Reactions of Alcohols (Cambridge International AS Chemistry)

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

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

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 concentrated H₂SO₄ or concentrated 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:

  • They form aldehydes which can undergo further oxidation to form carboxylic acids

• Secondary alcohols can be oxidised

  • They form ketones only

• Tertiary alcohols do not undergo oxidation

Oxidising agents

• The oxidising agents of alcohols include acidified K₂Cr₂O₇ or acidified KMnO₄

• Acidified potassium dichromate(VI), K₂Cr₂O₇, is an orange oxidising agent

  • Acidified means 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

• When a warm primary alcohol is treated with an oxidising agent, it is initially oxidised to an aldehyde

  • The aldehyde has a lower boiling point than the alcohol, so it can be distilled off as it forms to prevent further oxidation
  • If it is not distilled, continued heating under reflux with excess oxidising agent will convert the aldehyde into a carboxylic acid

• In contrast, secondary alcohols are oxidised to ketones, which cannot be further oxidised, so there is no need to distil them off immediately

The distillation setup to convert an alcohol to an aldehyde

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

Oxidation of propan-2-ol by acidified K₂Cr₂O₇

The oxidation of secondary alcohols by acidified potassium dichromate 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

Exam Hint The first part of an ester’s name comes from the alcohol, whereas the second part comes from the carboxylic acid

So, if ethanol and propanoic acid react together, this will make the ester ethyl propanoate