Revise Aldehydes and ketones for Cambridge 9701 (17.1)

Both aldehydes and ketones feature the carbonyl functional group, C=O. In aldehydes, the carbonyl group is bonded to at least one hydrogen atom, meaning it is always found at the end of a carbon chain. Their names end in '-al', e.g., ethanal. In ketones, the carbonyl group is bonded to two carbon atoms, so it is always found within a carbon chain. Their names end in '-one', e.g., propanone. The geometry around the carbonyl carbon is trigonal planar, with bond angles of approximately 120°. The C=O double bond consists of one sigma (σ) bond and one pi (π) bond.

The key difference in reactivity between aldehydes and ketones is their behaviour with oxidising agents. Aldehydes are easily oxidised to carboxylic acids, while ketones are resistant to oxidation. This difference is exploited in simple chemical tests to distinguish between them. We use mild oxidising agents so that other functional groups are not affected.

General Oxidation of an Aldehyde: RCHO(aq) + [O] $\rightarrow$ RCOOH(aq)

Two common reagents are Tollens' reagent and Fehling's solution. With Tollens' reagent, [Ag(NH₃)₂]⁺, an aldehyde is oxidised, and the Ag⁺ ions are reduced to metallic silver, forming a beautiful silver mirror on the inside of the test tube. With Fehling's solution, a deep blue solution containing Cu²⁺ ions, an aldehyde is oxidised, and the Cu²⁺ ions are reduced to Cu⁺ ions, which form a brick-red precipitate of copper(I) oxide, Cu₂O, upon warming. Ketones give no reaction with either reagent.

The most characteristic reaction of carbonyl compounds is nucleophilic addition. The electron-deficient ($\delta+$) carbonyl carbon is attacked by a nucleophile (an electron-pair donor). In this process, the weak π-bond of the C=O group breaks, and its electrons move onto the electronegative oxygen atom, forming a negatively charged intermediate. This intermediate is then protonated, usually by a water molecule or a weak acid, to form the final product.

Aldehydes and ketones react with hydrogen cyanide (HCN) to form hydroxynitriles (also called cyanohydrins). This reaction is important as it increases the length of the carbon chain by one carbon atom. For safety and to ensure a sufficient concentration of the nucleophile, the reaction is typically carried out using a mixture of sodium or potassium cyanide (NaCN or KCN) and a weak acid like H₂SO₄. The KCN provides the nucleophilic cyanide ion (:CN⁻), and the acid provides the H⁺ for the final protonation step.

Mechanism with propanone: CH₃COCH₃ + HCN $\rightarrow$ (CH₃)₂C(OH)CN

Aldehydes and ketones can be reduced to alcohols. A common reducing agent used in the lab for this purpose is sodium borohydride, NaBH₄. This reagent acts as a source of the hydride ion (:H⁻), which is a powerful nucleophile. The reaction is another example of nucleophilic addition. Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols.

Aldehyde reduction: RCHO + 2[H] $\rightarrow$ RCH₂OH (Primary Alcohol) Ketone reduction: RCOR' + 2[H] $\rightarrow$ RCH(OH)R' (Secondary Alcohol)