Distinguishing Aldehydes from Ketones

This lesson covers: 

1. How to distinguish aldehydes from ketones

Distinguishing aldehydes from ketones

Aldehydes can be distinguished from ketones because aldehydes can be easily oxidised into carboxylic acids, while ketones cannot.

Reagents are used that change colour when reduced as the aldehyde gets oxidised.

Three commonly used reagents are:

1. Tollens’ reagent
2. Fehling’s solution
3. Acidified potassium dichromate

Using Tollens' reagent

Tollens' reagent is a colourless solution containing complexed silver ions, [Ag(NH₃)₂]⁺, formed by the addition of aqueous ammonia to silver nitrate.

When warmed with an aldehyde, the Ag⁺ ions in Tollens' reagent undergo reduction to Ag, depositing silver metal as a silver mirror. No reaction occurs with ketones.

Full equation:

2[Ag(NH₃)₂]⁺ + RCHO + 3OH^- ➔ 2Ag + RCOO^- + 4NH₃ + 2H₂O

Using Fehling's solution

Fehling's solution contains blue copper(II) complex ions dissolved in aqueous sodium hydroxide.

When warmed with an aldehyde, the copper(II) ions are reduced to a red precipitate of copper(I) oxide, Cu₂O. No reaction occurs with ketones.

Full equation:

RCHO + 2Cu²⁺ + 5OH^- ➔ RCOO^- + Cu₂O + 3H₂O

Using acidified potassium dichromate(VI)

Heating an aldehyde with acidified potassium dichromate(VI) causes oxidation to a carboxylic acid.

The orange dichromate(VI) ions (Cr₂O₇^2-) act as the oxidising agent, being reduced to green Cr³⁺ ions. Ketones do not react.

Full equation:

3RCHO + Cr₂O₇^2- + 8H⁺ ➔ 3RCOOH + 2Cr³⁺ + 4H₂O

Summary of observations

The table below summarises the observations for each reagent when used with aldehydes and ketones: