6.9:
SN2 Reaction: Transition State
An SN2 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 SN2 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 SN2 mechanism.
Additionally, increased β-alkyl substitution of a primary halide increases steric repulsion, making the molecule incapable of undergoing an SN2 reaction.
6.9:
SN2 Reaction: Transition State
An SN2 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 SN2 reactions is as follows:
Methyl halide (highly reactive) > primary halide > secondary halide > β-substituted halide > tertiary halide (practically unreactive).