nucleophile

The word means nucleus-loving, and that is exactly the personality. A nucleophile is an electron-rich species looking for a positive place to give its electrons to — a generous partner carrying a spare pair of electrons (or an electron-rich double bond) and seeking a positively charged or electron-poor atom to bond with. Hydroxide (OH-), ammonia (:NH3), a bromide ion (Br-), even the pi cloud of an alkene — all are nucleophiles, drawn toward positive nuclei the way a magnet is drawn to its opposite pole.

Concretely, a nucleophile attacks by pushing one of its electron pairs into an electron-poor atom — most often a carbon bearing a partial positive charge — forming a new bond. Picture hydroxide approaching a carbon that already carries a leaving group: the oxygen's lone pair reaches in and forms a fresh O-C bond as the leaving group departs with the old bonding electrons. A nucleophile is essentially the same thing as a Lewis base (an electron-pair donor); the name nucleophile is used when we are talking about attack on carbon in a reaction mechanism, while base is used when the target is a proton.

Nucleophiles are one half of the two-word vocabulary that runs all of organic mechanism — nucleophile and electrophile. Almost every reaction can be read as electrons flowing from a nucleophile to an electrophile, and a curved arrow drawn from the nucleophile's electron pair to its target captures the whole event. Knowing what makes a good nucleophile — charge, electronegativity, polarizability, the solvent, and how crowded it is — lets you predict which species will attack, how fast, and where.