Alcohols and Esters: Question 4
Syllabus 33.2
Ethyl benzoate can be prepared by two different routes.
Route 1: benzoic acid is heated under reflux with ethanol, using concentrated sulfuric acid as a catalyst.
Route 2: ethanol is added dropwise, at room temperature, to benzoyl chloride.
Which statement correctly compares these two routes?
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Worked solution
Route 1: esterification of a carboxylic acid
Benzoic acid reacting with ethanol, catalysed by concentrated sulfuric acid, is a standard acid-catalysed esterification:
This reaction is reversible, reaching a position of equilibrium (which is why it needs a catalyst and reflux to proceed at a reasonable rate and yield), and its by-product is water.
Route 2: reaction with an acyl chloride
Benzoyl chloride is far more reactive than benzoic acid towards nucleophiles such as ethanol, because the chlorine atom is a much better leaving group than the hydroxyl group of a carboxylic acid. The reaction is fast and exothermic even at room temperature, needs no catalyst, and goes essentially to completion (it is not a reversible/equilibrium reaction):
The by-product here is hydrogen chloride gas (often seen as steamy white fumes), not water.
Why the other options are wrong
- A swaps the two routes’ behaviour and by-products around completely.
- C is wrong on both counts: the routes do not reach the same equilibrium (Route 2 is not an equilibrium at all), and Route 2 does not produce water.
- D reverses which route needs the catalyst and reflux: it is Route 1 (the carboxylic acid) that needs the sulfuric acid catalyst and reflux, while Route 2 (the acyl chloride) proceeds rapidly at room temperature without one.
Final answer
B. Route 1 is a reversible esterification producing water; Route 2 goes essentially to completion, producing hydrogen chloride gas.