Reactions of Alcohols (OCR A Level Chemistry)

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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

 

Hydroxy Compounds Combustion of Alcohols, downloadable AS & A Level Chemistry revision notes

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

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 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), K2Cr2O7, is an orange oxidising agent
      • When the alcohols are oxidised the orange dichromate ions (Cr2O72-) 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

 Carbonyl Compounds Oxidising Agents, downloadable AS & A Level Chemistry revision notes

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 

Carbonyl Compounds Synthesis of Aldehydes, downloadable AS & A Level Chemistry revision notes

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

Heating under Reflux (1), downloadable IB Chemistry revision notes

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

Forming ketones

Hydroxy Compounds Oxidation of Secondary Alcohols, downloadable AS & A Level Chemistry revision notes

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 & Substitution Reactions of Alcohols

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

Hydroxy Compounds Dehydration of Alcohols, downloadable AS & A Level Chemistry revision notes

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)

4-4-2-substitution-reactions-of-alcohols-1st-reaction

Substitution of alcohols to produce haloalkanes

Examiner Tip

An alternative method to produce the bromoalkane by reacting the alcohol with sodium or potassium bromide and concentrated sulfuric acid generating HBr is to use phosphorus tribromide, PBr3.

To form bromoethane from ethanol, PBr3 can be added drop wise to ethanol (this reaction is very vigorous).

PBr3 + 3C2H5OH → 3C2H5Br + H3PO3

The water removes excess PBr3 by the hydrolysis reaction

PBr3 + 3H2O → 3HBr + H3PO3

   

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.