3.2.10 Alcohols - Classification and reactions

Specification

Students should:
  • understand that alcohols can be classified as primary, secondary or tertiary
  • understand that tertiary alcohols are not easily oxidised
  • understand that primary alcohols can be oxidised to aldehydes and carboxylic acids and that secondary alcohols can be oxidised to ketones by a suitable oxidising agent such as acidified potassium dichromate(VI) (equations showing [O] as oxidant are acceptable)
  • be able to use a simple chemical test to distinguish between aldehydes and ketones (e.g. Fehling's solution or Tollens' reagent)

Classification of alcohols

Alcohols can also be categorised as primary (1º), secondary (2º) or tertiary depending on the number of carbon atoms that are attached to the carbon atom holding the OH group.

For example: propan-1-ol is a primary alcohol, while propan-2-ol is a secondary alcohol.

Methylpropan-2-ol is an example of a tertiary alcohol.

The classification is useful as each type of alcohol has different properties.


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Oxidation of alcohols

Alcohols are oxidised by strong oxidising agents. Potassium dichromate(VI) in the presence of dilute acid is the reagent of choice. The reaction is useful because the different types of alcohol behave differently.

1º alcohols

The reaction involves the orange solution of dichromate ions turning green as chromium(III) ions are formed.

3CH3CH2OH + Cr2O72- + 8H+ 3CH3CHO + 2Cr3+ + 7H2O

This redox formula may be simplified to:

CH3CH2OH + [O] CH3CHO + H2O

In this case ethanol is oxidised to ethanal. The ethanal can be further oxidised by exactly the same reagent to ethanoic acid, so it is important to remove it from the reaction vessel immediately.

Fortunately, this is possible as ethanal has a much lower boiling point than both ethanol and ethanoic acid. The reaction is performed in a distillation flask above the boiling point of ethanal and below the boiling point of the other compounds and the ethanal is allowed to distill off as it is formed.

the preparation of aldehydes by distillation of primary alcohols with acidified potassium dichromate(VI)

If we wish to oxidise a primary alcohol all the way to a carboxylic acid then we must make sure that the aldehyde does not escape before it can be oxidised. This is done using reflux apparatus.

CH3CH2OH + 2[O] CH3COOH + H2O

oxidation of a primary alcohol (or an aldehyde) to a carboxylic acid using reflux

2º alcohols

Secondary alcohols have only one hydrogen on the carbon atom that holds the alcohol group. This means that although they can be oxidised to ketones, the oxidation can go no further. Hence, there is no need to use distillation apparatus and the reaction may be carried out using reflux.

However, distalllation apparatus will still work as the boiling point of the ketone is always less than that of the parent alcohol.

Once again the dichromate ions turning green as chromium(III) ions are formed.

3CH3CH(OH)CH3 + Cr2O72- + 8H+ 3CH3COCH3 + 2Cr3+ + 7H2O

This equation can be also be simplified:

CH3CH(OH)CH3 + [O] CH3COCH3 + H2O

3º alcohols

Tertiary alcohols have no hydrogen atoms on the carbon atom that holds the alcohol group. This means that they cannot undergo oxidation.


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Testing for aldehydes and ketones

Aldehydes differ from ketones in that they are reducing agents. Ketones can't be oxidised, so they can't behave as reducing agents. There are two simple tests which allow us to distinguish between aldehydes and ketones:

Tollens reagent silver mirror test

Tollens reagent is a solution of the diamminesilver(I) complex ion, [Ag(NH3)2]+. It is sometimes called ammoniacal silver oxide in text books as this is how it is made. Just enough sodium hydroxide solution is added to silver nitrate solution so as to precipitate all of the silver as a brown precipitate of silver oxide.

Then concentrated ammonia is added to the suspended precipitate with shaking until it just dissolves.

When the suspected aldehyde is warmed with a sample of Tollen's reagent, a silver mirror forming on the walls of the vessel is indicative of an aldehyde.

The silver(I) complex is reduced by the aldehyde to silver(0), the element.

Tollen's test for an aldehyde

Fehling's solution test

Fehling's solution is actually a mixture of two solution that are kept apart until needed. These are called Fehling's A and Fehling's B solutions.

Fehling's A is a solution of copper(II) sulphate and Fehling's B is a mixture of sodium hydroxide and potassium sodium tartrate (2,3-dihydroxybutanedioate). When these two solutions are mixed together they form a copper(II) complex ion with the 2,3-dihydroxybutanedioate ligand. [Cu(tartrate)2]2+.

When the suspected aldehyde is warmed with a sample of this mixture, a red precipitate of copper(I) oxide is indicative of an aldehyde.

The copper(II) complex is reduced by the aldehyde to copper(I) oxide.


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