Nucleophilic acyl substitution reactions summary

Carboxylic acids and carboxylic acid derivatives can also be prepared by methods other than nucleophilic acyl substitution reactions. A summary of the methods used to synthesize these compounds is given in Appendix IV. 

In summary, the mechanism for a nucleophilic acyl substitution reaction involves two core steps— nucleophilic attack and loss of a leaving group. Notice that these are the same two steps involved in an 5 2 process. However, there is one important difference. In an Si,i2 process, the two steps occur in a concerted fashion (simultaneously), but in a nucleophihc acyl substitution reaction, the two steps must occur separately. It is a common mistake to draw these two steps as occurring together. 

The most important reactions of carboxylic acids are the conversions to various carboxylic acid derivatives, e.g. acid chlorides, acid anhydrides and esters. Esters are prepared by the reaction of carboxylic acids and alcohols. The reaction is acid catalysed and is known as Fischer esterification (see Section 5.5.5). Acid chlorides are obtained from carboxylic acids by the treatment of thionyl chloride (SOCI2) or oxalyl chloride [(COCl)2], and acid anhydrides are produced from two carboxylic acids. A summary of the conversion of carboxylic acid is presented here. All these conversions involve nucleophilic acyl substitutions (see Section 5.5.5). 

The reactions of carboxylic acids and their derivatives are summarized here. Many (but not all) of the reactions in this summary are acyl substitution reactions (they are principally the reactions referenced to Sections 17.5 and beyond). As you use this summary, you will find it helpful to also review Section 17.4, which presents the general nucleophilic addition-elimination mechanism for acyl substitution. It is instructive to relate aspects of the specific acyl substitution reactions below to this general mechanism. In some cases proton transfer steps are also involved, such as to make a leaving group more suitable by prior protonation or to transfer a proton to a stronger base at some point in a reaction, but in all acyl substitution the essential nucleophilic addition-elimination steps are identifiable. 

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