Final answer:
Allyl or benzyl groups can decrease the reaction rate in an SN2 reaction due to steric hindrance.
Step-by-step explanation:
In an SN2 reaction, the presence of allyl or benzyl groups on the substrate can decrease the reaction rate. This is because these groups increase steric hindrance, making it difficult for the nucleophile to approach the electrophilic carbon atom. This decrease in reactivity is due to the bulky nature of the allyl or benzyl groups and the resulting hindrance to the substitution process.
For example, if we compare the reaction rates of a simple primary alkyl halide (no allyl or benzyl groups) and the same alkyl halide with an allyl or benzyl group attached, we would expect the reaction rate to be slower in the latter case due to the added steric hindrance.