S_N2 Reaction - Reaction Mechanism

Reaction Mechanism

The reaction most often occurs at an aliphatic sp3 carbon center with an electronegative, stable leaving group attached to it - 'X' - frequently a halide atom. The breaking of the C-X bond and the formation of the new C-Nu bond occur simultaneously to form a transition state in which the carbon under nucleophilic attack is pentacoordinate, and approximately sp2 hybridised. The nucleophile attacks the carbon at 180° to the leaving group, since this provides the best overlap between the nucleophile's lone pair and the C-X σ* antibonding orbital. The leaving group is then pushed off the opposite side and the product is formed.

If the substrate under nucleophilic attack is chiral, this can lead, although not necessarily, to an inversion of stereochemistry called a Walden inversion (the nucleophile attacks the electrophilic carbon center, inverting the tetrahedron, much like an umbrella turning inside out in the wind).

In an example of the S_N2 reaction, the attack of OH− (the nucleophile) on a bromoethane (the electrophile) results in ethanol, with bromide ejected as the leaving group:

S_N2 attack occurs if the backside route of attack is not sterically hindered by substituents on the substrate. Therefore this mechanism usually occurs at an unhindered primary carbon centre. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, the substitution will involve an S_N1 rather than an S_N2 mechanism, (an S_N1 would also be more likely in this case because a sufficiently stable carbocation intermediary could be formed.)

In coordination chemistry, associative substitution proceeds via a similar mechanism as S_N2.