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Carboxylic Acids and Derivatives: Question 9

Syllabus 18.1, 33.1

Multiple choice A2 1 mark

Methyl propanoate, CH3CH2COOCH3\text{CH}_3\text{CH}_2\text{COOCH}_3, can be made either (i) by warming propanoic acid under reflux with methanol and a few drops of concentrated sulfuric acid, or (ii) by adding propanoyl chloride dropwise to excess methanol.

Which statement correctly compares these two routes to the same ester?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Route (i): direct esterification

CH3CH2COOH+CH3OH⇌CH3CH2COOCH3+H2O\text{CH}_3\text{CH}_2\text{COOH} + \text{CH}_3\text{OH} \rightleftharpoons \text{CH}_3\text{CH}_2\text{COOCH}_3 + \text{H}_2\text{O}

The concentrated sulfuric acid is a catalyst that speeds up the approach to equilibrium and (by removing water) can help shift its position, but the fundamental reaction remains a reversible, acid-catalysed equilibrium. Even under reflux, the mixture reaches an equilibrium containing significant amounts of unreacted acid and alcohol alongside the ester; a good yield of ester requires techniques such as using an excess of one reactant or removing the water (or ester) as it forms.

Route (ii): via the acyl chloride

CH3CH2COCl+CH3OH→CH3CH2COOCH3+HCl\text{CH}_3\text{CH}_2\text{COCl} + \text{CH}_3\text{OH} \rightarrow \text{CH}_3\text{CH}_2\text{COOCH}_3 + \text{HCl}

This is a nucleophilic addition-elimination reaction. Because chloride ion is such a good leaving group, and the electronegative chlorine makes the carbonyl carbon strongly electrophilic, this reaction proceeds rapidly even in the cold, without any added catalyst, and effectively goes to completion in one direction. The hydrogen chloride gas escapes from the mixture rather than reacting further, so there is no equilibrium to speak of.

Why A is correct, and the others are wrong

  • A correctly identifies route (i) as a slow, reversible, catalysed equilibrium with incomplete conversion, and route (ii) as a fast, uncatalysed, essentially irreversible reaction that goes to completion, exactly the distinction the topic draws between the “slow, reversible equilibrium of direct esterification” and the acyl chloride’s rapid addition-elimination.
  • B is wrong: the acid in route (i) is only a catalyst for a fundamentally slow equilibrium reaction, and does not make it faster than the acyl chloride’s spontaneous, highly electrophilic reaction in route (ii).
  • C is wrong because the two routes proceed by different overall processes (a reversible equilibrium versus an effectively one-way reaction) and do not give the same yield in the same time.
  • D is wrong: hydrogen chloride gas simply escapes from the reaction mixture in route (ii) and does not react with the ester product to regenerate the acyl chloride; route (ii) is not reversible in this way.

Final answer

A. Direct esterification (route (i)) is a slow, reversible, acid-catalysed equilibrium with incomplete conversion, whereas the acyl chloride route (ii) is fast, uncatalysed and goes to completion.