CIE AS Chemistry (9701) 2019-2021

Revision Notes

3.4.2 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

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