Alcohols

• Alcohols are a homologous series with the functional group -OH.

  General formula:

  CnH2+1OH
• Physical properties:

  • High boiling point, due to hydrogen bonding between alcohol molecules and weaker van der Waal's forces
  • Soluble in water
• Alcohols are produced industrially by hydration of alkenes in the presence of an (phosphoric) acid catalyst.

  For example: Ethene and steam and passed over a concentrated phosphoric acid catalyst. The reaction mixture is kept at 60 atm and 600K, ensuring a 100% yield.
• Ethanol is produced industrially by fermentation of glucose. This method is cheaper than the hydration of alkanes as it can be carried out at a lower temperature but it has to be fermented in batches, therefore it takes longer and produces a lower percentage yield.

  Conditions:

  • Anaerobic conditions
  • 30-40 degrees (celsius)
• Ethanol produced industrially by fermentation is separated by fractional distillation and can then be used as a biofuel. Biofuel is any fuel made from living organisms or their waste.

  Theoretically biofuel is a carbon-neutral fuel as there are no net annual emissions of carbon dioxide into the atmosphere. Although, the production is not, the energy required to heat the ethanol is produced by burning fossil fuels, which release carbon dioxide into the atmosphere, the plants being fermented require fertilisers and pesticides, which require energy to transport.
• Hydration of alkenes: Electrophilic addition

  
• Alcohols are classified as primary, secondary and tertiary depending on the position of the -OH group in the carbon chain.

  Primary = The -OH is positioned at the end of the chain.

  Secondary = The -OH is positioned along the chain, e.g having carbon atoms either side.

  Tertiary = The -OH is positioned at a branch in the chain with 3 R groups.
• Primary alcohols can be oxidised to aldehydes which can be further oxidised to carboxylic acids. Oxidising agents are represented as [O].

  1. The primary alcohol loses two hydrogen atoms, one for the -OH group and one from the saturated carbon. The primary alcohol must be in excess and the product is distilled immediately.
  2. An oxygen atom is added to the remaining hydrogen. The oxidising agent must be in excess and the reaction mixture is refluxed gently.

  A) Primary alcohol

  B) Aldehyde

  C) Carboxylic acid

  
• Secondary alcohols can be oxidised to ketones, which do not undergo further oxidation.

  A) Secondary alcohol

  B) [O]

  C) Ketone

  
• Tertiary alcohols are not easily oxidised as they do not have two hydrogen atoms directly attached to the carbon that is bonded to the -OH group. They can be oxidised using hot concentrated nitric acid as it breaks the strong C-C bond.
• Acidified potassium dichromate(VI) is a suitable oxidising agent, as the agent is reduced it will turn from orange to green.
• Alcohols to alkene: Elimination reaction or dehydration reaction

  Conditions:

  • Concentrated sulfuric acid (acting as a catalyst)
  • 170 degree celsius

  OR

  Conditions:

  • Passing the alcohol vapour over a heated aluminum oxide catalyst
  • 600 degree celsius
• Alcohols to alkene: Elimination reaction mechanism

  A) Alkene

  B) Alcohol

  C) Concentrated sulfuric acid