Organic Chemistry MCAT Practice Exam

Which group stabilizes carbocations?

1. Electron withdrawing groups

2. Electron donating groups

3. Hydrophobic groups

4. None of the above

The correct answer is: Electron donating groups

Electron donating groups stabilize carbocations through their ability to donate electron density to the positively charged carbon atom. A carbocation, which has a positive charge, is electron-deficient and thus seeks to stabilize that charge with electron-rich groups nearby. Donating groups, such as alkyl groups and certain other substituents, can provide this electron density via inductive effects or hyperconjugation. Inductive effects arise when electron-withdrawing groups pull electron density away from the carbocation, making it less stable. In contrast, electron donating groups push electron density toward the carbocation, effectively lowering its energy and increasing stability. Hyperconjugation occurs when adjacent C–H or C–C bonds can overlap with the empty p orbital of the carbocation, further contributing to stabilization. Overall, the presence of electron donating groups creates a more favorable environment for the carbocation, allowing it to exist longer before undergoing further reactions such as rearrangement or nucleophilic attack. This stabilizing effect is fundamental in understanding reaction mechanisms involving carbocations in organic chemistry.