Testing for Carbonyl Compounds (OCR A Level Chemistry)

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Using 2,4-Dinitrophenylhydrazine

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

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

Examiner Tip

The 2,4-DNPH test is especially useful as other carbonyl compounds such as carboxylic acids and esters do not give a positive result

Using Tollens' Reagent

  • Tollens' reagent contains the silver(I) complex ion [Ag(NH3)2]+
  • This is formed when aqueous ammonia is added to a solution of silver nitrate
    • Tollens' reagent is also known as ammoniacal silver nitrate

  • If gently warmed with Tollens' reagent, an aldehyde will become oxidised
  • The silver(I) complex ion solution, [Ag(NH3)2]+, is colourless
  • As the aldehyde is oxidised, it causes the [Ag(NH3)2]+ ions to become reduced to solid metallic silver, Ag
  • This is why a positive test result is called a "silver mirror"

Positive Test Result:

  • When Tollens' reagent is gently warmed with an aldehyde, the silver mirror is formed
    • This is the positive test result

  • When Tollens' reagent is gently warmed with a ketone, no silver mirror will be seen, as the ketone cannot be oxidised by Tollens' reagent, so no reaction takes place
    • This is a negative test result

3-5-4-tollens-reagent-new

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

Examiner Tip

Use 2,4-DNP to identify if the compound is a carbonyl

Then to differentiate an aldehyde from a ketone use Tollens' reagent.

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.