Aldehydes and Ketones

Specification Reference Organic Chemistry, Carbonyl compounds 17.1

Quick Notes

Aldehydes can be made by the oxidation of a primary alcohol with acidified K₂Cr₂O₇ or KMnO₄ (aldehyde collected by distillation).
Ketones can be made by the oxidation of a secondary alcohol with acidified K₂Cr₂O₇ or KMnO₄
Reactions of Aldehydes and Ketones:
- Reduction to Alcohols:
  Use NaBH₄ or LiAlH₄.
  Aldehydes form primary alcohols
  Ketones form secondary alcohols
- Reaction with HCN/KCN:
  Forms hydroxynitriles (adds –OH and –CN across the C=O group).
  Occurs via nucleophilic addition.
Tests for Carbonyl Compounds:
- 2,4-DNPH Test:
  Reacts with both aldehydes and ketones → orange precipitate.
- Fehling’s Solution / Tollens’ Reagent:
  Distinguish between aldehydes and ketones.
  Aldehydes are easily oxidised → give a positive result.
  (Fehling’s: red ppt, Tollens’: silver mirror).
  Ketones give no reaction.
- Iodoform Test (I₂/OH⁻):
  Detects a CH₃CO– group (typically methyl ketones or ethanal).
  Gives a yellow precipitate of CHI₃ (iodoform) with antiseptic smell.

Full Notes

Aldehydes and ketones and their reactions have been outlined in more detail here.
This page is just what you need to know for CIE A-level Chemistry :)

Aldehydes and ketones contain a carbonyl (C=O) functional group.

Making Aldehydes

Oxidation of Primary Alcohols

Aldehydes can be formed by the oxidation of Primary Alcohols

CIE A-Level Chemistry diagram showing oxidation of a primary alcohol to an aldehyde with distillation.

Reagents: Acidified K₂Cr₂O₇ or acidified KMnO₄
(oxidising agents provide the oxygen required and are shown as [O] in equations).
Conditions: Heat and distillation

Note: Distillation prevents further oxidation to a carboxylic acid.

Example CH₃CH₂OH + [O] → CH₃CHO + H₂O

Oxidation of Secondary Alcohols

Ketones can be made by the oxidation of Secondary Alcohols

CIE A-Level Chemistry diagram showing oxidation of a secondary alcohol to a ketone.

Reagents: Acidified K₂Cr₂O₇ or acidified KMnO₄
(oxidising agents that provide oxygen required, shown as [O] in equations).
Conditions: Heat under reflux or distil

Note: Ketones do not oxidise further under normal conditions.

Example CH₃CH(OH)CH₃ → CH₃COCH₃ + H₂O

Key Reactions

Aldehydes and ketones react with nucleophiles as the C=O bond is polar and nucleophiles are attracted to the Cδ⁺.

Nucleophiles are electron pair donors.

Reduction to Alcohols

Aldehydes and Ketones can be reduced to alcohols

CIE A-Level Chemistry reaction scheme showing reduction of aldehydes and ketones to alcohols using NaBH4 or LiAlH4.

Reagents: NaBH₄ (aqueous) or LiAlH₄ (dry ether)
Aldehydes form Primary alcohols
Ketones form Secondary alcohols

Explanation: H⁻ ion from the reducing agent adds to the carbonyl carbon, shown as [H] in equation.

Note - LiAlH₄ is a more powerful reducing agent than NaBH₄, because of this, if LiAlH₄ is used no water can be present and the reaction must be carried out in dry ether.

Example CH₃CHO + 2[H] → CH₃CH₂OH

Matt’s exam tip

Remember reduction in organic chemistry is the gaining of a carbon-hydrogen bond. To provide the hydrogen needed, we use reducing agents (such as NaBH₄ and LiAlH₄) and show hydrogen from a reducing agent in equations as [H].

Reaction with HCN/KCN (Forming Hydroxynitriles)

Carbonyls react with KCN in H₂SO₄ to form hydroxynitriles.

