A carbocation is an organic ion in which a carbon atom carries a positive charge. Carbocations are important because they appear as reactive intermediates in many reactions, including SN1 substitutions, E1 eliminations, and additions to alkenes. Their stability strongly affects reaction rate, product distribution, and whether rearrangements occur.
In general, more substituted carbocations are more stable because nearby carbon groups help spread out the positive charge.
The main stability order is tertiary greater than secondary greater than primary greater than methyl. Alkyl groups stabilize a carbocation through induction, which pushes electron density through sigma bonds, and hyperconjugation, which overlaps nearby C-H or C-C sigma bonds with the empty p orbital on the charged carbon. Carbocations can also become more stable by rearranging through hydride shifts or alkyl shifts.
Recognizing these effects helps predict the most likely intermediate and the major product of a reaction.
Key Facts
- Carbocation stability order: 3° > 2° > 1° > methyl.
- A carbocation carbon is usually sp2 hybridized, trigonal planar, and has an empty p orbital.
- Hyperconjugation stabilizes a carbocation when adjacent sigma bonds overlap with the empty p orbital.
- Inductive donation from alkyl groups helps reduce electron deficiency at the positively charged carbon.
- More adjacent alkyl groups usually means more hyperconjugation and greater carbocation stability.
- Carbocation rearrangements often form a more stable carbocation, such as 1° to 2° or 2° to 3°.
Vocabulary
- Carbocation
- A carbocation is an organic species in which a carbon atom has a positive formal charge and only six valence electrons.
- Tertiary carbocation
- A tertiary carbocation is a positively charged carbon bonded to three other carbon atoms.
- Hyperconjugation
- Hyperconjugation is stabilization caused by overlap between a nearby sigma bond and an empty p orbital.
- Inductive effect
- The inductive effect is the shift of electron density through sigma bonds due to differences in electron donating or withdrawing ability.
- Rearrangement
- A rearrangement is a structural shift, such as a hydride or alkyl shift, that forms a more stable carbocation.
Common Mistakes to Avoid
- Ranking carbocations by the number of hydrogens on the positive carbon, which is wrong because stability mainly depends on electron donation from neighboring groups.
- Forgetting the empty p orbital, which is wrong because hyperconjugation requires overlap with that orbital to stabilize the carbocation.
- Assuming primary carbocations always stay primary, which is wrong because they often rearrange if a hydride or alkyl shift can form a more stable cation.
- Treating all positively charged carbons as equally reactive, which is wrong because a more stable carbocation usually has lower energy and forms more easily.
Practice Questions
- 1 Rank these carbocations from most stable to least stable: (CH3)3C+, CH3CH2+, (CH3)2CH+, CH3+.
- 2 A secondary carbocation has 6 adjacent C-H sigma bonds that can hyperconjugate, while a tertiary carbocation has 9. Which is more stable, and by how many additional hyperconjugating C-H bonds?
- 3 In a reaction mechanism, a secondary carbocation forms next to a carbon that bears a hydrogen and can shift to produce a tertiary carbocation. Explain why a hydride shift is likely.