• 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

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

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

• 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?colour of the solution turns?opaque?red due to the formation of a?copper(I) oxide?precipitate
• Ketones?cannot be oxidised and therefore give a?negative test?when warmed with Fehling’s solution

The copper(II) ions in Fehling’s solution are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to copper(I) ions in the Cu₂O precipitate

Tollens’ reagent

• Tollen’s reagent?is an aqueous alkaline solution of silver nitrate in excess ammonia solution
  • Tollen’s 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

The?Ag⁺?ions in Tollens’ reagent are?oxidising agents,?oxidising the aldehyde to a carboxylic acid and getting reduced themselves to silver atoms