CIE AS Chemistry (9701) 2019-2021

Revision Notes

3.5.3 Carbonyl Compound Tests

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

Carbonyl Compounds 2,4-DNPH, downloadable AS & A Level Chemistry revision notes

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

 

Carbonyl Compounds Positive-Negative Test, downloadable AS & A Level Chemistry revision notes

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

Exam Tip

The 2,4-DNPH is especially useful as other carbonyl compounds such as carboxylic acids and esters do not give 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 (-COONa+)
  • 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

Carbonyl Compounds Fehlings Solution, downloadable AS & A Level Chemistry revision notes

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

Carbonyl Compounds Tollens Reagent, downloadable AS & A Level Chemistry revision notes

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

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