Chapter 8 · Chemistry
Nucleophilic Addition Reactions: One Carbonyl, Five Pathways
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Every nucleophilic addition reaction of aldehydes and ketones in one place: HCN, NaHSO₃, Grignard reagents, alcohols, and the addition–elimination of ammonia derivatives, with the product each one gives.
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Reducing and Oxidising Aldehydes and KetonesReduction is easy for both aldehydes and ketones, but in oxidation the aldehyde reacts at once and the ketone resists. The reason is one H atom.Chemistry · simulation
Key points
- HCN (with a base, so CN⁻ attacks) → cyanohydrin: CN and OH on the same carbon, e.g. CH₃CH(OH)CN.
- NaHSO₃ (HSO₃⁻ attacks through S) → bisulphite addition compound, a crystalline solid. Dilute acid or alkali gives the aldehyde back, so it separates and purifies aldehydes.
- Grignard reagent R–MgX in dry ether, then H₃O⁺ → alcohol with a new C–C bond: HCHO gives 1°, other aldehydes 2°, ketones 3°.
- Alcohols, dry HCl: one ROH → hemiacetal (OH + OR); a second ROH → acetal (OR + OR) + H₂O; ketone + ethylene glycol → cyclic ketal.
- Ammonia derivatives H₂N–Z (acid): addition, then loss of H₂O → C=N–Z (nucleophilic addition–elimination): oxime, hydrazone, phenylhydrazone, semicarbazone, 2,4-DNP hydrazone.
The five pathways
| Reagent (condition) | What attacks the C=O carbon | Product (from CH₃CHO) |
|---|---|---|
| HCN (base) | CN⁻, through C | CH₃CH(OH)CN, a cyanohydrin |
| NaHSO₃ | HSO₃⁻, through S | CH₃CH(OH)SO₃⁻Na⁺, the bisulphite addition compound |
| CH₃MgBr (dry ether), then H₃O⁺ | the CH₃ group (new C–C bond) | CH₃CH(OH)CH₃, propan-2-ol (2°) |
| CH₃OH (1 mol), dry HCl | O of CH₃OH | CH₃CH(OH)OCH₃, a hemiacetal |
| CH₃OH (2 mol), dry HCl | O of CH₃OH, twice; H₂O leaves | CH₃CH(OCH₃)₂, an acetal |
| H₂N–Z (acid) | N of the NH₂; then H₂O leaves | CH₃CH=N–Z |
Pure HCN reacts very slowly; a base turns it into CN⁻, the real nucleophile. For NaHSO₃ the equilibrium lies to the right for most aldehydes and to the left for most ketones, because the two alkyl groups of a ketone crowd the carbon.
Grignard reagents: which alcohol?
| Carbonyl compound | + CH₃MgBr, then H₃O⁺ | Alcohol |
|---|---|---|
| HCHO | CH₃CH₂OH | 1° (ethanol) |
| CH₃CHO | CH₃CH(OH)CH₃ | 2° (propan-2-ol) |
| CH₃COCH₃ | (CH₃)₃COH | 3° (2-methylpropan-2-ol) |
Count the carbon groups on the carbon that carries the OH: one, two or three.
Ammonia derivatives: C=O becomes C=N–Z
| Reagent H₂N–Z | Product |
|---|---|
| NH₂OH, hydroxylamine | oxime, >C=N–OH |
| NH₂NH₂, hydrazine | hydrazone, >C=N–NH₂ |
| C₆H₅NHNH₂, phenylhydrazine | phenylhydrazone, >C=N–NHC₆H₅ |
| NH₂NHCONH₂, semicarbazide | semicarbazone, >C=N–NHCONH₂ |
| 2,4-dinitrophenylhydrazine (2,4-DNP) | 2,4-DNP hydrazone, a yellow-orange precipitate |
The reaction is acid catalysed and reversible; rapid loss of water from the addition product drives it to >C=N–Z. Ammonia or a primary amine gives an imine (a Schiff base with an amine). The mechanism of the first step is on the page for the nucleophilic addition reel.
Try it yourself
Goal: Pick a carbonyl compound and a reagent, predict the product type, then react: the page builds the product, names the attacking species and says whether it is an addition or an addition–elimination.
- Pick a carbonyl compound and see how open and δ+ its carbon is.
- Pick a reagent from one of the five chambers.
- Predict the product type.
- Press React: compare your prediction with the product the chamber builds.
- Try the same reagent with a ketone and with formaldehyde.
Quick quiz
5 questions. Choose an answer for each, then submit to see your score and the explanations.
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