Question

Which one of the carbocations from the following is most stable?

• A.
• B.
• C.
• D.

Hint:

Carbocations are stabilized by resonance and electron-donating groups, while electron-withdrawing groups tend to destabilize them. Look for conjugated systems or groups that can delocalize the positive charge to predict carbocation stability.

Answer 1

The Correct Option is B

The stability of a carbocation is influenced by the following factors: 
- Inductive Effect: Electron-withdrawing groups stabilize a carbocation, while electron-donating groups destabilize it. 
- Resonance Effect: A carbocation can be stabilized by resonance if the positive charge can be delocalized. Let’s analyze each carbocation: 
1. Option (1): \( \,^{+}CH_2-CH_2-O-CH_3 \) This carbocation is a primary carbocation, and the oxygen atom attached to the molecule is an electron-withdrawing group. However, there is no resonance stabilization available, making this carbocation less stable compared to others. 
2. Option (2): \( \,^{+}CH_2-CH=O-CH_3 \) This carbocation has the possibility of resonance stabilization. The positive charge can be delocalized through the conjugation with the carbonyl group, making this carbocation the most stable among the four. 
3. Option (3): \( \,^{+}CH_2-CH_2-COOCH_3 \) While the ester group is electron-withdrawing, this carbocation doesn’t have as much resonance stabilization as the one in option (2), making it less stable. 
4. Option (4): \( \,^{+}CH_2-CH_2-F \) The fluorine atom is highly electronegative and is an electron-withdrawing group. This carbocation is a primary carbocation with no resonance stabilization, which makes it less stable than the one in option (2). 
Thus, Option (2) is the most stable carbocation due to resonance stabilization with the carbonyl group.