Intermediate Topic 764 of 1102 Organic Chemistry
Carbocations
Positively charged carbon species whose stability is tertiary above secondary above primary.
In-depth explanation of Carbocations
A carbocation is a species in which a carbon atom bears a positive charge and has only three bonds, leaving an empty p orbital. The cation carbon is sp2 hybridized and planar, and the stability order is tertiary greater than secondary, greater than primary, greater than methyl. Stability arises from hyperconjugation with adjacent C-H bonds and inductive donation from alkyl groups, while benzylic and allylic cations gain extra stability from resonance. Carbocations are reactive intermediates that can rearrange through hydride or alkyl shifts to more stable forms.
Going deeper: Carbocations
Key ideas: Trivalent carbon with a positive charge, sp2 hybridised and planar with an empty p orbital. Stability: allyl or benzyl > 3 > 2 > 1 > methyl, owing to hyperconjugation, induction and resonance. They rearrange by 1,2-hydride or alkyl shift to give a more stable cation. Formed in SN1, E1, electrophilic addition and Friedel-Crafts alkylation, and attacked by nucleophiles from either face.
Common mistakes and connections: A common mistake is ignoring rearrangement, for instance in 3-methyl-2-butanol with HBr. Another is drawing a primary carbocation as a likely intermediate, or confusing carbocation (three bonds) with carbonium ion (five coordinate). Links to SN1, Wagner-Meerwein shifts and the pinacol rearrangement.
Visualization of Carbocations
Gas-phase hydride ion affinity falls from methyl to tert-butyl: more substituted carbocations are more stable and easier to form. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The tert-butyl cation, (CH3)3C+, is a classic example of a relatively stable carbocation.
Frequently asked questions about Carbocations
What is Carbocations?
Positively charged carbon species whose stability is tertiary above secondary above primary.
Is Carbocations a beginner, intermediate or advanced topic?
Carbocations is a intermediate topic in Organic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Carbocations?
Study these earlier topics in Organic Chemistry first: Hyperconjugation, Electromeric Effect, Free Radicals.
What comes after Carbocations?
The next topics in Organic Chemistry are Carbanions and Carbenes.
Can I practise Carbocations with quizzes?
Yes. The interactive lesson includes a self-check question on Carbocations, and the Organic Chemistry quiz draws on every topic in this area.
Where Carbocations fits in Organic Chemistry
Carbocations is topic 36 of 148 in Organic Chemistry.