Testing for Carbonyl Compounds (Cambridge (CIE) AS Chemistry)

Testing for Carbonyls: 2,4-DNPH

• 2,4-dinitrophenylhydrazine (also known as 2,4-DNPH) is a reagent which detects the presence of carbonyl compounds (compounds with -C=O group)

• The carbonyl group of aldehydes and ketones undergoes a condensation reaction with 2,4-dinitrophenylhydrazine

  • A condensation reaction is a reaction in which two molecules join together and a small molecule (such as H₂O or HCl) is eliminated

• The product formed when 2,4-DNPH is added to a solution that contains an aldehyde or ketone is a deep-orange precipitate which can be purified by recrystallisation

• The melting point of the formed precipitate can then be measured and compared to literature values to find out which specific aldehyde or ketone had reacted with 2,4-DNPH

Example reaction of a ketone with 2,4-DNPH

Ketones and aldehydes react with 2,4-DNPH in a condensation reaction

Results of chemical tests using 2,4-DNPH

The test tube on the left shows a negative 2,4-DNPH test and the tube on the right shows a positive test

Identifying Carbonyls: Fehling's Solution & Tollens' Reagent

• The presence of an aldehyde group (-CHO) in an unknown compound can be determined by the oxidising agents Fehling’s and Tollens’ reagents

Fehling’s solution

• Fehling’s solution is an alkaline solution containing copper(II) ions which act as the oxidising agent

• When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Cu²⁺ ions are reduced to Cu⁺ ions 

  • In the alkaline conditions, the carboxylic acid formed will be neutralised to a carboxylate ion (the -COOH will lose a proton to become -COO^- )

  • The carboxylate ion (-COO^-) will form a salt with a positively charged metal ion such as sodium (-COO^-Na⁺)

• The clear blue solution turns opaque due to the formation of a red precipitate, copper(I) oxide

• Ketones cannot be oxidised and therefore give a negative test when warmed with Fehling’s solution

Colour change using Fehling's solution

The copper(II) ions in Fehling’s solution are oxidising agents. They can oxidise an aldehyde to a carboxylic acid and are themselves reduced to copper(I) ions in the Cu₂O precipitate

Tollens’ reagent

• Tollens' reagent is an aqueous alkaline solution of silver nitrate in excess ammonia solution

  • Tollens' reagent is also called ammoniacal silver nitrate solution

• When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Ag⁺ ions are reduced to Ag atoms

  • In the alkaline conditions, the carboxylic acid will become a carboxylate ion and form a salt

• The Ag atoms form a silver ‘mirror’ on the inside of the tube

• Ketones cannot be oxidised and therefore give a negative test when warmed with Tollens’ reagent

Colour change using Tollens' reagent

The Ag⁺ ions in Tollens’ reagent are oxidising agents. They can oxidise an aldehyde to a carboxylic acid and are themselves reduced to silver atoms