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.
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.
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.