Reactions of Alcohols (OCR AS Chemistry A): Revision Note

Combustion 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

• Energy density is the amount of energy released in kJ per kg of fuel

  • The energy density of ethanol is lower than gasoline

  • This means that ethanol fuelled cars need larger tanks or more frequent refuelling

• Blending ethanol with petrol or diesel:

  • Increases the energy density 

  • Makes flames more visible in fires, improving safety

• There are socio-economic concerns about using farm land to grow fuel crops

  • Some argue that the land should be used for food production

Oxidation of alcohols

• Primary alcohols can be oxidised to form aldehydes and then further oxidised to carboxylic acids

• Secondary alcohols can be oxidised to form ketones only

• Tertiary alcohols do not undergo oxidation

Oxidising agents

• Common oxidising agents for alcohols include:

  • Acidified potassium dichromate(VI) (K₂Cr₂O₇) – orange solution

  • Acidified potassium manganate(VII) (KMnO₄) – purple solution

• When alcohols are oxidised:

  • Orange Cr₂O₇^2- ions are reduced to green Cr³⁺ ions

  • Purple MnO₄^- ions are reduced to colourless Mn²⁺ ions

Forming aldehydes and carboxylic acids

Aldehyde formation

• A primary alcohol is gently heated with oxidising agent

  • This forms an aldehyde

• The aldehyde formed has a lower boiling point than the alcohol

  • So, the aldehyde can be distilled off as it forms

Carboxylic acid formation

• If the aldehyde is not distilled off, refluxing with excess oxidising agent produces a carboxylic acid

Forming ketones

• A secondary alcohol is gently heated with oxidising agent

  • This forms a ketone

• Since ketones cannot be further oxidised, there is no need to distil off the product during the reaction

Elimination & substitution reactions of alcohols

Elimination reaction of alcohols

• Alcohols can also undergo dehydration to form alkenes

  • This is called an elimination reaction

• These elimination reactions produce:

  • The desired alkene

  • Water as a small molecule byproduct

    • The water is formed from the -OH group and a hydrogen atom on the adjacent carbon atom

• Common reaction conditions include:

  • Passing the alcohol vapour over a hot catalyst of aluminium oxide (Al₂O₃) powder or pieces of porous pot

  • Heating the alcohol with excess hot, concentrated sulfuric acid or phosphoric acid catalyst

Substitution reactions of alcohols

• In substitution reactions, a hydroxy group (-OH) of the alcohol is replaced by a halogen

  • This forms a haloalkane 

• The halogen can be made in situ during the reaction:

  • For example, KBr with H₂SO₄ or H₃PO₄ produces HBr, which reacts with the alcohol to form the haloalkane