Return to Contents  Chapter 20: Carboxylic Acid Derivatives. Nucleophilic Acyl Substitution  Ch 20 contents

Nucleophilic Acyl Substitution : Less Reactive Systems

Step 1:
An acid / base reaction. Protonation of a lone pair on the O of the carbonyl group creates a more electrophilic, cationic system. 

Step 2:
The nucleophile attacks the electrophilic C of the carbonyl group causing the p bond in the C=O to break and allows for neutralisation of the positive charge. This converts the sp2 C=O into the sp3 tetrahedral intermediate

Step 3:
Electrons on the O are used to aid the loss of the leaving group (see note below). This reforms the p bond of the carbonyl group. 

Step 4:
An acid/base reaction. Deprotonation reveals the carbonyl of the product and regenerates the acid catalyst.

acid catalysed nucleophilic acyl substitution for less reactive systems

Note:  In some cases (especially esters and amides) the leaving group must also be activated prior to being lost. This can also be achieved by protonation. This adds a step to the process shown above.

Examples Reactions that follow this mechanism:

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© Dr. Ian Hunt, Department of Chemistry, University of Calgary