Fischer esterification

Meaning: An acid catalyst (H₂SO₄ or HCl) protonates the carbonyl oxygen of the carboxylic acid, making the carbonyl carbon more electrophilic. The alcohol oxygen attacks it, forming a tetrahedral intermediate. After proton transfers, water leaves and the carbonyl re-forms as an ester. Every step is reversible, so the mixture settles at an equilibrium whose Keq is often near 1.

Equation: RCOOH + R'OH ⇌ RCOOR' + H₂O (H⁺ catalyst), with Keq = [RCOOR'][H₂O] / ([RCOOH][R'OH]). R and R' are carbon groups, brackets are molar concentrations (mol/L), and Keq is dimensionless.

On the MCAT: Expect Le Châtelier reasoning. Using excess alcohol, or removing water as it forms, raises the ester yield. You may be asked to name the ester from a given acid and alcohol, to write the tetrahedral intermediate, or to trace an ¹⁸O label: label on the alcohol oxygen ends up in the ester, not in the water. The catalyst speeds approach to equilibrium but does not change Keq.

Common confusion: Students assume the alcohol's OH is lost, as in dehydration. Here the OH from the carboxylic acid leaves as water, and the alcohol keeps its oxygen in the ester. Also, acid-catalyzed hydrolysis is the reverse reaction, while base-promoted hydrolysis (saponification) is effectively irreversible because it forms a carboxylate.