The carbonyl group
Aldehydes and ketones contain the carbonyl group, C=O.
In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen: R–CHO (ethanal, CH₃CHO).
In a ketone, it is bonded to two carbon groups: R–CO–R′ (propanone, CH₃COCH₃).
In a carboxylic acid, it carries an –OH group: R–COOH (ethanoic acid, CH₃COOH).
The carbonyl carbon is sp² hybridised and planar. Oxygen is more electronegative, so the C=O bond is polar: the carbon is electrophilic (attacked by nucleophiles) and the oxygen is nucleophilic.
Boiling points of aldehydes and ketones are higher than those of hydrocarbons and ethers (dipole-dipole attraction) but lower than those of alcohols (no hydrogen bonding with each other). Lower members are water-soluble.
Preparing aldehydes and ketones
Oxidation of alcohols: primary → aldehydes (using PCC, so it stops there); secondary → ketones.
Dehydrogenation of alcohol vapour over copper at 573 K.
Ozonolysis of alkenes.
Hydration of alkynes: ethyne gives ethanal; other alkynes give ketones (with HgSO₄ and dilute H₂SO₄).
Rosenmund reduction: acyl chloride + H₂ over Pd on BaSO₄ (poisoned catalyst) gives an aldehyde.
Stephen reaction: a nitrile reduced with SnCl₂/HCl, then hydrolysed, gives an aldehyde.
Etard reaction: toluene + chromyl chloride (CrO₂Cl₂) gives benzaldehyde.
Friedel-Crafts acylation: benzene + acyl chloride with anhydrous AlCl₃ gives an aromatic ketone.
Grignard reagents with nitriles, then hydrolysis, give ketones.
Nucleophilic addition
A nucleophile attacks the electrophilic carbonyl carbon, and the carbon changes from sp² (planar) to sp³ (tetrahedral).
Aldehydes are more reactive than ketones, for two reasons:
Steric: two bulky groups in a ketone hinder the approach of the nucleophile.
Electronic: two alkyl groups push electrons towards the carbonyl carbon, making it less positive.
Reagent | Product |
|---|---|
HCN | cyanohydrins |
NaHSO₃ | crystalline bisulphite addition products (used to separate and purify aldehydes) |
Grignard reagent, then water | alcohols |
Alcohols (with dry HCl) | hemiacetals and acetals (ketones give ketals) |
Ammonia derivatives (H₂N–Z) | addition followed by elimination of water: oximes (with hydroxylamine), hydrazones, phenylhydrazones, and 2,4-DNP derivatives (orange-yellow precipitates, used to identify carbonyl compounds), semicarbazones |
Reduction
To alcohols with NaBH₄ or LiAlH₄.
To hydrocarbons (–CH₂–) by the Clemmensen reduction (zinc amalgam + conc. HCl) or the Wolff-Kishner reduction (hydrazine, then KOH in ethylene glycol on heating).
Telling aldehydes from ketones
Aldehydes are easily oxidised to carboxylic acids; ketones are not (under mild conditions). Mild oxidising agents distinguish them:
Test | Reagent | Aldehydes | Ketones |
|---|---|---|---|
Tollens' test | ammoniacal silver nitrate | silver mirror forms | no change |
Fehling's test | Fehling's solutions A + B (copper(II) complex) | red-brown precipitate of Cu₂O (aromatic aldehydes don't respond) | no change |
Iodoform test: compounds with a CH₃CO– group (methyl ketones, ethanal) or alcohols that can be oxidised to them (ethanol, propan-2-ol) give a yellow precipitate of iodoform (CHI₃) with NaOH and iodine.
Reactions involving α-hydrogens
Hydrogens on the carbon next to the carbonyl (α-hydrogens) are acidic, because the resulting anion is stabilised by resonance.
Aldol condensation: aldehydes and ketones with α-hydrogens, in dilute alkali, combine to form a β-hydroxy aldehyde (aldol) or β-hydroxy ketone (ketol), which loses water on heating to give an α,β-unsaturated compound.
2CH₃CHO → (dil. NaOH) → CH₃CH(OH)CH₂CHO → (heat) → CH₃CH=CHCHO (but-2-enal)
A reaction between two different carbonyl compounds is a cross aldol condensation.
Cannizzaro reaction: aldehydes without α-hydrogens (methanal, benzaldehyde) undergo self oxidation-reduction in concentrated alkali: one molecule is reduced to an alcohol and another oxidised to a carboxylic acid salt.
2HCHO + conc. KOH → CH₃OH + HCOOK
Aromatic aldehydes and ketones undergo electrophilic substitution at the meta position, as the carbonyl group is deactivating.
Carboxylic acids
Preparation:
Oxidation of primary alcohols and aldehydes.
Oxidation of alkylbenzenes with KMnO₄: the whole side chain becomes –COOH, giving benzoic acid.
Hydrolysis of nitriles, amides, esters and acid anhydrides.
Grignard reagent + dry ice (CO₂), then acid.
Physical properties: carboxylic acids have higher boiling points than alcohols of similar mass, because they form dimers held by two hydrogen bonds, even in the vapour.
Acidity. Carboxylic acids are weak acids but much stronger than alcohols and phenols. The carboxylate ion is stabilised by resonance, with the negative charge spread equally over two oxygen atoms. They react with NaHCO₃ to release CO₂, which phenols don't do, a useful test.
Electron-withdrawing groups increase acidity: CF₃COOH > CCl₃COOH > CHCl₂COOH > ClCH₂COOH > CH₃COOH. The closer the group to –COOH, the bigger the effect.
Electron-donating groups decrease acidity.
Formic acid (HCOOH) is stronger than acetic acid.
Reactions:
Esterification: with alcohols and an acid catalyst, giving esters (fruity smell).
With PCl₅, PCl₃ or SOCl₂: acid chlorides.
With NH₃: ammonium salts, which on heating give amides.
Reduction with LiAlH₄: primary alcohols.
Decarboxylation: sodium salts with soda lime give alkanes with one carbon less.
Kolbe electrolysis of sodium/potassium carboxylate solutions gives alkanes with twice the alkyl groups.
Hell-Volhard-Zelinsky (HVZ) reaction: acids with α-hydrogens react with Cl₂ or Br₂ and red phosphorus to give α-halo acids.
Aromatic acids undergo electrophilic substitution at the meta position, but do not undergo Friedel-Crafts reactions (the –COOH group deactivates the ring and binds the AlCl₃ catalyst).
Summary
The polar C=O is attacked by nucleophiles; aldehydes react faster than ketones.
Tollens' and Fehling's tests detect aldehydes; the iodoform test detects CH₃CO– groups.
Aldol condensation needs α-hydrogens and dilute alkali; Cannizzaro needs no α-hydrogens and concentrated alkali.
Carboxylic acids are acidic due to resonance in carboxylate; electron-withdrawing groups strengthen them; HVZ gives α-halo acids.