Organic Chemistry MCAT Practice Exam

Which of the following statements is true regarding the stability of carbocations?

1. More substituted carbocations are less stable

2. Less substituted carbocations are more stable

3. More substituted carbocations are more stable

4. All carbocations are equally stable

The correct answer is: More substituted carbocations are more stable

The correct answer is that more substituted carbocations are more stable. The stability of carbocations is primarily influenced by the presence of alkyl groups attached to the positively charged carbon atom. When a carbocation forms, it has an empty p-orbital that is prone to instability due to the positive charge. Alkyl groups, through hyperconjugation and inductive effects, help to stabilize the positive charge. Hyperconjugation occurs when a filled orbital (such as a C-H or C-C bond) donates electron density into the empty p-orbital of the carbocation. This electron donation reduces the positive character of the carbon and stabilizes the carbocation. Additionally, the inductive effect from nearby alkyl groups can help to disperse the positive charge away from the carbocation center, leading to greater stability. Therefore, as the number of substituents on the carbocation increases (i.e., more alkyl groups are attached), the overall stability of the carbocation also increases. Tertiary carbocations, which have three alkyl groups attached, are significantly more stable than secondary or primary carbocations for this reason. In contrast, less substituted carbocations, which have fewer alkyl groups, cannot benefit from these stabilizing