Acyl Chlorides

Acyl chlorides are derivatives of carboxylic acids in which the OH group has been replaced by a chlorine atom. Their functional group is -COCl and their general formula is RCOCl.They are named by replacing the -oic acid ending of the parent acid with -oyl chloride. CH₃COCl is ethanoyl chloride and CH₃CH₂COCl is propanoyl chloride.Worked example: name CH₃CH₂CH₂COCl. Count the carbons including the C of COCl: there are 4, so the parent acid is butanoic acid and the compound is butanoyl chloride.Why so reactive? The carbonyl carbon is bonded to two electronegative atoms, oxygen and chlorine. Both pull electron density away from it, so the carbon carries a large δ⁺ charge and is strongly attacked by nucleophiles. The chloride ion is also a good leaving group. Acyl chlorides are among the most reactive organic compounds you meet at A level: ethanoyl chloride fumes in moist air and reacts violently with water at room temperature.Acyl chlorides react with nucleophiles such as water, alcohols, ammonia and primary amines by nucleophilic addition-elimination. The acyl group, RCO, is transferred to the nucleophile, so these are acylation reactions. Every one of them also produces hydrogen chloride, seen as steamy white fumes. Because they are so reactive, acyl chlorides must be stored in dry conditions.