Reactions of Alcohols (CIE AS Chemistry)

• 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

 The complete combustion of ethanol

Complete combustion of alcohols produces 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 

The substitution of alcohols produces 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

 

The reaction of ethanol with sodium metal

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

The reflux setup to convert an alcohol to an aldehyde

The oxidation of ethanol by acidified K₂Cr₂O₇ forms ethanal by distillation and ethanoic acid by refluxing

 

Oxidation of propan-2-ol by acidified K₂Cr₂O₇

The oxidation of secondary alcohols by acidified K₂Cr₂O₇ forms a ketone, under 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 

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

 

Example esterification reaction

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