Organic Chemistry MCAT Practice Exam

Which statement about SN1 reactions is true?

1. They occur at primary carbons only

2. They avoid polar protic solvents

3. They involve carbocation formation

4. They are concerned with nucleophile concentration in rate law

The correct answer is: They involve carbocation formation

In SN1 reactions, the formation of a carbocation is a key step in the mechanism. The reaction proceeds through an initial step where the leaving group departs, resulting in the generation of a carbocation intermediate. This carbocation is typically stabilized by nearby alkyl groups or other inductive effects, which influence the stability of the intermediate and the favorability of the reaction. The more stable the carbocation, the more likely the SN1 reaction will occur. This mechanism is characteristic of tertiary or sometimes secondary substrates, as they can stabilize the positive charge of the carbocation more effectively compared to primary substrates, which tend to favor SN2 mechanisms. The stability of the carbocation plays a crucial role in determining the rate of the reaction, which is first-order with respect to the substrate and does not depend on the concentration of the nucleophile. In contrast, the other options either misrepresent the characteristics of SN1 reactions or mistakenly attribute them to different reaction types. For instance, SN1 reactions can occur at secondary and tertiary carbons, rather than being restricted to primary carbons. Additionally, they are more effectively facilitated in polar protic solvents, which help stabilize the carbocation and the leaving group. Regarding the rate law, the concentration of the nucle