Nucleophilic Addition to Carbonyls: Reactivity and Selectivity
Why aldehydes react faster than ketones, and why the same reasoning orders every carbonyl derivative.
Why the carbonyl carbon is electrophilic
The carbon–oxygen double bond is strongly polarised toward oxygen, leaving partial positive charge on carbon. A nucleophile attacks there, the pi electrons move onto oxygen, and a tetrahedral alkoxide intermediate forms which is then protonated.
The carbon changes from trigonal planar to tetrahedral in the process, which is why steric bulk around it matters so much.
Aldehydes versus ketones
| Factor | Aldehyde | Ketone |
|---|---|---|
| Alkyl groups on the carbonyl carbon | One | Two |
| Electron donation to the carbon | Less | More — reduces electrophilicity |
| Steric hindrance to attack | Less | More |
| Relative reactivity | Higher | Lower |
The wider reactivity order
Extending the same reasoning to derivatives gives the standard order:
The pattern follows from how much the substituent donates into the carbonyl. Chlorine donates poorly, so the carbon stays strongly electrophilic. Nitrogen donates strongly, so an amide is much less reactive. A carboxylate is least reactive of all, being already negatively charged.
Being asked to order a set of derivatives is a standard question, and reasoning from donation rather than recalling the list means an unfamiliar derivative can still be placed.
Catalysis
| Acid catalysis | Base catalysis | |
|---|---|---|
| What it does | Protonates the carbonyl oxygen | Deprotonates the nucleophile |
| Effect | Makes the carbon more electrophilic | Makes the nucleophile more reactive |
| Suits | Weak nucleophiles | Nucleophiles with an acidic proton |
The two act on opposite partners, which is why the choice depends on which one is limiting. A weak neutral nucleophile benefits from acid; a nucleophile that becomes far more reactive on deprotonation benefits from base.
Reactions with amines show an optimum pH for exactly this reason: too acidic and the amine is protonated and unreactive; too basic and the carbonyl is not activated. Explaining that optimum is a good question.
Reversibility
Many carbonyl additions are reversible, and the position of equilibrium depends on the stability of the product relative to the starting materials. Hydrate formation, for instance, is unfavourable for most ketones but favourable where strong electron withdrawal makes the carbonyl unusually electrophilic.
Where an addition is reversible, driving it forward requires removing the product — typically by dehydration to give a species that cannot revert. That is why several classical carbonyl reactions end in an elimination step.
Frequently asked questions
Why are aldehydes more reactive than ketones?
Because they have one fewer electron-donating alkyl group, so the carbonyl carbon is more electrophilic, and less steric bulk obstructing attack. Both factors act in the same direction.
Why is an amide so much less reactive than an ester?
Because nitrogen donates its lone pair into the carbonyl far more effectively than oxygen does, reducing the electrophilicity of the carbon substantially.
Why is there an optimum pH for reactions with amines?
Because acid activates the carbonyl but also protonates the amine. The optimum balances activation against loss of the nucleophile.
Why do some carbonyl additions need a dehydration step?
Because the addition itself is reversible. Eliminating water gives a product that cannot revert, driving the reaction to completion.
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