AS Level Chemistry · Carboxylic acids and derivatives. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
An ester is made by heating an alcohol with a carboxylic acid and a little concentrated sulfuric acid, which is the catalyst. Water is lost as the two molecules join, so this is a condensation reaction. It is reversible.
The name has two parts. The first part comes from the alcohol and the second part, ending in -oate, comes from the acid. Ethanol and methanoic acid give ethyl methanoate, HCOOCH2CH3.
Hydrolysis is the reverse: water splits the ester link. Heated with dilute acid, an ester gives the carboxylic acid and the alcohol in a reversible reaction. Heated with dilute alkali, it gives the carboxylate salt and the alcohol, and the reaction goes to completion.
Students name the ester the wrong way round. In CH3CH2COOCH3 the part with the C=O comes from the acid (propanoic acid) and the part joined to the single-bonded O comes from the alcohol (methanol), so it is methyl propanoate, not propyl methanoate.
Answer: 4.10 g of sodium ethanoate.
Try each one, then open the answer.
Answer: B. Alkaline hydrolysis goes to completion and the acid produced is immediately neutralised by NaOH to the salt: CH3COOC2H5 + NaOH → CH3COONa + C2H5OH. Acid hydrolysis would give ethanoic acid in an equilibrium.
Answer: D. 8.80 ÷ 88.0 = 0.100 mol of ester, which gives 0.100 mol of ethanol (Mr 46.0) = 4.60 g. 6.00 g is the mass of ethanoic acid formed.
Answer: B. In an ester name the alkyl group (ethyl) comes from the alcohol and the -oate part (methanoate) comes from the acid, so the reactants are ethanol and methanoic acid.
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