What is an acyl chloride and how is it formed?

An acyl chloride is a type of organic compound formed by replacing a hydroxyl group with a chlorine atom.

Acyl chlorides, also known as acid chlorides, are organic compounds that contain a carbonyl group (C=O) and a chlorine atom attached to the same carbon atom. They are highly reactive and are used in various chemical reactions such as the formation of esters, amides, and anhydrides.

Acyl chlorides are formed by the reaction of a carboxylic acid with thionyl chloride (SOCl2) or phosphorus pentachloride (PCl5). During this reaction, the hydroxyl group (-OH) of the carboxylic acid is replaced with a chlorine atom (-Cl) to form an acyl chloride. The reaction is exothermic and produces hydrogen chloride gas (HCl) as a by-product.

For example, the reaction between ethanoic acid and thionyl chloride produces ethanoyl chloride and hydrogen chloride gas:

CH3COOH + SOCl2 → CH3COCl + HCl + SO2

Acyl chlorides are used in the synthesis of many organic compounds and are also used as intermediates in the production of pharmaceuticals, agrochemicals, and polymers. However, they are highly reactive and must be handled with care as they can cause severe burns and respiratory problems.