Describe the reactions of aldehydes and ketones including nucleophilic addition of HCN, reduction, and the use of 2,4-DNPH, Tollens, Fehling and the tri-iodomethane test to identify and distinguish carbonyl compounds

What this dot point is asking

SEAB wants you to describe the reactions of aldehydes and ketones (nucleophilic addition of HCN, reduction), and the use of 2,4-DNPH, Tollens' reagent, Fehling's solution and the tri-iodomethane test to identify a carbonyl group and distinguish aldehydes from ketones. The HCN addition mechanism and the identification tests are guaranteed exam content.

The answer

The carbonyl group

Aldehydes ($\text{RCHO}$) and ketones ($\text{RCOR}'$) both contain the carbonyl group C=O. Oxygen is more electronegative than carbon, so the bond is polar: the carbon is slightly positive ($\text{C}^{\delta+}$) and open to attack by nucleophiles. The characteristic reaction is nucleophilic addition.

Nucleophilic addition of HCN

Aldehydes and ketones react with hydrogen cyanide (with a trace of base, e.g. KCN) to add across the C=O, forming a hydroxynitrile and lengthening the carbon chain by one:

1. The cyanide ion attacks the slightly positive carbonyl carbon; the C=O pi bond breaks, the electrons going onto oxygen, forming an alkoxide intermediate.
2. The alkoxide is protonated, giving the hydroxynitrile (e.g. ethanal gives 2-hydroxypropanenitrile).

Because the planar carbonyl can be attacked from either face equally, a chiral product forms as a racemic mixture of the two enantiomers.

Reduction

Aldehydes and ketones are reduced (e.g. by $\text{NaBH}_4$ or $\text{LiAlH}_4$) back to alcohols:

• aldehyde to a primary alcohol,
• ketone to a secondary alcohol.

Test 1: 2,4-DNPH (confirm a carbonyl)

2,4-dinitrophenylhydrazine (Brady's reagent) gives an orange or yellow precipitate with any aldehyde or ketone, confirming a C=O group. The melting point of the purified precipitate can identify the specific carbonyl compound.

Tests 2 and 3: Tollens and Fehling (distinguish aldehyde from ketone)

Aldehydes are easily oxidised to carboxylic acids; ketones are not. Two oxidising tests exploit this:

• Tollens' reagent (ammoniacal silver nitrate): an aldehyde gives a silver mirror; a ketone gives no reaction.
• Fehling's solution (copper(II) complex): an aldehyde gives a brick-red precipitate of copper(I) oxide; a ketone gives no reaction.

Test 4: tri-iodomethane (methyl ketones)

Warming with iodine and sodium hydroxide gives a pale yellow precipitate of $\text{CHI}_3$ for compounds containing the $\text{CH}_3\text{CO}-$ group (and the $\text{CH}_3\text{CH(OH)}-$ group). So ethanal and any methyl ketone (e.g. propanone) test positive.

Examples in context

Example 1. Building a chiral drug intermediate. The HCN addition to an aldehyde adds a carbon and creates a new chiral centre, but because the carbonyl is planar the product is a racemic mixture. SEAB uses this to test the link between a planar reactive site and the lack of stereoselectivity, an idea revisited when discussing why biological syntheses (using chiral enzymes) give a single enantiomer.

Example 2. The silver mirror in industry. The Tollens reaction, in which an aldehyde reduces silver ions to a thin silver mirror, was historically used to silver glass for mirrors and is still a vivid lecture demonstration. SEAB frames it as both an identification test and an example of an aldehyde acting as a reducing agent.

Try this

Q1. State the reagent and observation for a test that confirms a compound contains a carbonyl group. [2 marks]

• Cue. 2,4-DNPH (Brady's reagent); orange or yellow precipitate.

Q2. Name the product of reducing propanone with $\text{NaBH}_4$. [1 mark]

• Cue. Propan-2-ol (a secondary alcohol).

Q3. Explain why ethanal gives a silver mirror with Tollens' reagent but propanone does not. [2 marks]

• Cue. The aldehyde is readily oxidised to an acid, reducing the silver ions to silver; the ketone is not oxidised under these conditions.