Electrophiles and Nucleophiles

Every reagent that attacks an organic substrate during a reaction can be sorted into one of two broad categories, based on whether it is short of electrons or rich in them.

An electrophile (literally, 'electron lover') is any atom, ion or molecule that is electron deficient -- it either carries a full or partial positive charge, or has an atom with an incomplete octet -- and therefore seeks out and accepts a pair of electrons from another species to form a new covalent bond, behaving as a Lewis acid. Positively charged species such as a proton ($\text{H}^+$), a carbocation ($\text{R}^+$), the nitronium ion ($\text{NO}_2^+$) and a bromonium-type species ($\text{Br}^+$) are all electrophiles, as are electrically neutral molecules whose central atom has an incomplete octet and can therefore still accept a lone pair, such as boron trifluoride ($\text{BF}_3$) and aluminium chloride ($\text{AlCl}_3$).

A nucleophile (literally, 'nucleus lover') is the opposite: any atom, ion or molecule that is electron rich -- it either carries a full negative charge, or has one or more lone pairs of electrons available -- and therefore donates a pair of electrons to an electron-deficient centre to form a new bond, behaving as a Lewis base. Negatively charged ions such as hydroxide ($\text{OH}^-$), cyanide ($\text{CN}^-$) and bromide ($\text{Br}^-$) are classic nucleophiles, and so are electrically neutral molecules that carry an accessible lone pair, such as ammonia ($\text{NH}_3$), water ($\text{H}_2\text{O}$) and alcohols ($\text{ROH}$). …