3.4.5 Compounds Containing the Carbonyl Group - Carboxylic acids and esters
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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
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.
Biodiesel
This useful commodity can be made by esterification of long chain carboxylic acids using methanol, to give methyl esters.