An S_N2 reaction follows a concerted mechanism during which a nucleophile attacks the substrate from the back-side, with simultaneous loss of the leaving group from the side opposite to it.

This occurs through a transition state, which is represented within square brackets. The structure includes an electrophilic carbon attached to five groups: three substituents, one nucleophile, and one leaving group. The nucleophile and the leaving group are partially bonded, and both groups carry a partial negative charge. 

Since the transition state is not an intermediate, it cannot be isolated. It has a trigonal bipyramidal geometry with the three substituents in a planar arrangement. The nucleophile and the leaving group are placed 180° apart, perpendicular to the plane.

The substituents are at 90°angles from both the nucleophile and the leaving group. This increases crowding, leading to an unstable structure at the highest energy on the reaction pathway.

The energy of the transition state affects the rate of an S_N2 reaction. The higher the transition state energy, the larger the activation energy, and hence the lower the reaction rate.

Reaction rates are also influenced by steric hindrance, meaning the ease with which the nucleophile attacks the electrophilic center.

When alkyl substitution on an alpha-carbon increases, steric hindrance and, therefore, van der Waals repulsion with the incoming nucleophile increases.

For instance, a methyl halide and any primary halide present the least steric repulsion, and the reaction proceeds faster.

In comparison, a secondary halide with two bulky groups leads to an increased steric repulsion, which consequently slows down the reaction.

In a tertiary halide, the three bulky groups impede the nucleophilic attack, making the substrate practically nonreactive by an S_N2 mechanism.

Additionally, increased β-alkyl substitution of a primary halide increases steric repulsion, making the molecule incapable of undergoing an S_N2 reaction.