4.4.2 Reactions of Alcohols

• Alcohols react with oxygen in the air when ignited and undergo complete combustion to form carbon dioxide and water

alcohol + oxygen → carbon dioxide + water

 

Complete combustion of alcohols to produce carbon dioxide and water

• Lower alcohols burn with an almost invisible flame and make good fuels
• Ethanol can be produced sustainably as a fuel by the fermentation of sugars
• However, the energy density (the amount of energy in kJ per kg of fuel) is lower than gasoline so cars that run on ethanol must either have a larger fuel tank or fill up more often
• Blending ethanol with gasoline or diesel increases the energy density and makes it safer in case of fires as it is easier to see the flames compared to pure ethanol burning
• However, the are socio-economic concerns about using large quantities of farm land to produce crops for fermentation, which could be better used for food production

• 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 agents of alcohols include acidified K₂Cr₂O₇ or acidified KMnO₄

Oxidising agents

• The oxidising agents used to prepare aldehydes and ketones from alcohols include acidified potassium dichromate (K₂Cr₂O₇) and acidified potassium manganate (KMnO₄)
  • The acidified potassium dichromate(VI), K₂Cr₂O_7, is an orange oxidising agent
    • When the alcohols are oxidised the orange dichromate ions (Cr₂O₇^2-) are reduced to green Cr³⁺ ions

  • The acidified potassium manganate(VII), KMnO₄ is a purple oxidising agent
    • When the alcohols are oxidised the purple manganate ions (MnO₄^-) are reduced to colourless Mn²⁺ ions

 

The oxidising agents change colour when they oxidise an alcohol and get reduced themselves

Forming aldehydes and carboxylic acids

• For aldehydes, a primary alcohol 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 

Oxidation of ethanol by acidified K₂Cr₂O₇ to form an aldehyde by distillation

• If the aldehyde is not distilled off, further refluxing with excess oxidising agent will oxidise it to a carboxylic acid

Further oxidation of the aldehyde via reflux can be done to produce a carboxylic acid

Forming ketones

Oxidation of propan-2-ol by acidified K₂Cr₂O₇ to form a ketone

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

Elimination Reaction of Alcohols

• Alcohols can also undergo dehydration to form alkenes
  • This is an example of an elimination reaction
  • Elimination reactions involve a small molecule leaving the parent molecule as a byproduct
  • In this case, the small molecule is a water molecule
  • The water molecule is formed from the -OH group and a hydrogen atom from the adjacent carbon atom

• Alcohol vapour is passed over a hot catalyst of aluminium oxide (Al₂O₃) powder or pieces of porous pot
  • Excess hot, concentrated sulfuric acid or phosphoric acid is used as a catalyst

Dehydration of ethanol using aluminium oxide as a catalyst forms ethene gas, which can be collected over water

• The reaction and mechanism for the dehydration of propan-2-ol is shown below

Dehydration of propan-2-ol mechanism

Substitution Reactions of Alcohols

• In the substitution of alcohols, a hydroxy group (-OH) is replaced by a halogen to form an haloalkane 
• The substitution of the alcohol group for a halogen can be achieved by reacting the alcohol with:
  • HX (rather than using HBr, KBr is reacted with H₂SO₄ or H₃PO₄ to make HBr that will then react with the alcohol)

Substitution of alcohols to produce haloalkanes