Acyl chloride

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Acyl chlorides are organic compounds containing the functional group -C(=O)Cl, with the general formula R-COCl, where R is a side chain.

General properties

Acyl chlorides are volatile fuming liquids that react with water to form hydrogen chloride and the carboxylic acid of the respective acyl chloride. Due to the Cl substituent, acyl chlorides do not form hydrogen bonds, which causes them to have have lower boiling and melting points than their corresponding carboxylic acid.

Acyl chlorides readily hydrolyze in water to give the corresponding carboxylic acid and hydrochloric acid.

RCOCl + H2O → RCO2H + HCl

Acyl chlorides can react with the salt of the corresponding carboxylic acid to give the anhydride of the said acid.

For example, acetyl chloride will react with sodium acetate to give acetic anhydride:

CH3COCl + CH3COONa → (CH3CO)2O + NaCl


Due to their hazards, acyl chlorides are difficult if not impossible to get by the amateur chemist.

Some acyl chlorides, as acetyl chloride, may be monitored in many countries due to their use in the production of acetic anhydride, a DEA List II substance.


With deoxychlorinating reagents

Acyl chlorides can be prepared by reacting a carboxylic acid or its salt with a chlorodehydrating agent, such as PCl3, PCl5 or SOCl2. Phosgene can also be used, but its great toxicity makes this route dangerous.

Another method using a different chlorodehydrating agent, involves the use of oxalyl chloride and DMF.[1]

By oxidative chlorination of non-enolizable aldehydes

A more accessible route, albeit less known, is the direct chlorination of benzaldehyde, yielding benzoyl chloride[2], which can then be easily converted into more volatile acyl chlorides, such as acetyl chloride.[3]

With acetonitrile and hydrogen chloride

Acetyl chloride is produced by the reaction of acetic acid and acetonitrile with anhydrous hydrogen chloride. This method depends on the formation of the iminoyl chloride in situ, and is driven by the formation of highly stable acetamide and the volatility of acetyl chloride.

AcOH + MeCN + HCl → AcNH2 + AcCl (g)

Caution must be taken to avoid exposure to highly toxic acetyl chloride vapor.


Acyl chlorides are corrosive and very toxic. Wear proper protection when working with them.

See also


  1. R. Salmon, Encyclopedia of Reagents for Organic Synthesis, (Ed.) 2001.

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