electron acceptor

Meaning: In organic reactions, the electron acceptor is the electrophile, or Lewis acid. It has a partial positive charge or a low-energy empty orbital (the LUMO), so it can accept an electron pair from a nucleophile. Common acceptors are carbonyl carbons, carbocations, H⁺, and carbons bonded to a good leaving group. Electron-withdrawing groups such as C=O, NO₂ and halogens make nearby atoms better acceptors. In arrow-pushing, the arrow starts at the donor (nucleophile) and points toward the acceptor. In redox language, an electron acceptor is the oxidizing agent and is itself reduced. In biochemistry, O₂ is the terminal electron acceptor of the electron transport chain.

On the MCAT: A passage shows a molecule and asks which atom a nucleophile attacks, or asks you to rank electrophiles. A greater partial positive charge, less resonance donation into the acceptor, and a better leaving group all make a stronger acceptor. For example, acyl chlorides are more reactive than anhydrides, which are more reactive than esters, which are more reactive than amides. Questions on nucleophilic addition and substitution at carbonyls rely on this idea.

Common confusion: A Lewis acid does not need to donate H⁺. It only needs to accept an electron pair, so BF₃ and carbocations count. Do not mix up the roles. The nucleophile donates the pair, and the acceptor receives it. In redox, the acceptor is the species that gets reduced, not oxidized.