How are acid chlorides prepared?

Acid chlorides, also called acyl chlorides, are prepared from carboxylic acids by replacing the hydroxyl group with a chlorine atom.

Methods of preparation:

1. Using thionyl chloride (SOCl₂): This is the most common method. The reaction is convenient because the by-products are gases that escape easily, leaving behind a pure acid chloride.
2. Using phosphorus trichloride (PCl₃): Carboxylic acids can be treated with phosphorus trichloride, which converts them into acid chlorides. A phosphorus-containing compound remains as a by-product.
3. Using phosphorus pentachloride (PCl₅): Another way is to react the carboxylic acid with phosphorus pentachloride. This also produces acid chloride along with other phosphorus-based by-products and hydrogen chloride gas.

Key points:

• Acid chlorides are highly reactive compared to carboxylic acids.
• They are useful intermediates for preparing esters, amides, and anhydrides.
• Thionyl chloride is usually preferred in the laboratory because the reaction is clean and the by-products are easily removed.