Conversion of carboxylic acids into acid chlorides with SOCl2

by James

Description: Thionyl chloride converts carboxylic acids into acid chlorides. The reaction liberates HCl and SO2 gas.
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Notes: Acid bromides can also be made through the reaction of carboxylic acids with thionyl bromide (SOBr2)

Examples:

Notes:

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

Mechanism:

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.

Notes:

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