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Chemistry: AQA A Level, topic 5.6.1

Testing Alcohols

We can test alcohols to decide whether they are primary, secondary or tertiary.

The test

The test

  • We must warm the alcohol and then add acidified potassium dichromate (V).
  • Primary: turns green.
  • Secondary: turns green.
  • Tertiary: nothing happens.
The problem

The problem

  • The issue with this test is that it gives the same result for primary and secondary alcohols.
  • A rough way to differentiate between the two is that, generally, primary alcohols will react faster - the green colour will form quicker.
  • But this is unreliable.
Overcoming the problem

Overcoming the problem

  • A better way to differentiate is by testing the products of the first test.
  • In the first test, the acidified potassium dichromate (V) oxidises:
  • Primary alcohols to aldehydes.
  • Secondary alcohols to ketones.
  • So we can carry out an aldehyde/ketone test to determine which alcohol was initially present.
  • The aldehyde/ketone test uses either Fehling's solution or Tollen's reagent - it has been covered before.

Aldehyde or Ketone

We can determine whether a chemical is either an aldehyde or a ketone through two possible tests.

Aldehydes and ketones

Aldehydes and ketones

  • Aldehydes and ketones have very similar functional groups and so are hard to tell apart.
  • The one feature they have that is different is their oxidising ability.
  • So we can use oxidising agents to identify which is present.
Oxidising ability

Oxidising ability

  • Aldehydes can be oxidised further (twice) to carboxylic acids.
  • This is because they still have a C-H bond that can be broken for oxidation.
  • Ketones cannot be further oxidised (only once).
  • You would have to break a C-C bond.
Fehling's solution

Fehling's solution

  • Our first test is Fehling’s solution.
  • It contains Cu2+ and is deep blue.
  • When you add an aldehyde, the Cu2+ oxidises the aldehyde and is reduced to Cu+.
  • This causes a colour change from deep blue to brick red.
  • With a ketone, you get no colour change because it can’t be oxidised.
Tollen's reagent

Tollen's reagent

  • Tollen’s reagent contains the diamine silver 1+ ion: [Ag(NH3)2]+.
  • This is a clear solution.
  • This reagent also oxidises an aldehyde, but will do nothing to a ketone.
  • When it oxidises an aldehyde, it is reduced to solid silver and is said to form a ‘silver mirror’ on the surface of the test tube.
Overall test results

Overall test results

  • Fehling's solution:
  • Aldehyde: Deep blue → brick red.
  • Ketone: Remains deep blue.
  • Tollen's reagent:
  • Aldehyde: Clear → silver mirror.
  • Ketone: Remains clear.

Test yourself on Reactions

  • If we add acidified potassium dichromate (V) to an alcohol, which of the following will give us a definite result?

  • If a solution turns green from acidified potassium dichromate (V), what could we add to improve our identification of the sample?

  • Why can't ketone be further oxidised?

  • What colour will the resulting solution be if a ketone is tested with Fehling's solution?

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