CIE A-Level Chemistry reaction scheme showing aldehyde/ketone reacting with HCN/KCN to form a hydroxynitrile.

Reagents: HCN, KCN as catalyst, heat

Explanation: Adds CN⁻ and H⁺ across the C=O group to give an extra C–C bond.

Note: HCN is generated in situ by reacting KCN with H₂SO₄ because pure HCN is too toxic and volatile to handle safely. The KCN supplies CN⁻ ions, while H₂SO₄ provides H⁺ ions. This combination produces the same product — a hydroxynitrile — without needing to use HCN directly.

Example CH₃CHO + HCN → CH₃CH(OH)CN

Nucleophilic Addition Mechanism (HCN)

Aldehydes and ketones react by nucleophilic addition mechanisms - a nucleophile starts the reaction by attacking the Cδ⁺ in the C=O bond.

CIE A-Level Chemistry mechanism diagram of CN− nucleophilic addition to a carbonyl forming a hydroxynitrile.

Steps:

CN⁻ attacks the electrophilic carbon in the carbonyl group.
Intermediate forms with a negative charge on oxygen.
H⁺ (from HCN or acid) adds to O⁻, forming the hydroxynitrile.

Test with 2,4-DNPH

Carbonyl containing compounds (aldehydes and ketones) form an orange precipitate when added to a solution of 2,4-DNPH (Brady’s reagent).

CIE A-Level Chemistry photo/chart showing orange/yellow precipitate with 2,4-DNPH for carbonyl compounds.

Reagent: 2,4-dinitrophenylhydrazine
Observation: Bright orange or yellow precipitate
Explanation: Forms a hydrazone derivative with C=O compounds (both aldehydes and ketones).

CIE A-Level Chemistry scheme of hydrazone formation when carbonyl reacts with 2,4-DNPH.

Distinguishing Aldehydes from Ketones

Because aldehydes can be further oxidised, they can be distinguished from ketones using oxidation reactions.

The two main tests used are Fehling's Solution and Tollens' Reagent.

Test	Aldehyde (CHO)	Ketone
Fehling’s	Red ppt of Cu₂O	No change
Tollens’	Silver mirror forms (Ag⁰)	No silver formed

Fehling’s Solution Test:

CIE A-Level Chemistry test showing brick-red Cu2O precipitate with Fehling’s for aldehydes.

Aldehyde: Forms a brick-red precipitate (Cu₂O).
Ketone: No change.

Tollens’ Silver Mirror Test:

CIE A-Level Chemistry test showing silver mirror formation with Tollens’ reagent for aldehydes.

Aldehyde: Forms a silver mirror (Ag).
Ketone: No change.

Iodoform Test (CH₃CO– group)

The iodoform test detects compounds with the CH₃CO– group (methyl ketones) or ethanal.

CIE A-Level Chemistry reaction showing iodoform (CHI3) formation as a yellow precipitate.

Reagent: I₂ in alkali (e.g. NaOH)
Positive test: Yellow precipitate of CHI₃ (iodoform)

Example CH₃COCH₃ + 3I₂ + 4OH⁻ → CHI₃ + CH₃COO⁻ + 3I⁻ + 3H₂O

Summary

Aldehydes can be made by oxidising primary alcohols and collected by distillation to prevent further oxidation.
Ketones can be made by oxidising secondary alcohols; do not oxidise further under normal conditions.
Both reduce to alcohols with NaBH₄ or LiAlH₄ (aldehydes form primary alcohols, ketones form secondary alcohols).
HCN/KCN adds across C=O via nucleophilic addition to form hydroxynitriles (adds –CN and –OH).
Tests:
- 2,4-DNPH: orange/yellow precipitate with carbonyls.
- Fehling’s/Tollens’: aldehydes positive (red ppt / silver mirror), ketones no reaction.
- Iodoform (I₂/OH⁻): yellow CHI₃ with CH₃CO– (methyl ketones) or ethanal.