Testing for Carbonyl Compounds (OCR A Level Chemistry A): Revision Note

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-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

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

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