Reactions of Aldehydes and Ketones: MDCAT Chemistry notes
Reactions of Aldehydes and Ketones for MDCAT: nucleophilic addition, reduction, Tollens', Fehling's, iodoform and 2,4-DNPH tests, aldol, Cannizzaro.
Nucleophilic addition
The carbonyl carbon is electron-poor, so the characteristic reaction is nucleophilic addition: a nucleophile attacks the carbon, the $\pi$ bond shifts onto oxygen, and the alkoxide formed then picks up $\mathrm{H^+}$.
- HCN (NaCN + acid): $\mathrm{CN^-}$ adds to give a cyanohydrin, e.g. HCHO → $\mathrm{HOCH_2CN}$.
- Sodium bisulphite ($\mathrm{NaHSO_3}$): aldehydes and methyl ketones give crystalline adducts, decomposed back by dilute acid or alkali. Used to separate and purify carbonyl compounds from non-carbonyl ones.
- Grignard reagents: HCHO gives a 1$^\circ$ alcohol, other aldehydes give 2$^\circ$, ketones give 3$^\circ$ alcohols after hydrolysis.
- Alcohols (dry HCl): aldehyde + 2ROH → acetal, $\mathrm{RCH(OR')_2}$, a diether (gem-dialkoxy compound).
- 2,4-DNPH: addition then loss of water gives a yellow-orange precipitate with both aldehydes and ketones.
Reduction
| Reagent | Aldehyde gives | Ketone gives |
|---|---|---|
| $\mathrm{H_2}$/Ni or Pt (catalytic), $\mathrm{NaBH_4}$, $\mathrm{LiAlH_4}$ | 1$^\circ$ alcohol (HCHO → methanol) | 2$^\circ$ alcohol (acetone → 2-propanol) |
| Zn(Hg) + conc. HCl (Clemmensen) | Alkane (C=O → $\mathrm{CH_2}$) | Alkane (C=O → $\mathrm{CH_2}$) |
| $\mathrm{NH_2NH_2}$/KOH (Wolff–Kishner) | Alkane | Alkane |
Oxidation and identification tests
Aldehydes are easily oxidized to carboxylic acids (acidified $\mathrm{K_2Cr_2O_7}$: ethanal → ethanoic acid). Ketones resist mild oxidants. Ketones are made by oxidizing secondary alcohols.
| Test | Reagent | Positive result | Given by |
|---|---|---|---|
| Tollens' (silver mirror) | Ammoniacal $\mathrm{AgNO_3}$ | Silver mirror / black ppt | All aldehydes (methanal, ethanal, butanal…) |
| Fehling's | Alkaline Cu(II) tartrate complex | Blue → brick-red $\mathrm{Cu_2O}$ | Aliphatic aldehydes |
| Benedict's | Alkaline Cu(II) citrate complex | Red $\mathrm{Cu_2O}$ | Aliphatic aldehydes |
| Sodium nitroprusside | $\mathrm{Na_2[Fe(CN)_5NO]}$ + alkali | Red colour | Ketones |
| 2,4-DNPH | 2,4-dinitrophenylhydrazine | Yellow-orange ppt | Aldehydes and ketones |
| Iodoform | $\mathrm{I_2}$ + NaOH or $\mathrm{Na_2CO_3}$ | Yellow crystals of $\mathrm{CHI_3}$ | $\mathrm{CH_3CO-}$ compounds (acetone, acetaldehyde) and $\mathrm{CH_3CH(OH)-}$ alcohols |
Tollens' and Fehling's reagents therefore distinguish aldehydes from ketones. In the iodoform reaction a methyl ketone such as acetone gives $\mathrm{CHI_3}$ and the sodium salt of an acid with one carbon fewer.
Reactions involving the $\alpha$-hydrogen
Aldol condensation
With dilute NaOH, a compound having an $\alpha$-H loses it to form a carbanion (enolate). This attacks the carbonyl carbon of a second molecule to give a $\beta$-hydroxy carbonyl compound, the aldol.
- 2 $\mathrm{CH_3CHO}$ → 3-hydroxybutanal (aldol).
- 2 acetone → 4-hydroxy-4-methyl-2-pentanone (diacetone alcohol).
- On heating, aldols undergo dehydration to $\alpha,\beta$-unsaturated carbonyl compounds.
Cannizzaro reaction
Aldehydes with no $\alpha$-H (HCHO, benzaldehyde) with concentrated (50%) NaOH disproportionate: one molecule is reduced to an alcohol, the other oxidized to a carboxylate salt. 2HCHO → $\mathrm{CH_3OH}$ + HCOONa.
Polymerization and phenol
- With a little conc. $\mathrm{H_2SO_4}$, HCHO trimerizes to trioxane (metaformaldehyde) and $\mathrm{CH_3CHO}$ to paraldehyde.
- Phenol + HCHO first gives an addition product (adduct, hydroxymethylphenol), which condenses further to Bakelite.
Common MDCAT traps
- Silver mirror = Tollens'; red $\mathrm{Cu_2O}$ = Fehling's/Benedict's.
- Fehling's uses tartrate; Benedict's uses citrate.
- 2,4-DNPH is positive for both aldehydes and ketones; it cannot distinguish them.
- Dilute alkali gives aldol; 50% NaOH with no $\alpha$-H gives Cannizzaro.
- $\mathrm{NaBH_4}$/$\mathrm{LiAlH_4}$ are reducing agents; $\mathrm{K_2Cr_2O_7}$ is the oxidizing agent.
Quick revision
- Carbonyl compounds undergo nucleophilic addition.
- Ketone + $\mathrm{NaBH_4}$ → 2$^\circ$ alcohol.
- Clemmensen: Zn(Hg)/conc. HCl.
- Acetals are diethers.