Alkanoyl halides are obtained by reacting carboxylic acids with PBr ₃ . ₂ can also be used .
Thus, ethanoic acid [1] is transformed into ethanoyl bromide [2] by reaction with phosphorus tribromide. Ethanoic acid by reaction with thionyl chloride forms compound [3]

The reaction mechanism consists of the nucleophilic attack of the carboxylic acid on the phosphorus or sulfur of the reagent, producing an addition-elimination mechanism that leads to the alkanoyl halide.

[a] Acid attack on phosphorus tribromide.
[b] Nucleophilic addition step.
[c] Elimination step

This mechanism is repeated three times, replacing all the bromines of the phosphorus tribromide with hydroxy groups.

Thionyl chloride has a mechanism analogous to that of phosphorus tribromide, converting carboxylic acids to alkanoyl chlorides. The byproducts formed in this reaction are sulfur dioxide and hydrogen chloride.

[a] Acid attack on thionyl chloride.
[b] Nucleophilic addition
[c] Elimination