Alcohols

Physical properties

  • When comparing alcohols with alkanes containing the same number of carbon atoms alcohols are less volatile, have higher melting points and greater water solubility than the corresponding alkanes. 
  • Differences become much smaller as the length of the carbon chain increases. 
  • Alcohols have polar O-H bonds because of the difference in the electronegativity of the oxygen and hydrogen atoms therefore the alcohols are polar. 
  • The intermolecular forces will be very weak but there will also be much stronger hydrogen bonds. 
  • Hydrogen bonds require large amounts of energy to overcome them so alcohols have high melting points. 
  • Alcohols are soluble in water as the polar OH group can form hydrogen bonds with the water molecule. 
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Combustion of alcohols

  • Alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water.
  • The reaction is exothermic releasing a large quantity of energy in the form of heat. 

C2H5OH + 3O2 ---> 2CO2 + 3H2O

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Oxidation of primary alcohols

  • On gentle heating of alcohols under distillation with acidified potassium dichromate, an aldehyde is formed. 
  • If the primary alcohol is heated strongly under reflux with an excess of potassium dichromate a carboxylic acid is formed. 

C4H9OH + [O] ---> C4H8O + H2O 

Butan-1-ol + oxidising agent ---> butanal + water

C4H9OH + 2[O] ---> C3H7COOH + H2O

Butan-1-ol + oxidising agent ---> butanoic acid + water

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Oxidation of secondary alcohols

  • Secondary alcohols are heated under reflux to form ketones.
  • It is not possible to further oxidise ketones using acidified potassium dichromate. 

C3H7OH + [O] ---> C3H6O + H2O

Propan-2-ol + oxidising agent ---> Propanone 

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Dehydration of alcohols

  • Alcohols are heated under reflux in the presence of an acid catalyst such as concentrated sulfuric acid or concentrated phosphoric acid. 
  • Produces an alkene. 
  • It is an example of an elimination reaction. 
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Substitution reactions of alcohols

  • Alcohols react with hydrogen halides to form haloalkanes. 
  • The alcohol is heated under reflux with sulfuric acid and a sodium halide. 

C3H7OH + HBr ---> C3H7Br + H2O

Propan-2-ol + hydrogenbromide ---> 2-bromopropane + water

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