Recall that in a nucleophilic substitution reaction, a nucleophile donates its electrons to an electrophile.
Electrophiles are electron-seeking reagents — either neutral or positively charged — containing an empty atomic orbital or a low-energy antibonding orbital.
A positive electrophile, like the proton — with a vacant, low-energy 1s orbital — is very reactive. Consequently, a nucleophile like the hydroxide ion attacks the proton, neutralizing the charge and forming water.
Another positive electrophile — the carbocation — has a vacant p orbital, making it reactive towards a nucleophilic attack.
A neutral electrophile, like the Lewis acid boron trifluoride, has an empty p orbital that can accept electrons from the nucleophile, thus forming a bond and resulting in a stable complex.
In a neutral molecule like chlorobutane, the electrophilic center results from the electron-withdrawing inductive effect of the more electronegative substituent attached to the molecular chain.
In an organic electrophile with a double-bonded electronegative atom — like the carbonyl group — the C=O bond dipole renders a partial positive charge to the carbon atom.
In a reaction, the nucleophile deposits its electrons into the lower energy antibonding π orbital of the electrophile. As a result, the C–O π bond breaks, and the electrons move on to the oxygen atom.
In an electrophile, like HCl, which consists of a single-bonded electronegative atom, the dipole of the σ bond forces the nucleophilic electrons to move into the lower energy HCl antibonding σ orbital, and thus breaks the bond.
Molecules like halogens, with σ bonds and no dipoles, also make good electrophiles. In bromine, for example, poor overlaps between the atomic orbitals of bromine atoms result in a weak Br–Br bond.
Thus, a nucleophile attacks the lower energy σ antibonding orbital, breaking the Br–Br bond and making a new bond.
