acyl

Meaning: An acyl group is the fragment R–C(=O)– left when the –OH of a carboxylic acid is removed. It is the core of the carboxylic acid derivatives: acyl (acid) chlorides, anhydrides, esters, amides, and the carboxylic acids themselves. The acyl carbon is electrophilic because the carbonyl oxygen pulls electron density away, so nucleophiles attack it. The usual mechanism is nucleophilic acyl substitution: the nucleophile adds to give a tetrahedral intermediate, then the C=O reforms and the leaving group departs.

Equation: R–CO–X + Nu⁻ → R–CO–Nu + X⁻, where R is an alkyl or aryl group, X is the leaving group (Cl, OCOR, OR, NH₂), and Nu is the nucleophile.

On the MCAT: Expect questions that rank derivative reactivity: acyl chloride > anhydride > ester ≈ carboxylic acid > amide. A more reactive derivative can be converted into a less reactive one, but not the reverse without special conditions. Passages also show amide or ester formation, or ask you to name the product of hydrolysis. Acetyl (CH₃CO–) and acetyl-CoA are common examples, and acyl groups appear in biochemistry in fatty acyl-CoA.

Common confusion: Students mix up acyl with alkyl or with acetyl. Acyl contains the C=O, and alkyl does not. Acetyl is just the specific acyl group with R = CH₃. Resonance donation from the leaving group's lone pair into the carbonyl, as in amides, lowers reactivity.