Aldehydes & Ketones

Inside the wall of a vacuum (thermos) flask there is a thin, shining layer of metallic silver. It was put there by the very reaction you meet in this chapter — an aldehyde reducing Tollens' reagent (ammoniacal silver nitrate) and depositing pure silver on the glass.

• Tollens' test — warm an unknown with [Ag(NH₃)₂]⁺. An aldehyde gives a bright silver mirror; a ketone gives nothing. It cleanly separates the two families.

The aldehyde is oxidised to a carboxylate while the silver(I) is reduced to the metal. Watch the flask silver itself in the panel →

Unscrew a bottle of nail-polish remover and the sharp, clean solvent smell that rushes out is propanone (acetone) — the simplest and most familiar ketone. A ketone has its carbonyl carbon bonded to two carbon groups, so the C=O sits in the middle of the chain: R–CO–R′.

• Carbonyl group >C=O — sp², planar, and polar: oxygen is δ−, carbon is δ+. Aldehyde = R–CHO (C=O at the end); ketone = R–CO–R′ (C=O in the middle).

Aldehydes take the ending -al; ketones take -one. Acetone is a brilliant solvent for paints, varnishes and plastics — which is exactly why it strips nail polish.

On the shelf of every biology lab stands a glass jar with a specimen suspended inside it. The clear liquid is formalin — a 40% aqueous solution of methanal (formaldehyde), the simplest aldehyde, R = H. It cross-links proteins, so it preserves tissue and is also used to make Bakelite resins.

How do we make these carbonyls? The main route is to oxidise an alcohol:

• 1° alcohol → aldehyde (mild, controlled oxidation; distil it off before it over-oxidises to an acid).
• 2° alcohol → ketone (the ketone resists further oxidation, so it is the final product).
• 3° alcohols are not oxidised — there is no H on the C–OH carbon.

Think of the carbonyl carbon as a magnet. Because oxygen hogs the electrons, the carbon is left δ+ (electron-poor) — and like a magnet it attracts an electron-rich nucleophile (Nu⁻). The nucleophile snaps onto the carbon, the π-electrons fold up onto the oxygen as an alkoxide O⁻, and that then grabs an H⁺.

• Nucleophilic addition — Nu⁻ adds to the δ+ carbon (HCN → cyanohydrin, NaHSO₃, Grignard, alcohols → acetals). Aldehydes react faster than ketones — a ketone's two alkyl groups both donate electrons (less δ+) and crowd the carbon.

In a hospital lab, a drop of urine or blood is warmed with a deep-blue reagent. If it turns brick-red, the patient's blood sugar is high — because glucose carries an aldehyde group. This is Benedict's / Fehling's test, and it works for the same reason Tollens' does: an aldehyde is oxidised easily, a ketone resists.

The blue copper(II) is reduced to a brick-red precipitate of copper(I) oxide, Cu₂O. A ketone leaves the solution blue.

When a chef gently heats sugar until it turns golden-brown caramel, carbonyl molecules are joining together and losing water — an everyday glimpse of the aldol condensation. Any carbonyl with an α-hydrogen (a H on the carbon next to C=O) can do it: a base pulls off that H, and the resulting carbanion adds to a second molecule's carbonyl.

Lift a perfume strip to your nose and many of the notes you smell are carbonyl compounds. Benzaldehyde gives the scent of bitter almonds; cinnamaldehyde is the smell of cinnamon; and the famous "aldehyde" top-notes of classic perfumes are long-chain aldehydes. The carbonyl group is not just a lab curiosity — it is a smell, a flavour and a solvent.

• Benzaldehyde — bitter-almond smell; used in flavourings and perfumes.
• Ethanal — a feedstock for making acetic acid and many chemicals.
• 2,4-DNP (Brady's) — gives an orange precipitate with all carbonyls; the quick test that a C=O group is present, before Tollens'/Fehling's tells aldehyde from ketone.

1. The polar carbonyl >C=O (δ+ C, δ− O); aldehyde R–CHO vs ketone R–CO–R′.
2. Naming (-al / -one); preparation by oxidising 1°/2° alcohols.
3. Nucleophilic addition — the δ+ carbon is the target; aldehydes > ketones.
4. Oxidation: aldehydes easy, ketones resist — the basis of every test.
5. Tests: Tollens' (silver mirror), Fehling's/Benedict's (brick-red), iodoform, 2,4-DNP.
6. Aldol condensation; uses — formalin, acetone, perfume aldehydes.