Testing for Aldehydes & Ketones
This lesson covers:
- Using 2,4-dinitrophenylhydrazine (2,4-DNPH) to identify aldehydes and ketones
- How to distinguish aldehydes from ketones
- Using alkaline iodine to identify methyl ketones
2,4-DNPH derivatives identify carbonyls
When 2,4-dinitrophenylhydrazine (2,4-DNPH) is dissolved in methanol and concentrated sulfuric acid, it reacts with carbonyl groups (C=O) found in aldehydes and ketones to form a bright orange precipitate.
This reaction only occurs with aldehydes and ketones due to their C=O group. Other carbonyls like carboxylic acids (COOH) and esters (COO) do not react.
The orange derivative crystals produced are filtered and then purified by recrystallisation. After purification, the melting point of the derivative is measured and compared to known values in a database. By matching the measured melting point to a known value, the original carbonyl compound can be identified.
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:
- Tollens’ reagent
- Fehling’s solution
- 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 (Cr2O72-) act as the oxidising agent, being reduced to green Cr3+ ions. Ketones do not react.
Full equation:
3RCHO + Cr2O72- + 8H+ ➔ 3RCOOH + 2Cr3+ + 4H2O
Summary of observations
The table below summarises the observations for each reagent when used with aldehydes and ketones:
| Reagent | With an aldehyde | With a ketone |
|---|---|---|
| Tollens’ reagent | Silver mirror forms on test tube walls | Solution remains colourless |
| Fehling’s solution | Brick-red precipitate forms | Solution remains blue |
| Acidified potassium dichromate | Colour change from orange to green | Solution remains orange |
Alkaline iodine solution identifies methyl ketones
A solution of iodine in sodium hydroxide can detect the presence of methyl ketones (RCOCH3).
On warming with a methyl ketone, a yellow precipitate of triiodomethane (CHI3) forms.
This yellow precipitate can be confirmed as triiodomethane by collecting the crystals and determining their melting point, which is 119°C.
The reaction involves two main steps:
- Step 1 - Halogenation, where the hydrogen atoms on the methyl group are replaced by iodine atoms.
- Step 2 - Hydrolysis, leading to the formation of the yellow precipitate of triiodomethane (CHI3).
Full equation:
RCOCH3 + 3I2 + 4OH- ➔ RCOO- + CHI3 + 3I- + 3H2O