electrophile
Summary
An electrophile is an electron-pair acceptor: a species with a positive or partial positive charge, or an empty orbital, that bonds to a nucleophile's electron pair. On the MCAT, carbonyl carbons, carbocations and carbons bearing leaving groups are the usual electrophilic sites, and arrows point toward them.
Detail
Meaning: An electrophile ("electron-loving") is a Lewis acid in organic reactions. It accepts a pair of electrons from a nucleophile to form a new covalent bond. Common electrophiles are carbocations, H⁺, Lewis acids such as BF₃ and AlCl₃, and atoms with a partial positive charge (δ+), such as the carbon of a C=O group or the carbon bonded to a halogen in an alkyl halide. Electrophilicity generally rises with more positive charge, with nearby electron-withdrawing groups, and with less steric crowding. Among carbonyl compounds, acyl chlorides are more electrophilic than anhydrides, which are more electrophilic than esters, which are more electrophilic than amides. Aldehydes are generally more reactive than ketones.
On the MCAT: You may be asked to identify the electrophilic atom in a mechanism, to draw or read curved arrows (they run from the nucleophile's lone pair or π bond to the electrophile), or to rank reactivity. Settings include SN1 and SN2, nucleophilic addition to carbonyls, nucleophilic acyl substitution, and electrophilic aromatic substitution, where the benzene ring is the nucleophile and an electrophile such as NO₂⁺ is attacked.
Common confusion: The electrophile is the electron-poor site, not the entire molecule, and it need not carry a full charge. A neutral carbonyl carbon is a strong electrophile. Students also swap the terms: the nucleophile donates the electron pair, and the electrophile accepts it.
Sources
- Organic Chemistry by Paula Yurkanis Bruice
- Organic Chemistry by David Klein
- Organic Chemistry by John McMurry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.