L10 - Reactions of Alcohols

objectives

• write equations, state conditions and predict products to represent the reactions of alcohols, including but not limited to:
  • combustion
  • dehydration
  • substitution with HX

Dehydration of Alcohols to form Alkenes

Reagent/catalyst:

• conc. sulphuric acid (H₂SO₄) or conc. phosphoric acid (H₃PO₄)

Conditions:

• reflux at 180°C

Product:

• alkene

Equation

C₂H₅OH(l) ——> CH₂ = CH₂(g) + H₂O(l)

Mechanism:

Note 1

• There must be an H on a carbon atom adjacent the carbon with the OH

Note 2

• Alcohols with the OH in the middle of a chain can have two ways of losing water.
• In Step 3 of the mechanism, a proton can be lost from either side of the carbocation. This gives a mixture of alkenes from unsymmetrical alcohols...

Combustion of Alcohols

• Alcohols make useful fuels

C₂H₅OH(l) + 3O₂(g) ———> 2CO₂(g) + 3H₂O(l)

• Advantages
  • have high enthalpies of combustion
  • do not contain sulphur so there is less pollution
  • can be obtained from renewable resources

Substitution with Halogen Halides (HX)

Reagent(s):

conc. hydrobromic acid HBr(aq) or sodium (or potassium) bromide and concentrated sulphuric acid

Conditions:

reflux

Product:

haloalkane

Equation:

C₂H₅OH(l) + conc. HBr(aq) ———> C₂H₅Br(l) + H₂O(l)

Mechanism: