elimination & substitution reactions of alcohols

• elimination reactions of alcohols?

  • Alcohols can also undergo dehydration to form alkenes
  • This is an example of an elimination reaction
  • Elimination reactions involve a small molecule leaving the parent molecule as a byproduct
  • In this case, the small molecule is a water molecule
  • The water molecule is formed from the -OH group and a hydrogen atom from the adjacent carbon atom
• elimination reaction process (dehydration)?

  • Alcohol vapour is passed over a hot catalyst of aluminium oxide (Al2O3) powder or pieces of porous pot
  • Excess hot, concentrated sulfuric acid or phosphoric acid is used as a catalyst
• substitution reactions of alcohols?

  • In the substitution of alcohols, a hydroxy group (-OH) is replaced by a halogen to form an haloalkane 
  • 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)
• What are the reagents, conditions and product of elimination of an alcohol?

  reagents: concentrated H2SO4 or concentrated H3PO4

  conditions: 180 oC

  product: alkene (and water)
• elimination?

  alkenes can be formed by the dehydration of alcohols

  a molecule of water is removed from the molecule

  the alcohol is heated with concentrated phosphoric acid