Chapter 20: Carboxylic Acid Derivatives. Nucleophilic Acyl Substitution

Overview of Nucleophilic Acyl Substitution

Overall nucleophilic acyl substitution is most simply represented as follows:

A nucleophile is an electron rich species that will react with an electron poor species (Nu in scheme).
An acyl group is R-C=O (where R can be alkyl or aryl).... note the acyl group in both the starting material and the product.
A substitution note that the leaving group (LG) is replaced by the nucleophile (Nu).

There are two fundamental events in a nucleophilic acyl substitution reaction:

• formation of the new s bond to the nucleophile, Nu.
• breaking of the s bond to the leaving group, LG.

Overall, these events are the same as those in a simple nucleophilic substitution (chapter 8), note the fundamental similarity in the two general processes.

The difference in nucleophilic acyl substitution is that when the nucleophile adds to the electrophilic C, it becomes tetrahedral and an intermediate forms, then the leaving group departs as shown below:

QUESTION:

• In nucleophilic substitution (e.g. for alkyl halides) the nucleophile cannot attack until the leaving group leaves, why is there a difference ? ANSWER

© Dr. Ian Hunt, Department of Chemistry