Chemistry A2 · Carboxylic acids and derivatives. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
An acyl chloride contains the group –COCl, for example ethanoyl chloride, CH3COCl. It is made from a carboxylic acid by replacing the –OH group with –Cl, using PCl3 and heat, PCl5 or SOCl2.
The carbonyl carbon is joined to two electronegative atoms, O and Cl, so it is strongly δ+. Nucleophiles attack it easily. Water, alcohols, phenol, ammonia and amines all react at room temperature, and each reaction also gives HCl.
The mechanism is addition–elimination. A lone pair on the nucleophile attacks the carbon and the π bond of C=O breaks. Then C=O reforms, the C–Cl bond breaks to release Cl−, and H+ is lost.
Students give the conditions for making an ester from a carboxylic acid. An acyl chloride needs no heat and no acid catalyst, the reaction goes to completion, and the second product is HCl, not water.
Answer: 2.04 g of ethyl propanoate
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