An S_N2 reaction of an alkyl halide is a single-step process in which bond formation between the nucleophile and the substrate and bond breaking between the substrate and the halide occurs simultaneously through a transition state without forming an intermediate.

When the nucleophile approaches the electrophilic carbon with its lone pairs, the halide acts as a leaving group and moves away with the electron-pair bonded to the carbon. Dotted partial bonds represent the bonds being formed or broken in the transition state to depict this mechanism, and the structure is enclosed within square brackets.

The transition state is highly unstable and reacts quickly to reach the product state, which is energetically more favored. The geometry of the transition state is trigonal bipyramidal with reduced bond angles. This results in steric crowding that leads to van der Waals repulsion and an increase in the activation energy to form the transition state. This ultimately influences the rate of the reaction. Hence, the higher the energy of activation, the slower is the rate of reaction.

Substitution on alkyl halides increases steric hindrance leading to less access for a nucleophilic attack. It also increases the crowding and the energy of the transition state. Thus, the order of reactivity for an alkyl halide undergoing S_N2 reactions is as follows:

Methyl halide (highly reactive) > primary halide > secondary halide > β-substituted halide > tertiary halide (practically unreactive).