electrophile

The mirror image of a nucleophile. The word means electron-loving, and an electrophile is an electron-poor atom or molecule craving a pair of electrons to complete itself. It is the partner with the empty hands, reaching out for the electrons a nucleophile is eager to give. A bare proton (H+), a carbocation (a carbon missing a pair and bearing a positive charge), a carbonyl carbon made positive by its neighboring oxygen — these are electrophiles, the targets that get attacked.

Concretely, an electrophile accepts an electron pair to form a new bond. Think of the carbon in a carbonyl group, C=O: the oxygen is greedy and pulls the shared electrons toward itself, leaving the carbon with a partial positive charge — a soft target sitting there, electron-hungry. A nucleophile homes in on exactly that carbon and pushes its electron pair in. An electrophile is essentially the same idea as a Lewis acid (an electron-pair acceptor); we say electrophile when describing what gets attacked in a mechanism.

Electrophile and nucleophile are the two poles every organic reaction runs between. Reading a molecule, your first instinct should be to spot the electron-poor sites (the electrophilic carbons, the positive centers) and the electron-rich sites (the nucleophiles, the lone pairs and pi bonds), because electrons always flow from rich to poor. A partial positive charge, drawn with the Greek delta-plus, is your visual flag for where a nucleophile is likely to attack.