AQA A Level chemistry - A2 Unit 4: Section 3.4.5 Compounds Containing the Carbonyl Group

3.4.5 Compounds Containing the Carbonyl Group - Carboxylic acids and esters

Students should:

know that carboxylic acids are weak acids but will liberate CO2 from carbonates
know that carboxylic acids and alcohols react, in the presence of a strong acid catalyst, to give esters
know that esters can have pleasant smells
know the common uses of esters (e.g. in solvents, plasticizers, perfumes and food flavourings)
know that vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol)
know that esters can be hydrolysed
understand that vegetable oils and animal fats can be hydrolysed to give soap, glycerol and long chain carboxylic (fatty) acids
know that biodiesel is a mixture of methyl esters of long chain carboxylic acids

Acidic properties

Carboxylic acids are able to release hydrogen ions in solution, as the carboxylate ion is stabilised by resonance delocalisation. However, the equilibrium does not go very far and a typical organic acid dissociates about 1% in aqueous solution.

CH₃COOH CH₃COO^- + H⁺

This does not mean that carboxylic acids are unable to react as acids. The hydrogen ions available are able to react with active metals, carbonates, hydroxides etc. removing them from the equilibrium pulling the reaction to the right hand side.

CH₃COOH + Mg (CH₃COO)₂Mg + H₂

CH₃COOH + NaOH CH₃COONa + H₂O

2CH₃COOH + Na₂CO₃ 2CH₃COONa + H₂O + CO₂

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Esterification

Carboxylic acids can react with alcohols in the presence of concentrated sulfuric acid forming esters.

CH₃COOH + CH₃CH₂OH CH₃COOCH₂CH₃ + H₂O

The reaction is reversible and arrives at equilibrium. The purpose of the strong sulfuric acid is also to absorb the water formed, helping the reaction to the right hand side.

The properties of esters

Esters are pleasant, fruity smelling compounds used in the perfumery and flavouring industries. They also find use as solvents and plasticizers.

Natural esters

Apart from the natural scents and flavours esters also form an important part of biochemistry as fats and oils. Animal fat is a triester of propan-1,2,3-triol and stearic acid, C₁₇H₃₅COOH.

Hydrolysis of esters

As the esterification reaction is reversible, esters can be broken apart by waming with an aqueous solution of base. This forms the basis of soap-making, a reaction that was discovered thousands of years ago, probably when hot fat from a fire mixed with ashes (containing potassium carbonate) and water.

propyl-1,2,3-tristearate (animal fat) + sodium hydroxide propan-1,2,3-triol (glycerol) + sodium stearate (soap)

This same reaction can be carried out with natural oils to give other soap-like compounds, such as sodium palmitate from palm oil.

Soap

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Biodiesel

This useful commodity can be made by esterification of long chain carboxylic acids using methanol, to give methyl esters.

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