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.
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Analogy
A wobbly table gets steadier the more legs it has: each alkyl group is another support.
Example
(CH₃)₃C⁺ tertiary: most stable(CH₃)₂CH⁺ secondaryCH₃CH₂⁺ primaryCH₃⁺ 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.