3.4.2 Reactions of Alcohols

• Alcohols are reactive molecules which undergo a wide range of 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

Complete combustion of alcohols to produce carbon dioxide and water

Substitution of alcohols

• In the substitution of alcohols, a hydroxy group (-OH) is replaced by a halogen to form an halogenoalkane
• 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)
  • PCl₃ and heat
  • PCl₅ at room temperature
  • SOCl₂

Substitution of alcohols to produce halogenoalkanes

Reaction with Na

• When an alcohol reacts with a reactive metal such as sodium (Na), the oxygen-hydrogen bond in the hydroxy group breaks
• Though the reaction is less vigorous than sodium reacting with water, hydrogen gas is given off and a basic compound (alkoxide) is formed
  • If the excess ethanol is evaporated off after the reaction a white crystalline solid of sodium alkoxide is left

Alcohol + sodium → sodium alkoxide + hydrogen

• The longer the hydrocarbon chain in the alcohol, the less vigorous the reaction becomes

Alcohols react with Na to form a basic sodium alkoxide salt and hydrogen gas

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
• Acidified potassium dichromate(VI), K₂Cr₂O₇, is an orange oxidising agent 
  • Acidified means that that the potassium dichromate(VI) is in a solution of dilute acid (such as dilute sulfuric acid) 
  • For potassium dichromate(VI) to act as an oxidising agent, it itself needs to be reduced
  • This reduction requires hydrogen (H⁺) ions which are provided by the acidic medium
    • When alcohols are oxidised the orange dichromate ions (Cr₂O_7^2-) are reduced to green Cr³⁺ ions

• Acidified potassium manganate(VII), KMnO₄, is a purple oxidising agent 
  • As with acidified K₂Cr₂O₇ the potassium manganate(VII) is in an acidic medium to allow reduction of potassium manganate(VII) to take place
    • When alcohols are oxidised, the purple manganate ions (MnO₄^-) are reduced to colourless Mn²⁺ ions

• Warm primary alcohol is added to the oxidising agent
• The formed aldehyde has a lower boiling point than the alcohol reactant so it can be distilled off as soon as it forms 
• If the aldehyde is not distilled off, further refluxing with excess oxidising agent will oxidise it to a carboxylic acid
• Since ketones cannot be further oxidised, the ketone product does not need to be distilled off straight away after it has been formed

Oxidation of ethanol by acidified K₂Cr₂O₇ to form an aldehyde by distillation and carboxylic acid by refluxing

Oxidation of propan-2-ol by acidified K₂Cr₂O₇ to form a ketone by distillation

Dehydration of alcohols

• Alcohols can also undergo dehydration to form alkenes
  • Dehydration is a reaction in which a water molecule is removed from a larger molecule
  • A dehydration reaction is a type of elimination reaction

• Alcohol vapour is passed over a hot catalyst of aluminium oxide (Al₂O₃) powder OR pieces of porous pot or pumice as well as concentrated acid can be used as catalysts

Dehydration of ethanol using aluminium oxide as a catalyst forms ethene gas, which can be collected over water

Esterification of Alcohols

• Esterification is a condensation reaction between a carboxylic acid and an alcohol to form an ester and a water molecule
• For esterification to take place, the carboxylic acid and alcohol are heated under reflux with a strong acid catalyst (such as H₂SO₄ or H₃PO₄)

Carboxylic acid + alcohol → ester + water

• The reaction is reversible so an equilibrium mixture can be established with all the reactants and products
• Esters have sweet, fruity smells

Esterification of ethanol and ethanoic acid using a strong acid catalyst to form ethyl ethanoate and water