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Acid (Acyl) Halides

Acid (acyl) halides, R–CO–X, are the most reactive carboxylic acid derivatives: the halogen hardly donates electrons to the carbonyl and is an excellent leaving group. That is why they are used to prepare almost all the other derivatives.

Examples of acyl halide names: cyclobutanecarbonyl fluoride, 2-(2-chloroformylcyclopentyl)acetic acid, acetyl chloride and 3-bromo-2-methylbutanoyl bromide

Nomenclature

 Plot of boiling points against molecular weight for alkanes, esters, acyl halides, acids and amides: amides and acids boil at much higher temperatures

Physical and Bond Properties

 Protonation of an acid derivative R–CO–L at the carbonyl oxygen and resonance forms of the cation, with the positive charge on the oxygen, the carbon and the L group

Acid-Base Properties

 Preparation of an acid chloride from the carboxylic acid with thionyl chloride

Preparation

 Reactivity of acyl halides: with water they give acids, with alcohols esters, with amines amides, with carboxylates anhydrides, with cuprates ketones and with H₂/catalyst or LiAl(OR)₃H aldehydes

Reactivity