acyl chloride
/ AY-sil /
If a carboxylic acid is the sleepy, well-behaved member of the family, the acyl chloride is its hot-tempered, hyper-reactive cousin. It is the most reactive of the common carboxylic-acid derivatives, so eager to react that it fumes in moist air and reacts violently with water. Chemists love it precisely because it gets things done.
An acyl chloride has the structure R-CO-Cl: the carbonyl carbon (C=O) is bonded to a chlorine instead of the -OH of an acid. That chlorine is a superb leaving group — chloride is the very stable conjugate base of a strong acid (HCl) — and chlorine is also electron-withdrawing, which makes the carbonyl carbon especially electron-poor and inviting to nucleophiles. Together these two features make the acyl chloride the high-energy starting point of acyl chemistry. You typically make one from a carboxylic acid using thionyl chloride (SOCl2), which neatly converts -OH into -Cl while the byproducts SO2 and HCl escape as gases.
Because they are so reactive, acyl chlorides are the workhorse for building less reactive derivatives: react them with an alcohol and you get an ester; with an amine and you get an amide; with a carboxylate and you get an anhydride. The rule of thumb is one-directional: you can always go from a more reactive derivative down to a less reactive one, and the acyl chloride sits at the top of that ladder, so it can be converted into any of the others.
CH3COOH + SOCl2 gives CH3COCl (acetyl chloride) + SO2 + HCl. The fresh acetyl chloride then reacts instantly with methanol to give methyl acetate (an ester) and HCl.
Thionyl chloride turns an acid into an acyl chloride; the acyl chloride then acylates almost anything.
Acyl chlorides react with water (and even atmospheric moisture) to give back the carboxylic acid plus HCl, so they must be kept dry and handled in a fume hood; the irritating HCl fumes are the giveaway.