Which Alkyl Halide Reacts Fastest in SN2 Reaction?

What factors determine the rate of reaction in an SN2 reaction?

A) Electronegativity of the halogen attached

B) Degree of steric hindrance

C) Type of leaving group

D) Molecular weight of the alkyl halide

Answer:

The rate of reaction in an SN2 reaction primarily depends on the degree of steric hindrance encountered by the nucleophile.

In an SN2 reaction, the rate of reaction is determined by how easily a strong nucleophile can access the carbon atom bonded to the leaving group (halogen). This accessibility is influenced by the degree of steric hindrance present in the alkyl halide.

Methyl and primary alkyl halides are more reactive in SN2 reactions because they have the least steric hindrance, allowing the nucleophile to attack the carbon atom from the opposite side of the leaving group. In contrast, secondary and tertiary alkyl halides have higher degrees of steric hindrance and react slower in SN2 reactions. Tertiary alkyl halides typically do not undergo SN2 reactions at all due to the significant steric hindrance.

Additionally, the type of leaving group attached to the carbon atom also plays a role in the rate of reaction. Better leaving groups, such as bromides compared to chlorides, can enhance the reaction rate by stabilizing the transition state.

Therefore, in the context of the given options, propyl bromide reacts fastest towards an SN2 reaction because it is a primary alkyl halide with less steric hindrance and has a better leaving group (bromide) compared to chlorides or secondary halides.