Qualitative Investigations to Test for the Presence of Aldehydes, Ketones and Carboxylic Acids

This is part of the HSC Chemistry course under Module 8 Section 2: Analysis of Organic Substances.

HSC Chemistry Syllabus

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

  • carbon–carbon double bonds 

  • hydroxyl groups

  • carboxylic acids (ACSCH130)

Qualitative Investigations for Aldehydes, Ketones and Carboxylic Acids

This video explores how qualitative chemical tests such as oxidation can distinguish between an aldehyde and ketone. The video also explores qualitative tests for the presence of carboxylic acids.

 

How to Qualitatively Test for Presence of Aldehydes and Ketones

Acidified dichromate (Cr2O72–/H+) Oxidation Test

  • This test is used to differentiate between:
    • tertiary alcohols from primary and secondary alcohols
    • ketones and aldehydes.
  • When Cr6+ ions (in dichromate) are reduced to form Cr3+, the solution turns from orange to green.

 $$\frac{1}{2}Cr_2I_7^{2-} + 7H^+ + 3e^- \rightleftharpoons Cr^{3+} + \frac{7}{2}H_2O$$

  •  Dichromate will oxidise primary and secondary alcohols but not tertiary alcohols
    • Solution turns green when added to primary or secondary alcohols
    • Solution remains orange when added to tertiary alcohols.
  • Dichromate will oxidise an aldehyde into a carboxylic acid but does not react with ketone.
    • Solution turns green in the presence of an aldehyde.
    • Solution remains orange in the presence of a ketone.

 

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

 

Observation for different types of alcohol

 

 

 

  

Acidified Permanganate (MnO4–/H+) Oxidation Test

  • This test is used to differentiate between:
    • tertiary alcohols from primary and secondary alcohols
    • ketones and aldehydes.
  • When Mn7+ ions (in permanganate) are reduced to form Mn2+, the solution turns from purple to colourless

$$MnO_4^- + 8H^+ + 5e^- \rightleftharpoons Mn^{2+} + 4H_2O$$ 

  •  Permanganate will oxidise primary and secondary alcohols but not tertiary alcohols
    • Solution decolourises when added to primary or secondary alcohols
    • Solution remains purple when added to tertiary alcohols.

 

  • Permanganate will oxidise an aldehyde into a carboxylic acid but does not react with ketone.
    • Solution decolourises in the presence of an aldehyde
    • Solution remains purple in the presence of a ketone

 

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

 

 

How to Qualitatively Test for Presence of Carboxylic 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