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Conversion of carboxylic acids into acid chlorides with SOCl2

Description: Thionyl chloride converts carboxylic acids into acid chlorides. The reaction liberates HCl and SO2 gas.

Notes: Acid bromides can also be made through the reaction of carboxylic acids with thionyl bromide (SOBr2)



The byproducts of each of these reactions are HCl and sulfur dioxide (SO2).


The reaction begins with 1,2 addition of the carbonyl oxygen to the sulfur (Step 1, arrows A, B and C) followed by 1,2-elimination of chloride (Step 2, arrows D and E). The chloride ion then performs a 1,2-addition on the carbonyl (Step 3, arrows F and G) to give a tetrahedral intermediate which does a second 1,2-elimination of SO2 (Step 4, arrows H, I, and J). Finally, the carbonyl oxygen is deprotonated to give the neutral acid chloride.