Organic Chemistry MCAT Practice Exam 2025 – The All-in-One Guide to Master Your Test Preparation!

In an SN2 reaction, the mechanism involves a nucleophile attacking the substrate at the same time that the leaving group departs. This means that the nucleophile must approach and collide with the substrate's electrophilic carbon in a backside attack.

When the steric hindrance of the substrate increases, the bulky groups around the carbon atom make it more difficult for the nucleophile to effectively approach and attack. Steric hindrance refers to the physical obstruction caused by the presence of multiple atoms or groups around a reactive center. High steric hindrance around the electrophilic carbon atom can significantly slow down the reaction because the nucleophile has less access to the carbon due to the blocks presented by the surrounding substituents.

As a result, when steric hindrance increases, the reaction rate decreases because the nucleophile competes with the bulky groups for space around the reaction site. This steric effect is a critical characteristic of SN2 reactions, which are highly sensitive to steric factors, unlike SN1 reactions that proceed via a carbocation intermediate and are less influenced by steric hindrance.

Thus, in the context of this question, increasing the steric hindrance of the substrate indeed results in a decrease in the reaction rate.