The SN2 Mechanism: Bimolecular Nucleophilic Substitution

The $S_N2$ mechanism (Substitution, Nucleophilic, Bimolecular) is the pathway by which primary haloalkanes, and to a lesser extent secondary ones, react with good nucleophiles. It proceeds in a single, concerted step: the incoming nucleophile $\text{Nu}^-$ attacks the electrophilic carbon from the side directly opposite the leaving group $\text{X}$ -- the "backside" -- at the same time as the $\text{C--X}$ bond begins to break. There is no discrete intermediate; instead the reaction passes through one transition state in which the carbon is simultaneously and partially bonded to both the incoming nucleophile and the departing leaving group, with the remaining three substituents momentarily flattened into a plane perpendicular to the $\text{Nu}{\cdots}\text{C}{\cdots}\text{X}$ axis.

Kinetics. Because both the substrate and the nucleophile are involved in the single rate-determining step, the reaction is second order overall, first order in each: $\text{rate} = k[\text{RX}][\text{Nu}^-]$. Doubling the nucleophile concentration doubles the rate.

Stereochemistry -- Walden inversion. Because the nucleophile must approach from the face opposite the leaving group, the three substituents that are not directly involved flip through to the other side as the reaction proceeds, exactly as an umbrella turns inside out in a strong gust. The net result is that the product's spatial arrangement is the mirror image of the starting material's at that carbon -- complete inversion of configuration, known as Walden inversion. If the leaving group and the incoming nucleophile occupy the same CIP priority rank (as $\text{Br}$ and $\text{OH}$ typically do, both outranking the alkyl groups and hydrogen), this spatial inversion is also seen as a flip of the $R/S$ descriptor: an $(R)$ substrate reacting cleanly by $S_N2$ gives the $(S)$ product, and vice versa. …