Benzene and phenol
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Is the reaction between ethanoic anhydride and a phenolic compound for the preparation of aspirin a nucleophilic substitution reaction? If so, which is the nucleophile?
Jenkin replies
Yes, overall it is nucleophilic substitution but it is better described as nucleophilic addition followed by elimination. It is not a straight substitution reaction.
The phenolic compound, 2-hydroxybenzoic acid, is the nucleophile: a lone pair of electrons on the oxygen atom of the –OH group attacks one of the carboxyl carbon atoms of the ethanoic anhydride.
It is an example of esterification.and can take place under acidic or alkaline conditions. The phenolic compound can be dissolved in alkali in which case a phenoxide ion is formed and will act as the nucleophile. Under acidic canditions, a C=O group of the acid derivative is protonated and the C atom, now having a positive charge is attacked by oxygen of the phenolic –OH.
You will find this on page 365 of the Nuffield Students' Book; a different example but the same essential mechanism.
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updated: 21 August 2003
