Nucleophilic substitution is a fundamental class of chemical reactions in which an electron-rich species, known as a nucleophile, selectively replaces a leaving group attached to a carbon atom. In organic chemistry, a nucleophile is an atom or molecule that seeks out positive charge, usually because it has a lone pair of electrons or a negative charge itself.
The leaving group is typically an electronegative atom, like a halogen, that withdraws electron density from the carbon and can easily break away with a pair of electrons. When the nucleophile attacks the electron-poor carbon, it forces the leaving group to detach, resulting in a completely new substituted molecule.
These substitution reactions generally proceed via two primary mechanisms, designated as SN1 and SN2. The SN2 mechanism is a concerted, one-step process. The nucleophile attacks the carbon atom directly from the backside, pushing the leaving group off at the exact same time.
This back-attack results in an inversion of the molecule’s three-dimensional geometry, similar to an umbrella flipping inside out in the wind. SN2 reactions happen fastest with small, unhindered carbon molecules because the nucleophile needs clear physical space to strike the target atom.
Conversely, the SN1 mechanism is a step-by-step process. First, the leaving group detaches on its own, leaving behind a highly reactive, positively charged intermediate called a carbocation. In the second step, the nucleophile attacks this exposed carbocation.
Because the carbocation is flat, the nucleophile can attack from either side, resulting in a mixture of different geometric products. SN1 reactions are favored when the central carbon is surrounded by bulky groups, because these bulky groups actually help stabilize the temporary positive charge formed during the first step of the reaction.