2.9.2 Reactions of Alcohols

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

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 + 3PI₃ → 3C₂H₅I + H₃PO₃

• This reaction results in the formation of phosphoric 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=CH₂ + H₂O

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