Classifying & Testing for Alcohols (Cambridge (CIE) AS Chemistry): Revision Note

Exam code: 9701

Philippa Platt

Last updated

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)

Primary, secondary and tertiary alcohols

Diagram of alcohol structures: primary alcohol (propan-1-ol), secondary alcohol (propan-2-ol), tertiary alcohols (2-methylpropan-2-ol, 2-methylbutan-1,2,3-triol).
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

 Testing for alcohols

Three test tubes with different coloured liquids: propan-1-ol (green), propan-2-ol (dark green), and 2-methyl-propan-2-ol (orange), labelled primary, secondary, tertiary alcohols.
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) is a yellow precipitate 

  • It is formed from the reaction of iodine in an alkaline solution 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 with iodine atoms, forming a -CI3 group

    • The intermediate compound is hydrolysed by an alkaline solution to form a sodium salt (RCO2- Na+) and a yellow precipitate of CHI3

The iodoform reaction

Chemical reaction of methyl ketone with iodine and NaOH to form sodium carboxylate and tri-iodomethane in two steps: halogenation and hydrolysis.
The reaction of methyl ketones with iodoform results in the formation of a yellow CHI3 precipitate

Iodoform & alcohols

  • The position of a secondary alcohol group in a molecule can be identified using the iodoform test with iodine in alkaline solution

    • If the –OH group is attached to a carbon atom that is next to a methyl group, the alcohol can be oxidised by the alkaline iodine to form a methyl ketone (RCOCH3)

    • The methyl ketone then undergoes:

      • Halogenation, where the three hydrogen atoms of the methyl group are replaced by iodine atoms (–CI3), followed by

      • Hydrolysis, forming a sodium carboxylate salt and a yellow precipitate of iodoform (CHI3)

  • A positive result (yellow precipitate) indicates that the alcohol has the structure CH₃CH(OH)R

  • No precipitate means the secondary alcohol is not adjacent to a methyl group and therefore cannot form a methyl ketone intermediate

Using the iodoform test

Chemical reaction diagram showing oxidation of butan-2-ol to butanone, followed by halogenation and hydrolysis steps producing sodium propanoate and tri-iodomethane.
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 iodine

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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

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