ester

Meaning: An ester has the group R–C(=O)–O–R′. Like other carboxylic acid derivatives, it reacts by nucleophilic acyl substitution: a nucleophile adds to the carbonyl carbon to form a tetrahedral intermediate, then the alkoxide leaves. Reactivity runs acid chloride > anhydride > ester > amide. Esters have no O–H, so they cannot donate hydrogen bonds and boil lower than acids or alcohols of similar mass. Cyclic esters are lactones, and triglycerides and phospholipids contain ester linkages.

Equation: RCOOH + R′OH ⇌ RCOOR′ + H₂O (acid catalyst, H⁺). R and R′ are carbon groups, so no SI units apply. Because this is an equilibrium, excess alcohol or removing water shifts it toward the ester.

On the MCAT: Expect product prediction. Acid hydrolysis gives acid plus alcohol. Saponification (ester + OH⁻ → carboxylate + alcohol) goes to completion because the carboxylate is deprotonated. Transesterification swaps R′ using a different alcohol. LiAlH₄ reduces an ester to two alcohols, and excess Grignard reagent gives a tertiary alcohol. In IR, the C=O stretch is near 1735 cm⁻¹, slightly higher than a ketone.

Common confusion: Students mix up the ester with the ether (R–O–R′, no carbonyl) and with the anhydride. Naming also trips people up: the alcohol-derived alkyl group is named first, then the acid-derived part, as in ethyl acetate (CH₃COOCH₂CH₃).