Alcohols

The Chemistry of Alcohols AS Level AQA

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Introduction

  • The general formula: CnH(2n+1)OH
  • The OH group is the hydroxyl group
  • The C-O-H angle is 105 degrees.
  • Classification of alcohols:

Primary- OH group at the end of a carbon chain (surrounded by1 R group and 2 H atoms)

Secondary- OH group in the body of a chain (surrounded by2 R groups and 1 H atom)

Tertiary- OH group at a branch in the chain (surrounded by 3 R groups)

  • Physical properties: High melting point due to hydrogen bonding between molecules. Shorter chain alcohols are soluble in water because of H-bonds between OH groups and water.
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Production of Ethanol (crude oil)

  • First, crude oil is cracked, producing ethene.
  • Ethene is then hydrated, which means that water is added to the double bond
  • Conditions: 

Catalyst- Phosphoric acid absorbed on silica.

Temperature- 570K

Pressure- 6500 kPa

  • This gies a conversion of ethanol of only 5%, but the excess ethene is separated and used gain until 95% of the reaction mixture is obtained,
  • The reaction is exothermic.
  • Advantages: fast process, continuous flow process and produces pure ethanol
  • Disadvantages: requires fairly high technology, uses lots of energy and uses non-renewable raw materials (crude oil)
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Production of ethanol (fermentation)

  • Sugar (glucose) is broken down into ethanol by enzymes present in yeast.
  • Carbon dioxide is also another product.
  • This is a process called anaerobic respiration.
  • Conditions:

Temperature: 37 degrees

Absence of air to prevent oxidation of ethanol to ethanoic acid

Yeast enzymes

  • Once the fermenting solution contains about 15% of ethanol the enzymes are unable to function and the fermentation stops.
  • Advantages: low technology process, does not use much energy, uses renewable raw materials (sugar cane)
  • Disadvantages: Batch process, relatively slow process and produces impure ethanol
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Uses of ethanol

  • alcoholic drinks
  • cosmetics
  • drugs
  • detergents
  • inks
  • motor fuel

Carbon neutrality

  • Carbon dioxide released when something is burnt is balanced by the carbon dioxide absorbed by the plant from which it is originally obtained.
  • Ethanol made from fermentation is renewable!!
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Reactions of alcohols (combustion and oxidation)

Combustion

  • Produces carbon dioxide and water on complete combustion.

Oxidation

  • Primary alcohols are oxidised to aldehydes, secondary alcohols are oxidised to ketones. Aldehydes can further be oxidised to carboxylic acids.
  • Tertiary alcohols are not easily oxidised. This is because a C-C bond needs to break, rather than a C-H bond.
  • Solution of acidified potassium dichromate is the oxidising agent. Orange dichromate ions turn to green chromium ions.
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Oxidising ethanol to ethanal

  • Use dilute acid and less potassium dichromate..
  • Heat the mixture under reflux
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