Acid chloride, alcohols from nucleophilic acyl substitution

The reaction occurs by a nucleophilic acyl substitution pathway in which the carboxylic acid is first converted into a chlorosulfite intermediate, thereby replacing the -OH of the acid with a much better leaving group. The chloro-sulfitc then reacts with a nucleophilic chloride ion. You might recall from Section 17.6 Hint an analogous chlorosulfite is involved in reaction of an alcohol with SOCb to yield an alkyl chloride. 

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). 

Acid anhydrides are not as reactive as acid chlorides, but they are still activated toward nucleophilic acyl substitution. An anhydride reacts with an alcohol to form an ester. Notice that one of the two acid units from the anhydride is expelled as the leaving group. 

The mechanism is similar to that for the formation of chlorides from alcohols and thionyl chloride. The hydroxyl group is converted to a good leaving group by thionyl chloride, followed by a nucleophilic acyl substitution in which chloride is the nucleophile (compare with Sec. 7.10). Phosphorus pentachloride and other reagents can also be used to prepare acyl chlorides from carboxylic acids. 

This chapter will revisit the lUPAC nomenclature system for aldehydes, ketones, and carboxylic acids, as well as introduce nomenclature for the four main acid derivatives acid chlorides, anhydrides, esters, and amides. The chapter will show the similarity of a carbonyl and an alkene in that both react with a Br0nsted-Lowry acid or a Lewis acid. The reaction of a carbonyl compound with an acid will generate a resonance stabilized oxocarbenium ion. Ketones and aldehydes react with nucleophiles by what is known as acyl addition to give an alkoxide product, which is converted to an alcohol in a second chemical step. Acid derivatives differ from aldehydes or ketones in that a leaving group is attached to the carbonyl carbon. Acid derivatives react with nucleophiles by what is known as acyl substitution, via a tetrahedral intermediate. 

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