Carboxylic Acid Derivatives
When treating carboxylic acids with thionyl chloride (SOCl2), phosphorus trichloride (PCl3), or phosphorus pentachloride (PCl5) the corresponding acyl chloride is obtained. Thionyl chloride is the most important reagent for the synthesis of acyl chlorides.
In applying thionyl chloride, the carboxylic acid's hydroxy group is converted into a comparatively more effective leaving group ( + ). The first reaction step is an esterification of the carboxylic acid with thionyl chloride which yields a chlorosulfonic ester. The carbonyl carbon is consequently activated in preparation for a nucleophilic attack by the protonation of the carbonyl oxygen. Subsequently, a chloride that has been previously eliminated from thionyl chloride during esterification is nucleophilically added to the carbonyl carbon. The resulting intermediate finally eliminates a chloride anion, sulfur dioxide, and a proton, yielding the acyl chloride.
Acyl chlorides possess a good leaving group - the chloride anion - and the carbonyl carbon is much more electrophilic than that of carboxylic acids. Thus, acyl chlorides are key starting products in the syntheses for further carboxylic acid derivatives, such as esters, carboxamides, and carboxylic acid anhydrides. The reaction of acyl halides with water is vigorous and yields the corresponding carboxylic acid and hydrogen chloride.