What a learner can do afterwards
- Ranks carbocations by stability and gives both the inductive and the hyperconjugative reason
- Predicts a hydride or methyl shift and draws the product it leads to
- Recognises an unexpected product as the fingerprint of a rearrangement
- Explains how a neighbouring group changes the rate and the stereochemistry of a substitution
1 · Read
A carbocation is a true intermediate with a finite lifetime. Rank them tertiary above secondary above primary above methyl, since that order decides which substitution route from the last stop can even run. Alkyl groups push electron density toward the positive centre through sigma bonds, which calms the shortage. Each neighbouring C-H bond also helps through hyperconjugation, where its electrons overlap with the empty p orbital and spread the charge.
A cation does not sit still. When a stabler spot sits next door, a hydrogen on the neighbouring carbon migrates with its bonding pair to the cationic centre. That is a hydride shift. A methyl group can migrate the same way. Either move parks the charge on a more substituted carbon and builds a new skeleton.
A neopentyl halide ionizes to a primary cation, yet the product looks rearranged. A methyl shift moves the charge to a tertiary centre, and the nucleophile captures that instead. Whenever you meet an unexpected skeleton, read it as the fingerprint of a rearrangement. Direct attack would have kept the old shape.
Watch next door. A lone pair or pi bond beside the centre can reach over as the leaving group departs. That neighbouring group speeds ionization and shields one face, so the nucleophile must attack from the back. Both the rate and the stereochemistry change, which is how you spot its hand.
Cations rank by donation and hyperconjugation, shift toward stabler carbons, and bend to neighbours that reach in.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.