Chemical Tests for Aldehydes, Ketones and Carboxylic Acids

This is part of the HSC Chemistry course under the topic Analysis of Organic Substances. This section explores a range of chemical tests that can be conducted in a school laboratory to identify and distinguish between various organic functional groups.

HSC Chemistry Syllabus

Conduct qualitative investigations to test for the presence in organic molecules of the following functional groups:

Chemical Test For Aldehydes, Ketones and Carboxylic Acids

This video discusses how chemical tests such as oxidation can distinguish between an aldehyde and ketone. The video also explores the identification of an organic acid such as carboxylic acid.

 

Dichromate (Cr2O72–) oxidation test

  • This test is used to identify ketones from aldehydes (or vice versa).
  • Dichromate is a strong oxidising agent that will oxidise an aldehyde into a carboxylic acid but does not react with ketone. When Cr6+ ions (in dichromate) are reduced to form Cr3+, the solution turns from orange to green.
  • Acidified dichromate solutions will turn green in the presence of an aldehyde.
  • Acidified dichromate solutions will remain orange in the presence of a ketone.

 

Cr6+ (orange) reduces to form Cr3+(green)

 

 

Tollens reagent (silver mirror test)

  • This test is used to identify ketones from aldehydes (or vice versa).
  • Tollens reagent contains Ag+ ions which are strong oxidising agents that will oxidise an aldehyde into a carboxylic acid but does not react with ketone. When Ag+ ions are reduced to form Ag, a layer of silver will coat the test tube. This creates a mirror-like appearance.
  • Tollens reagent will create a mirror-like appearance in the presence of an aldehyde.
  • Tollens reagent will remain colourless in the presence of a ketone.

 

  

Tollen’s reagent

Permanganate (MnO4) oxidation test

  • This test is used to identify ketones from aldehydes (or vice versa).
  • Acidified KMnO4 solution contains Mn7+ ions which are strong oxidising agents that will oxidise an aldehyde into a carboxylic acid but does not react with ketone. When Mn7+ ions (purple) are reduced to form Mn2+, the solution changes from purple to colourless. 
  • Acidified permanganate solutions will decolourise (from purple) in the presence of an aldehyde.
  • Acidified permanganate solutions will remain purple in the presence of a ketone.

 

Mn7+ (purple) reduces to form Mn2+ (colourless)

 

 

Identifying Organic Acids

  • This test is used to identify a carboxylic acid from aldehydes and ketones.
  • Carboxylic acids undergo acid-base reaction with carbonate and hydrogen carbonates to produce salt, water and carbon dioxide. No acid and base reactions occur between an aldehyde/ketone and carbonates.

 

  

  • When Na2CO3 or NaHCO3 is added to a test tube, formation of bubbles indicates the production of carbon dioxide, which in turn indicates the presence of a carboxylic acids.
  • Production of CO2 can be further confirmed by using limewater test.

 

CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)

 

Using pH indicators

  • A solution containing carboxylic acid will have a pH < 7 at 25ºC. This means a blue litmus paper will turn red, bromothymol blue will be yellow.
  • Ketones and aldehydes do not contain acidic hydrogens so their solutions will be neutral.

 

 

Limewater test of carbon dioxide