Reactions of Alcohols (Edexcel International A Level Chemistry)

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Alcohols - Reactions

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

Conversions to halogenoalkanes

  • These reactions involve replacing the hydroxyl group in an alcohol molecule with a halogen atom(known as halogenation)
  • Different methods are required for each halogen

Chlorination

  • Phosphorus(V) chloride is added to the alcohol resulting in a vigorous reaction at room temperature
    • This means the mixture doesn't need heating
  • This reaction is used as a qualitative test for the presence of the -OH group 
    • If you add PCl5 to an unknown liquid the evolution of steamy fumes (HCl gas) is evidence for the presence of the -OH group
  • Two inorganic products are formed: phosphoryl chloride and hydrogen chloride

CH3CH2CH2OH + PCl5 → CH3CH2CH2Cl + POCl3 + HCl

  • Chlorination of tertiary alcohols can be carried out in a different way by mixing (shaking) with hydrochloric acid at room temperature
  • An example equation for the reaction of 2-methyl propan-2-ol is:

(CH3)3COH + HCl → (CH3)3CCl + H2O

  • This reaction does not work for primary and secondary alcohols

Bromination

  • This reaction is carried out using a warmed mixture of potassium bromide and 50% concentrated sulfuric acid with the reacting alcohol
  • More concentrated sulfuric acid would would oxidise bromide ions to bromine resulting in different products
  • The reaction can be written as two equations as the inorganic reactants first react together to form hydrogen bromide and  potassium sulfate

2KBr + H2SO4  → K2SO4 +2HBr

  • The resulting hydrogen bromide then reacts with the alcohol, for example the reaction with butan-1-ol would be as follows:

CH3CH2CH2CH2OH + HBr → CH3CH2CH2CH2Br + H2O

Iodination

  • This reaction is carried out using a mixture of red phosphorus and iodine with the alcohol whilst heating under reflux
  • Similar to bromination, the reaction can be written as two equations as the inorganic reactants first react to form phosphorus(III) iodide

2P + 3I2 → 2PI3

  • The reaction for the iodination of ethanol would be:

3C2H5OH + PI3 → 3C2H5I + H3PO3

  • This reaction results in the formation of phosphorous acid as shown above

Dehydration to Alkenes

  • Dehydration is done by heating the alcohol with concentrated phosphoric acid
  • The reaction is similar to the elimination reaction of a halogenoalkene
    • The OH group and hydrogen of adjacent carbons are removed forming a C=C bond
  • The equation for the dehydration of ethanol would be

CH3CH2OH → CH2=CH+ H2O

  • Phosphoric acid does not appear in the equation as the water formed dilutes the concentrated phosphoric acid

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.