Enols, Enolates and Alpha Substitution
A carbonyl compound has a second reactive site, the carbon next to the group, which becomes nucleophilic when a proton is removed. That flip from electrophile to nucleophile is what lets carbonyl compounds react with each other.
What a learner can do afterwards
- Explains alpha acidity from delocalisation of the resulting anion onto oxygen
- Draws keto and enol forms and predicts which dominates for a simple ketone and for a diketone
- Chooses between a base that converts nearly all the ketone to enolate and one that sets up a small equilibrium, with the consequence for the product
- Predicts the site of alkylation or halogenation for an unsymmetrical ketone
1 · Read
A carbonyl compound has a second reactive site, the alpha carbon next to the group. Its hydrogens are far more acidic than ordinary C-H bonds because the resulting anion shares its charge onto oxygen through delocalisation. Pull one off with base and you hold an enolate, an anion with its charge spread over carbon and oxygen.
A ketone in a bottle is always shifting a little into its enol form, with the alpha hydrogen moved to oxygen and a double bond beside it. For a simple ketone the keto form wins by a huge margin. A 1,3-diketone holds far more enol: its enol conjugates with the second carbonyl and hydrogen-bonds internally.
The choice of base matters enormously. Compare the ketone with the conjugate acid of the base: the gap between those acidities drives deprotonation. A far larger conjugate-acid pKa converts nearly all the ketone to enolate. Near-equal acidities leave sizable amounts of both sides, not a tiny enolate sliver. Bulk alone is not enough, since a hindered but weak base still leaves most ketone untouched.
An unsymmetrical ketone offers two different alpha positions. A bulky strong base grabs the more exposed hydrogen under fast conditions, while a small base with time to equilibrate favours the more substituted enolate. Match the base to the site you want.
Delocalisation makes the alpha proton acidic, and the base you pick decides how much enolate you hold and where.
2 · Watch
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Where it sits
Learn first
Nothing; this is a starting point.
Optional connections
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.