Esterification Carboxylic acids react with alcohols, in the presence of a strong acid catalyst, to form esters and water. C C C O O C H H H H H H H H Esters have two parts to their names, eg methyl propanoate. The bit ending in –yl comes from the alcohol that has formed it and is next to the single bonded oxygen. The bit ending in –anoate comes from the carboxylic acid and includes the C in the C=O bond. Carboxylic Acid + Alcohol Ester + water H+ The reaction is reversible. The reaction is quite slow and needs heating under reflux, (often for several hours or days). Low yields (50% ish) are achieved. An acid catalyst (H2SO4 ) is needed. + + H2O H3C C O OH C C O O C C H H H H H H H H H+ C O H H H C H H H CH3CO2H + CH3CH2OH CH3CO2CH2CH3 + H2O Ethanoic acid Ethanol Ethyl Ethanoate Uses of Esters Esters can have pleasant smells Esters are sweet smelling compounds that can be used in perfumes and flavourings. For use in perfumes they need to be non toxic, soluble in solvent such as ethanol, volatile (turns into gas easily), and not react with water. Esters can be used as solvents for polar organic substances Ethyl ethanoate is used as a solvent in glues and printing inks Although polar, they do not form hydrogen bonds (reason: there is no hydrogen bonded to a highly electronegative atom) thus, they have much lower b.p. than the hydrogen-bonded carboxylic acids they came from. They are also almost insoluble in water Esters can be used as plasticisers for polymers Often pure polymers have limited flexibility because the polymer chains cannot move over each other. Incorporating some plasticiser into the polymer allows the chains to move more easily and the polymer can become more flexible. Hydrolysis of esters Esters can be hydrolysed and split up by either heating with acid or with sodium hydroxide. i) with acid reagents: dilute acid (HCl) conditions: heat under reflux This reaction is the reverse reaction of ester formation. When an ester is hydrolysed a carboxylic acid and an alcohol are formed. CH3CH2CO2CH2CH3 + H2O CH3CH2CO2H + CH3CH2OH ethyl propanoate H+ This reaction is reversible and does not give a good yield of the products. ii) with sodium hydroxide reagents: dilute sodium hydroxide conditions: heat under reflux CH3CH2CO2CH3 + NaOH CH3CH2CO2 – Na+ + CH3OH methyl propanoate sodium propanoate methanol The carboxylic acid salt product is the anion of the carboxylic acid. The anion is resistant to attack by weak nucleophiles such as alcohols, so the reaction is not reversible. This reaction goes to completion. Fats and oils are ESTERS of glycerol and long chain carboxylic acids (fatty acids) Fats and soaps Vegetable oils and animal fats can be hydrolysed to give soap, glycerol and long chain carboxylic (fatty) acids H2C HC H2C OH OH OH + 3 KOH + 3 CH3 (CH2 )16 CO2 –K+ Propane-1,2,3-triol long chain carboxylic (fatty) acids Soap Long chain carboxylic (fatty) acids, produced by the hydrolysis of fats, act as soaps. The polar CO2 – end is hydrophilic and mixes with water. The long non-polar hydrocarbon chain is hydrophobic and mixes with grease. So this allows the grease and water to mix and be washed away. Glycerol forms hydrogen bonds very easily and is readily soluble in water. It is used in cosmetics, food and in glues. Vegetable oils can be converted into biodiesel by reaction with methanol in the presence of a (strong alkali) catalyst Biodiesel Biodiesel is a mixture of methyl esters of long chain carboxylic acids CH2 HC CH2 O O O C C C O O O (CH2)14CH3 (CH2)14CH3 (CH2)14CH3 + 3 CH3OH H2C HC H2C OH OH OH + 3 CH3 (CH2 )14CO2CH3 It can be argued that the biodiesel produced from this method is classed as carbon–neutral as any carbon dioxide given off when the biofuel is burnt would have been extracted from the air by photosynthesis when the plant grew. This does not take into account any energy needed to irrigate plants, extract the oil, heat the reaction with methanol mixture or process the fuel. If the energy for this process comes from fossil fuels then the biofuel produced is not carbon neutral. It also does not take into account the effect on land available for food production.
188.8.131.52 Carboxylic acids and esters (A-level only)
The structures of:
Carboxylic acids and alcohols react, in the presence of an acid catalyst, to give esters.
Common uses of esters (eg in solvents, plasticisers, perfumes and food flavourings).
Vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol).
Esters can be hydrolysed in acid or alkaline conditions to form alcohols and carboxylic acids or salts of carboxylic acids.
Vegetable oils and animal fats can be hydrolysed in alkaline conditions to give soap (salts of long-chain carboxylic acids) and glycerol.
Biodiesel is a mixture of methyl esters of long-chain carboxylic acids.
Biodiesel is produced by reacting vegetable oils with methanol in the presence of a catalyst.