Reactions of Alcohols

Alcohols take part in several important types of reaction. This lesson gives an overview; each is covered in detail later.1. Combustion: alcohols burn in plenty of oxygen to form carbon dioxide and water, releasing a lot of energy, so they are used as fuels. For ethanol:C₂H₅OH + 3O₂ → 2CO₂ + 3H₂OWhen balancing, remember the O atom already in the alcohol. For propan-1-ol:C₃H₇OH + 4½O₂ → 3CO₂ + 4H₂O(Products contain 6 + 4 = 10 O atoms; one comes from the alcohol, so 9 O atoms, 4.5 O₂, are needed.)2. Halogenation: the -OH group is replaced by a halogen atom, forming a halogenoalkane. Reagents include phosphorus(V) chloride, PCl₅.3. Dehydration: heating with a concentrated acid catalyst removes a molecule of water, forming an alkene. This is an elimination reaction.4. Oxidation: primary and secondary alcohols are oxidised by heating with acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄. The dichromate(VI) ions are reduced, so the solution changes colour from orange to green. Primary alcohols give aldehydes and then carboxylic acids; secondary alcohols give ketones; tertiary alcohols are not oxidised.Together these reactions make alcohols very useful intermediates: they can be converted into halogenoalkanes, alkenes, aldehydes, ketones and carboxylic acids.