Organic Chemistry: Basic Principles
Carbocation Stability Order
A carbocation is a carbon with a positive charge. It gets more stable as more alkyl groups attach to it, so the order is primary < secondary < tertiary.
A carbocation is short of electrons on one carbon. Alkyl groups such as CH₃ push electron density towards that carbon through the inductive effect, spreading the positive charge out and calming it down.
More alkyl groups means more of this electron push. A tertiary carbocation (three alkyl groups) is the most stable, and a primary one (a single alkyl group) is the least. When the charge can spread by resonance, as in a benzyl or allyl cation, it is stabilised even more.
Where students slip up
Students often judge stability by size or steric crowding, and guess that a bulky primary cation is as stable as a tertiary one. It is not. Stability here is about electron donation, not bulk. Count the alkyl groups attached directly to the positive carbon: more groups, more stable.
Hold onto this
More alkyl groups on the positive carbon, more stable: 3° > 2° > 1°.
Why it helps in the exam: this directly predicts SN1 rate. The faster reaction forms the more stable carbocation, so tertiary halides undergo SN1 fastest. A very frequent reasoning question.
Where this connects
This one ranking quietly decides many reactions: which product forms in dehydration, which halide reacts by SN1, and why rearrangements happen. A less stable carbocation will even shift a hydrogen or a methyl group across to become more stable.
Quick check
Which carbocation is the most stable?
- ACH₃⁺ (methyl)
- BPrimary (1°)
- CSecondary (2°)
- DTertiary (3°)