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 deprotonates completely 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 diketone with a second carbonyl to share the load holds far more enol.
The choice of base matters enormously. A strong hindered base drives deprotonation to completion, banking the full enolate. A weaker base sets up only a small equilibrium concentration, and each choice leads to different products.
An unsymmetrical ketone offers two different alpha positions. A bulky base grabs the more exposed hydrogen, 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
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.