Nucleophilic addition and tests for carbonyl compounds

Nucleophilic addition of HCN (spec 15.8 iii)

Reagent: HCN with KCN present (KCN supplies the CN- nucleophile, because HCN is a weak acid and barely dissociates).

Mechanism:

1. A lone pair on the C of CN- attacks the δ+ carbonyl carbon; the C=O π bond breaks and the electrons move onto O, giving an O- intermediate.

2. The O- takes H+ from HCN (or water), regenerating CN-.

Propanone + HCN -> 2-hydroxy-2-methylpropanenitrile, (CH3)2C(OH)CN. Ethanal + HCN -> 2-hydroxypropanenitrile, formed as a racemic mixture. HCN is very toxic, so KCN with a little acid is used in a fume cupboard.

2,4-DNPH (spec 15.8 iv)

2,4-dinitrophenylhydrazine (Brady's reagent) gives an orange (or yellow-orange) precipitate with any aldehyde or ketone. To identify the compound, recrystallise the precipitate, measure its melting temperature and compare it with data-book values for 2,4-DNPH derivatives. No equation is needed.

Iodine in alkali (spec 15.8 v)

Warm with iodine and sodium hydroxide solution. A pale yellow precipitate of triiodomethane (iodoform), CHI3, with an antiseptic smell, shows a CH3CO- group (methyl ketones and ethanal). Alcohols with a CH3CH(OH)- group also give a positive result, because the iodine oxidises them to CH3CO- compounds first.

Worked example

Three liquids are propanal, propanone and pentan-3-one. Use tests to identify each.

1. 2,4-DNPH: all three give orange precipitates, so all are carbonyls.
2. Tollens' reagent: only propanal gives a silver mirror.
3. Iodine in alkali: only propanone (CH3CO-) gives yellow CHI3.

Answer: Propanal: silver mirror. Propanone: iodoform. Pentan-3-one: neither.

Check yourself

1. In the reaction of propanal with HCN and KCN, which species is the nucleophile?
   • HCN
   • K+
   • H+
   • CN-
   Show answer

   CN-. The cyanide ion's lone pair attacks the δ+ carbon.

2. Why is KCN added in the reaction of carbonyl compounds with HCN?
   • KCN oxidises the carbonyl compound
   • HCN is a weak acid, so KCN provides enough CN- ions
   • KCN is a dehydrating agent
   • KCN removes water
   Show answer

   HCN is a weak acid, so KCN provides enough CN- ions. HCN barely dissociates, so there is too little CN- without KCN.

3. What is the product of propanone with HCN?
   • 2-hydroxypropanenitrile
   • Butanenitrile
   • Propan-2-ol
   • 2-hydroxy-2-methylpropanenitrile
   Show answer

   2-hydroxy-2-methylpropanenitrile. CN and OH both end up on the central carbon: (CH3)2C(OH)CN.

4. What is seen when a ketone is added to 2,4-DNPH?
   • An orange precipitate forms
   • A silver mirror forms
   • A pale yellow precipitate with an antiseptic smell forms
   • The solution turns green
   Show answer

   An orange precipitate forms. 2,4-DNPH reacts with any C=O in aldehydes and ketones.

5. How is the 2,4-DNPH derivative used to identify a particular carbonyl compound?
   • Its colour is compared with a chart
   • Its mass is measured
   • It is recrystallised and its melting temperature compared with data-book values
   • It is dissolved in water and its pH measured
   Show answer

   It is recrystallised and its melting temperature compared with data-book values. Each derivative has a characteristic melting temperature.

6. Which compound gives a positive result with iodine in alkali?
   • Pentan-2-one
   • Pentan-3-one
   • Propanal
   • Methanal
   Show answer

   Pentan-2-one. Pentan-2-one has a CH3CO- group.

7. Which alcohol gives a pale yellow precipitate with iodine and sodium hydroxide?
   • Methanol
   • Propan-1-ol
   • Ethanol
   • 2-methylpropan-2-ol
   Show answer

   Ethanol. Ethanol (CH3CH2OH) has a CH3CH(OH)- unit (R = H) and is oxidised to ethanal.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson Edexcel IAL Chemistry (2018) scheme of work

Last updated 7 November 2026. Spotted a mistake? Tell us.