Acyl chlorides are derivatives of carboxylic acids in which the OH group is replaced by a chlorine atom. They contain the COCl functional group, with general formula RCOCl.Naming: take the name of the parent acid, remove '-oic acid' and add '-oyl chloride'.
• CH3COCl: ethanoyl chloride
• CH3CH2COCl: propanoyl chloride
• C6H5COCl: benzoyl chlorideThey are made from carboxylic acids using phosphorus(V) chloride.Reactivity: acyl chlorides are very reactive. The carbonyl carbon is bonded to two electronegative atoms, oxygen and chlorine, which both pull electron density away from it. This makes the carbon strongly δ+, so it is easily attacked by nucleophiles such as water, alcohols, ammonia and amines. The chlorine then leaves as a chloride ion. These reactions are called addition-elimination reactions: the nucleophile adds to the carbonyl carbon and then HCl is eliminated.Properties: the small acyl chlorides are colourless liquids that fume in moist air, because they react with water vapour and give off HCl. They have sharp, choking smells, so they are handled in a fume cupboard and stored away from water.Worked example: name CH3CH2CH2COCl. It has four carbon atoms including the COCl carbon, so the parent acid is butanoic acid and the compound is butanoyl chloride. Its molecular formula is C4H7OCl, so Mr = 4(12.0) + 7(1.0) + 16.0 + 35.5 = 106.5.
• CH3COCl: ethanoyl chloride
• CH3CH2COCl: propanoyl chloride
• C6H5COCl: benzoyl chlorideThey are made from carboxylic acids using phosphorus(V) chloride.Reactivity: acyl chlorides are very reactive. The carbonyl carbon is bonded to two electronegative atoms, oxygen and chlorine, which both pull electron density away from it. This makes the carbon strongly δ+, so it is easily attacked by nucleophiles such as water, alcohols, ammonia and amines. The chlorine then leaves as a chloride ion. These reactions are called addition-elimination reactions: the nucleophile adds to the carbonyl carbon and then HCl is eliminated.Properties: the small acyl chlorides are colourless liquids that fume in moist air, because they react with water vapour and give off HCl. They have sharp, choking smells, so they are handled in a fume cupboard and stored away from water.Worked example: name CH3CH2CH2COCl. It has four carbon atoms including the COCl carbon, so the parent acid is butanoic acid and the compound is butanoyl chloride. Its molecular formula is C4H7OCl, so Mr = 4(12.0) + 7(1.0) + 16.0 + 35.5 = 106.5.