CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.4.3 Classifying and Testing for Alcohols

Classifying Alcohols

  • Primary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to one other carbon atom (or alkyl group)
  • Secondary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to two other carbon atoms (or alkyl groups)
  • Tertiary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to three other carbon atoms (or alkyl groups)

Hydroxy Compounds Classifying Alcohols, downloadable AS & A Level Chemistry revision notes

Classifying primary, secondary and tertiary alcohols and alcohols with more than one alcohol group

  • Only primary and secondary alcohols can get oxidised when mildly oxidised with acidified K2Cr2O7
    • Primary alcohols get mildly oxidised to aldehydes
    • Secondary alcohols get mildly oxidized to ketones
  • Tertiary alcohols do not undergo oxidation with acidified K2Cr2O7
  • Therefore, only the oxidation of primary and secondary alcohols will change the colour of K2Cr2O7 solution as the orange Cr2O72- ions are reduced to green Cr3+ ions

 

Hydroxy Compounds Tests Alcohols, downloadable AS & A Level Chemistry revision notes

Only propan-1-ol and propan-2-ol, which are primary and secondary alcohols respectively, can get oxidised, turning the orange solution green; no colour change is observed with 2-methyl-propan-2-ol, which is a tertiary alcohol

Test for Alcohols

  • Tri-iodomethane (also called iodoform) forms a yellow precipitate with methyl ketones
    • Methyl ketones are compounds that have a CH3CO-group
    • Ethanal also contains a CH3CO- group and therefore also forms a yellow precipitate with iodoform
  • The reagent is heated with an alkaline solution of iodine
  • This reaction involves a halogenation and hydrolysis step
    • In the halogenation step, all three H-atoms in the -CH3 (methyl) group are replaced for iodine atoms, forming –CI3
    • The intermediate compound is hydrolysed by alkaline solution to form a sodium salt (RCO2 Na+) and a yellow precipitate of CHI3

Hydroxy Compounds Iodoform Test, downloadable AS & A Level Chemistry revision notes

The reaction of methyl ketones with iodoform results in the formation of a yellow CHI3 precipitate

Iodoform & alcohols

  • The position of a secondary alcohol can be deduced by reacting the compound with alkaline I2
  • If the -OH group is on the carbon atom next to a methyl group, it will firstly get oxidised to CH3CH(OH)- by the alkaline solution
  • This will result in the formation of a methyl ketone RCOCH3
  • The methyl ketone will then first get halogenated and then hydrolysed to form the sodium salt and the yellow precipitate
  • If no yellow precipitate is formed, then this means that the secondary alcohol is not on a carbon next to a methyl group

Hydroxy Compounds Iodoform Test Alcohol, downloadable AS & A Level Chemistry revision notes

The secondary alcohol butan-2-ol will firstly get oxidised to the methyl ketone butanone which will form a yellow precipitate when reacted with alkaline I2

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top