# Carbocation stability

A carbocation is a carbon with three bonds and a positive charge. More alkyl groups on the carbon make it more stable by pushing electron density to it (inductive effect) and by sharing electrons from nearby C–H bonds (hyperconjugation). Order: tertiary > secondary > primary > methyl.

Chemistry (university) · Part 10: Organic reactions and mechanisms · Card 4 of 10 · Last updated 2026-10-19

## Analogy

A wobbly table gets steadier the more legs it has: each alkyl group is another support.

## Example

**Rank stability**

(CH₃)₃C⁺ tertiary: most stable
(CH₃)₂CH⁺ secondary
CH₃CH₂⁺ primary
CH₃⁺ methyl: least stable

## In practice

Carbocation stability explains why SN1 favours tertiary halides and why addition to alkenes follows Markovnikov's rule.

## Practice

### Flashcards

- **Rank carbocation stability.** Tertiary > secondary > primary > methyl.

### Interview questions

**Compare SN1 and SN2 in terms of rate law, substrate, stereochemistry and solvent.**

- SN2: second order, one step, inversion, primary substrates
- SN1: first order, two steps, racemisation, tertiary substrates
- SN2 favours strong nucleophiles, polar aprotic
- SN1 favours polar protic solvents

**Predict the major product of propene with HBr and explain it using carbocation stability.**

- Alkene pi bond attacks H⁺
- Secondary cation forms in preference
- Br⁻ attacks the cation
- Product: 2-bromopropane (Markovnikov)

## Questions people ask

### What is a carbocation rearrangement?

A carbocation can shift a hydrogen or alkyl group from a neighbouring carbon to become more stable, for example turning a secondary cation into a tertiary one. This is why some reactions give a product with a different carbon skeleton than you might expect.

### Why are allylic and benzylic cations extra stable?

In an allylic or benzylic cation, the positive charge sits next to a pi system and spreads over several atoms by resonance. Spreading the charge lowers the energy. These cations can be more stable than a simple secondary one and sometimes rival tertiary cations.

## Related concepts

- [SN2 reaction](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-10/chemistry-university-part-10-concept-2)
- [SN1 reaction](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-10/chemistry-university-part-10-concept-3)
- [Electrophilic addition to alkenes](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-10/chemistry-university-part-10-concept-6)
- [Nucleophiles, electrophiles and curved arrows](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-10/chemistry-university-part-10-concept-1)

Web page: https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-10/chemistry-university-part-10-concept-4
Study this card in the app: https://studyvisually.com/app/learn?category=chemistry-university&card=chemistry-university-part-10-concept-4