3.5.5 Elimination Reactions of Alcohols

• Alcohols can also undergo dehydration to form alkenes

• Alcohol vapour is passed over a hot (600 C) catalyst of aluminium oxide (Al₂O₃) powder or pieces of porous pot

  • Excess hot, concentrated sulfuric acid or phosphoric acid is used as a catalyst

• Phosphoric(V) acid is used an alternative dehydrating agent

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

• This is an example of an elimination reaction

  • Elimination reactions involve a small molecule being formed as a by-product

  • The water molecule is formed from the -OH group of the alcohol and the H⁺ from the acid catalyst

  • The neighbouring hydrogen is then lost as a H⁺ ion, which regenerates the acid catalyst

• The reaction and mechanism for the dehydration of propan-1-ol is shown below

Dehydration of propan-1-ol mechanism

• Alkenes produced by this method can be used to produce addition polymers without using monomers derived from crude oil