salt water in a beaker. 2. Pour the mixture into a separating funnel and isolate the upper layer of crude ester. 3. Place the crude ester layer in a beaker and add saturated sodium hydrogen carbonate to remove any acid. Stop when fizzing ceases. 4. Isolate the ester once more using a separating funnel. 5. Dry the ester using crystals of anhydrous calcium chloride. Filter the mixture. 6. Set up a distillation apparatus and distil the dried ester. Collect the distillate in a clean beaker. The boiling points of the esters are: ethyl acetate (77ºC) 1-propyl acetate (102ºC) 1-butyl acetate (127ºC) 1-pentyl acetate (147ºC) ethyl butanoate (120ºC) 7. Compare the odours of the esters to those of the original alkanol and alkanoic acid. Name the ester you have prepared and draw its structural formula.