Identify Aldehyde from Ketone - Oxidation of aldehyde

Both aldehyde and ketone contain

Active group of aldehyde is and ketone's active group is

Differences of aldehyde and ketone

Aldehyde and ketone both have similar and difference chemical properties. Aldehyde can be oxidized to carboxylic acid easily while ketone are not.


Due to different active groups, aldehyde and ketone indicate different reactions for same reagent. But sometimes they show same changes (like colour changes) to some reagents.


The best way to identify aldehyde from ketone is checking the oxidizing ability of aldehyde or ketone compound. Observe the colour change which is related to the aldehyde compound.


Oxidation of aldehyde

Aldehyde are oxidized to carboxylic acids by both strong and mild oxidizing agents.



Strong oxidizing agents

Aldehyde are oxidized to alcohols by strong oxidizing agents and their colour changes will notify strong oxidizing agents are being reduced.

  • H+ / KMnO4: purple colour is changed to light pink or colourless.
  • H+ / K2CrO4: yellow colour is changed to green
  • H+ / K2Cr2O7: orange colour is changed to green

Mild strong oxidizing agents

  • Tollen's Reagent
  • Fehlingh's reagent

Aldehyde and tollen's reagent

Tollen's reagent is prepared by dissolving silver hydroxide, the white precipitate in ammonia solution.

When aldehyde is oxidized to carboxylic acid, Ag+ ion is reduced to Ag which is deposited in the bottom of the test tube. Deposited silver is shown as a silver mirror.

But we cannot identify formic acid from aldehyde from tollen's reagent because formic acid is oxidized to carbon dioxide. So both aldehyde and formic acid reacts with tollen's reagent and we cannot observe a different thing in two cases.


How do you identify aldehyde from formic acid?

Formic acid is the simplest carboxylic acid compound. We can use different characteristics of carboxylic acids and aldehydes to identify aldehyde from formic acid.

  • Add sodium and observe changes. In one compound, a gas is emitted. Formic acid reacts with sodium and emits hydrogen gas that can be used to identify aldehyde from formic acid. Also ketones can be identified from this method.


Reduction of aldehydes and ketones to alcohols by LiAlH4 and Lucas' test

From this method, we try to produce primary and secondary alcohols and then check their differences to identify aldehyde and ketone.

  • Aldehydes are reduced to primary alcohols by LiAlH4
  • Ketones are reduced to secondary alcohols by LiAlH4
Reduction of aldehydes and ketones to alcohols by LiAlH4, NaBH4, Na/ethanol, H2/Ni

Lucas' reagent test to identify primary alcohols and secondary alcohols

As lucas' reagent, we use anhydrous zinc chloride and concentrated hydrochloride acid (ZnCl2/HCl). This reagent is much suitable for, identify primary alcohols and secondary alcohols.

But, from good observation, you can identify primary alcohols and secondary alcohols too.


Identify primary alcohols and secondary alcohols from Lucas' reagent

  • Add Lucas' reagent to two alcohol products given from aldehyde and ketone.
  • Wait about 10 minutes. One solution become turbid or cloudy. That cloudy appearance is given by secondary alcohol. No visible reaction is occurred in other solution. So we can identify primary and secondary alcohols.
  • So primary aldehyde is given by aldehyde. Now aldehyde and ketone are also identified.


Below table shows some experiments can be done to identify aldehyde and ketone

Experiments to identify aldehyde and ketones

Reagent Aldehyde Ketone
Brady's Reagent Orange colour precipitate Orange colour precipitate
NH2OH Colourless precipitate, does not dissolve in water Colourless precipitate, does not dissolve in water
Acidic KMnO4 Purple colour is changed to light pink or colourless No difference
Acidic K2Cr2O7 Orange colour is changed to green No difference
Tollen's Reagent Silver deposits, (silver mirror) No reaction
Fehlinghs's Reagent (blue colour) Aliphatic aldehydes are oxidized. A brick red colour, Cu2O forms. No reaction


separate aldehyde and ketone