Identifying Hydroxyl Groups: Lucas' Test & Oxidation Test

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 (ASCH130)

Identifying Hydroxyl Groups: Lucas' Test & Oxidation Tests

This video explores the Lucas test and oxidation tests using acidified dichromate and permanganate solutions to distinguish between primary, secondary and tertiary alcohols.

What are primary, secondary and tertiary alcohols?

 

How to Distinguish Between Alcohols, Aldehydes and Ketones

  • Oxidation states of carbon atoms in organic molecules
    • Every bond between C and another C does not alter the oxidation state
    • Every bond between C and H will decrease the oxidation state by 1
    • Every bond between C and a more electronegative element will increase the oxidation state by 1

 

 

  • The oxidation of a primary (1º) alcohol yields aldehyde as an intermediate and carboxylic acid as the final product.

 

 

  • The oxidation of a secondary (2º) alcohol produces ketone. Ketone cannot be oxidised further as this would break a C–C bond which requires too much energy.

 

 

 

  • Tertiary (3º) alcohols cannot be oxidised because this would otherwise involve breaking a C–C bond (sigma bond) which requires too much energy.

 

 

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)

 

 

Silver Mirror Test (Tollen’s Reagent)

  • This test is used to identify:
    • tertiary alcohols from primary and secondary alcohols
    • between ketones and aldehydes.

 

  • Ag(NH3)2+ (Tollen’s reagent) is reduced to form solid silver

 

$$Ag(NH_3)_2^+(aq) + e^- \rightarrow Ag(s) + 2NH_3(aq)$$

 

  • Tollen’s reagent will oxidise primary and secondary alcohols but not tertiary alcohols.
    • When added to primary or secondary alcohols, a silver coating forms
    • When added to tertiary alcohols, no change occurs

 

  • Tollen’s reagent will oxidise aldehydes into carboxylic acids but not ketones.
    • When added to aldehydes, a silver coating forms
    • When added to ketones, no change occurs

 

  

Tollen’s reagent

 

Lucas Test for Alcohols

  • The Lucas test differentiates between primary, secondary and tertiary alcohols
  • Lucas’ reagent: concentrated HCl, ZnCl2 (catalyst)
  • The product of substitution of alcohol with HCl has lower solubility in Lucas’s reagent. Thus, the turbidity of solution increases when the substitution reaction takes place. The rate at which the turbidity appears is indicative of the reaction rate.
  • Observations:
    • Tertiary alcohols react fastest: turbidity quickly appears
    • Secondary alcohols: takes longer time to turn turbid than tertiary alcohols i.e. 3-5 minutes
    • Primary alcohols: no visible reaction at 25ºC. Reaction can be made more apparent with heat

 

Image shows a more turbid solution of Lucas’ reagent when a substitution with HCl reaction has occurred.