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Oxidation of Alcohols - 2

Chemistry: AQA A Level, topic 5.5.3

Primary Alcohol Oxidation Products

Looking further into the oxidation of primary alcohols, we shall determine how to produce an aldehyde or a carboxylic acid.

Oxidation of primary alcohols

Oxidation of primary alcohols

  • As we have seen before, there are two possible oxidations of primary alcohols:
  • One produces an aldehyde.
  • The other produces a carboxylic acid.
Producing an aldehyde

Producing an aldehyde

  • To make an aldehyde, you carry out the reaction under distillation.
  • This condenses any reacted aldehyde, which forms as a gas, into a new flask.
Producing a carboxylic acid

Producing a carboxylic acid

  • To make a carboxylic acid, you carry out the reaction under reflux.
  • This involves use of a reflux condenser.
  • The reflux condenser condenses any gases and forces them back into the reaction vessel.
  • This puts the aldehyde back in the flask to be oxidised again.

Testing Oxidation Products

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 to carboxylic acids.
  • This is because it still has a C-H bond that can be broken for oxidation.
  • Ketones cannot be further oxidised.
  • 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 diammine 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 Oxidation of Alcohols - 2

  • What is the apparatus shown here?

  • What is the purpose of a reflux condenser?

  • Oxidation under distillation produces which of the following?

  • A ketone cannot be further oxidised because of which of the following reasons?

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

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