Oxidation of Alcohols (Oxford AQA International A Level Chemistry)

Classification of Alcohols

• Alcohols are compounds that contain at least one hydroxy (-OH) group

• The general formula of alcohols is C_nH_2n+1OH

• They can be classed as primary, secondary and tertiary

Primary alcohols

• A primary alcohol contains a carbon atom, bonded to the -OH group, which is attached to one other carbon atom (or alkyl group)

Secondary alcohols

• A secondary alcohol contains a carbon atom, bonded to the -OH group, that is attached to two other carbon atoms (or alkyl groups)

Tertiary alcohols

• A tertiary alcohol contains a carbon atom, bonded to the -OH group, that is attached to three other carbon atoms (or alkyl groups)

Oxidation of Alcohols

• Primary alcohols can be oxidised to form aldehydes which can undergo further oxidation to form carboxylic acids

• Secondary alcohols can be oxidised to form ketones only

• Tertiary alcohols do not undergo oxidation

• The oxidising agent of alcohols is acidified potassium dichromate(VI), K₂Cr₂O₇ 

• The colour change is from orange to green

  • For potassium dichromate(VI) to act as an oxidising agent, it itself needs to be reduced

  • This reduction requires hydrogen (H⁺) ions which are provided by an acid, typically sulfuric acid

  • When alcohols are oxidised, the orange dichromate ions (Cr₂O₇^2-) are reduced to green Cr³⁺ ions

Primary alcohols

Oxidation of primary alcohols

Formation of a carboxylic acid (using ethanol)

• This reaction is performed using reflux apparatus

• A primary alcohol, such as ethanol, is added to the oxidising agent and warmed

CH₃CH₃OH (l) + 2[O] → CH₃COOH (g) + H₂O (l)

Formation of an aldehyde (using ethanol)

• This reaction is performed using distillation apparatus

• A primary alcohol, such as ethanol, is added to the oxidising agent and warmed

• The aldehyde product has a lower boiling point than the alcohol reactant so it can be distilled off as soon as it forms 

CH₃CH₂OH (l) + [O] → CH₃CHO (g) + H₂O (l)

Secondary alcohols

Oxidation of Secondary Alcohols

• Secondary alcohols, such as propan-2-ol, are oxidised to ketones

• Since ketones cannot be further oxidised, the ketone product does not need to be distilled off straight away after it has been formed

CH₃CH(OH)CH₃ (l) + [O] → CH₃COCH₃ (g) + H₂O (l)

Tertiary alcohols

• Tertiary alcohols do not undergo oxidation

  • This is because there must be a hydrogen on the functional group carbon, which breaks off to form water

  • There are only C-C bonds on the functional group carbon in a tertiary alcohol

Distinguishing Between Aldehydes and Ketones

• Aldehydes can be oxidised further to carboxylic acids

• Ketones can not be oxidised

• This difference allows us to use chemical tests to distinguish between aldehydes and ketones

Fehling’s solution

• Fehling’s solution is an alkaline solution containing copper(II) ions which act as the oxidising agent

• When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Cu²⁺ ions are reduced to Cu⁺ ions 

Cu²⁺ + e^- → Cu⁺

• The clear blue solution turns opaque due to the formation of a red precipitate of copper(I) oxide

• Ketones cannot be oxidised and therefore give a negative test when warmed with Fehling’s solution

Tollens’ reagent

• Tollens' reagent is an aqueous alkaline solution of silver nitrate in excess ammonia solution

  • Tollens' reagent is also called ammoniacal silver nitrate solution

• When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Ag⁺ ions are reduced to Ag atoms

Ag⁺ + e^- → Ag

• The Ag atoms form a silver ‘mirror’ on the inside of the tube

• Ketones cannot be oxidised and therefore give a negative test when warmed with Tollens’ reagent