Aldehydes and ketones

MDCAT Chemistry · Chemistry · Aldehydes and ketones. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Aldehydes (RCHO) and ketones (RCOR′) both contain the carbonyl group, C=O. The carbon is sp² and planar, and oxygen pulls the electrons towards itself, so the carbon is slightly positive. Nucleophiles attack this carbon and add across the C=O bond: this is nucleophilic addition.

Aldehydes react faster than ketones. A ketone has two alkyl groups, which push electrons onto the carbonyl carbon and also crowd it.

An aldehyde has an H atom on the carbonyl carbon, so it is easily oxidised to a carboxylic acid. Ketones resist mild oxidising agents. Most tests that tell the two apart use this difference.

Rules to remember

General formula C_nH_2nO. An aldehyde and a ketone with the same formula are functional group isomers (propanal and propanone)
2,4-DNPH: yellow or orange precipitate with both aldehydes and ketones (test for C=O)
Tollens' reagent: silver mirror with aldehydes only. Fehling's solution: brick-red Cu₂O with aliphatic aldehydes only (not benzaldehyde, not ketones)
Iodoform test (I₂ + NaOH, yellow CHI₃): positive for CH₃CO–, so ethanal and methyl ketones. Alkaline sodium nitroprusside: red colour with ketones
HCN (with a little NaCN) adds to give a hydroxynitrile (cyanohydrin); the attacking species is CN⁻. Saturated NaHSO₃ gives white crystals with aldehydes and small methyl ketones
Reduction (NaBH₄, LiAlH₄ or H₂/Ni): aldehyde → primary alcohol, ketone → secondary alcohol. Zn/Hg + concentrated HCl (Clemmensen) changes C=O to CH₂
Dilute NaOH: aldol condensation if there is an α-hydrogen. Concentrated NaOH: Cannizzaro reaction if there is no α-hydrogen (methanal, benzaldehyde)

Common mistake

Students use 2,4-DNPH to tell an aldehyde from a ketone. It gives a precipitate with both, so it only shows that C=O is present; use Tollens' reagent or Fehling's solution to pick out the aldehyde.

Worked example

Which single test distinguishes pentan-2-one, CH₃COCH₂CH₂CH₃, from pentan-3-one, CH₃CH₂COCH₂CH₃?

Both are ketones, so both give a precipitate with 2,4-DNPH and neither reacts with Tollens' reagent or Fehling's solution.
Pentan-2-one has a CH₃CO– group; pentan-3-one has C₂H₅ groups on both sides of C=O and no CH₃CO– group.
The iodoform test needs CH₃CO–, so only pentan-2-one gives the yellow precipitate of CHI₃.

Answer: The iodoform test (I₂ with NaOH): yellow precipitate with pentan-2-one only

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