Acid (Acyl) Halides Formation

The transformation of a carboxylic acid into an acid (acyl) halide gives a compound whose C=O group is much more reactive owing to the presence of the halogen atom.

The reaction is usually performed with thionyl chloride, a reagent already used to replace the OH group of alcohols by Cl.

Click here to recall other uses of thionyl chloride.

The acid OH attacks the sulfur of thionyl chloride, which expels a chloride ion; a protonated intermediate with an excellent leaving group forms

The sulfur atom of thionyl chloride is strongly electrophilic because it is bonded to three very electronegative atoms; it thus behaves as a strong Lewis acid that can be attacked by the OH of an alcohol or a carboxylic acid. A molecule of HCl is expelled in the process.

A kind of organic-inorganic anhydride is then formed, which is immediately protonated by the expelled HCl. This intermediate is well prepared for chloride attack since it contains an excellent leaving group and protonation strongly enhances the electrophilicity of the C=O carbon.

Addition of chloride to the protonated carbonyl group, giving a tetrahedral intermediate

At this stage chloride attacks the C=O group.

This is the addition step where the former C=O carbon goes from sp² to sp³ in a tetrahedral intermediate.

Elimination of the leaving group: the acid chloride forms and HOSOCl decomposes into HCl and gaseous SO₂

In the last elimination process, the leaving group created at the first step leaves. The release of SO₂ gas shifts the various equilibria to the formation of the acid chloride.

As usual, the loss of the leaving group in the elimination step allows the carbon to recover the sp² hybridization.

Acid (acyl) chlorides are very reactive compounds (very sensitive to moisture, which must be avoided at all times) used as precursors of other functions derived from carboxylic acids.