The polar C=O invites nucleophiles. Cyanide ions add to make hydroxynitriles, and a set of bench tests tells you whether you have a carbonyl at all, an aldehyde or a ketone, and whether it carries a CH3CO– group.
The C=O bond is polar, so the carbon is δ+ and is attacked by nucleophiles. With HCN and a KCN catalyst, the CN− ion attacks the carbon, the π electrons move onto oxygen, and the O− then takes H+ from HCN: a 2-hydroxynitrile forms. Because the C=O group is planar, attack from either face is equally likely, so a chiral product is a racemic mixture. 2,4-DNPH gives an orange-yellow precipitate with any aldehyde or ketone. Only aldehydes reduce Fehling's (blue to brick-red Cu2O) and Tollens' (silver mirror) and turn acidified dichromate green. Alkaline iodine gives yellow CHI3 with the CH3CO– group.
Step through the mechanism in the simulator. Watch the cyanide ion approach the planar carbonyl from above and from below, and compare the two products.
| Carbonyl | Product with HCN/KCN | Chiral? |
|---|---|---|
| ethanal, CH3CHO | CH3CH(OH)CN, 2-hydroxypropanenitrile | yes: racemic mixture |
| propanone, CH3COCH3 | (CH3)2C(OH)CN, 2-hydroxy-2-methylpropanenitrile | no: two CH3 groups |
| butanone, CH3COCH2CH3 | CH3C(OH)(CN)CH2CH3 | yes: racemic mixture |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What type of reaction occurs between HCN and ethanal?
What is seen when a ketone is added to 2,4-dinitrophenylhydrazine?
Which reagent gives a brick-red precipitate when warmed with an aldehyde?
Which compound gives a pale yellow precipitate with alkaline iodine?
Run each reagent against an aldehyde, a ketone and a methyl ketone in the simulator and record the observations.
| Test | Positive for | Observation |
|---|---|---|
| 2,4-DNPH (Brady's reagent) | any aldehyde or ketone (C=O) | orange or yellow precipitate |
| Fehling's solution, warm | aldehydes only | blue solution gives a brick-red precipitate of Cu2O |
| Tollens' reagent, [Ag(NH3)2]+, warm | aldehydes only | silver mirror (or grey-black precipitate) of Ag |
| acidified K2Cr2O7, warm | aldehydes only | orange to green |
| alkaline I2, warm | CH3CO– (ethanal, methyl ketones) | pale yellow precipitate of CHI3 |
Pick a mystery carbonyl in the simulator, choose your tests in a sensible order and name the compound from the results.
| C5H10O isomer | 2,4-DNPH | Tollens' | Alkaline I2 |
|---|---|---|---|
| pentanal | orange ppt | silver mirror | no ppt |
| pentan-2-one | orange ppt | no change | yellow ppt |
| pentan-3-one | orange ppt | no change | no ppt |
HCN adds by nucleophilic addition, not electrophilic: the attacking species is CN−, and it bonds through carbon. The product is racemic because the C=O is planar, not because the reaction is SN1. 2,4-DNPH does not distinguish aldehydes from ketones; Fehling's and Tollens' do. Fehling's turns brick-red, Tollens' gives silver; do not swap them. Propanal gives no CHI3: it has no CH3CO– group.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Why is the 2-hydroxypropanenitrile made from ethanal and HCN optically inactive?
Compound Q gives an orange precipitate with 2,4-DNPH and a silver mirror with Tollens' reagent, but no precipitate with alkaline iodine. What could Q be?
What is the role of KCN in the reaction of HCN with propanone?
What is the maximum mass of 2-hydroxypropanenitrile (Mr 71.0) from 2.20 g of ethanal (Mr 44.0)?
This skill is now recorded on your profile. Keep the chain going.