Alcohols

Ethanol — the alcohol in alcoholic drinks — can be oxidised in three ways. In each case the oxygen of the air or a chemical oxidising agent attacks the hydroxyl carbon, turning it into a carbonyl (C=O) carbon.

(a) Combustion of ethanol

• Ethanol burns in a plentiful supply of oxygen with an almost invisible pale-blue flame, giving carbon dioxide and water:

C₂H₅OH(l) + 3 O₂(g) → 2 CO₂(g) + 3 H₂O(l)

• The reaction is strongly exothermic, which is why ethanol is used as a fuel — for laboratory spirit burners, in cooking burners (methylated spirit), and as an additive to motor fuel (E10 petrol)
• A common mistake when balancing this equation is forgetting that ethanol already contains one oxygen atom of its own

(b) Aerobic oxidation by bacteria

• Bacteria in the genus Acetobacter can use atmospheric oxygen to oxidise ethanol to ethanoic acid (the acid in vinegar):

ethanol + oxygen → ethanoic acid + water

C₂H₅OH(aq) + O₂(g) → CH₃COOH(aq) + H₂O(l)

• A bottle of wine left uncorked on the kitchen counter for a few days develops a sharp vinegary taste because of exactly this microbial oxidation
• The same reaction is the industrial route to vinegar — wine or cider is left open to Acetobacter-rich air on purpose to sour it

(c) Oxidation with potassium dichromate

• In the school lab, ethanol is oxidised by heating it with acidified potassium dichromate(VI) solution (orange K₂Cr₂O₇ in dilute H₂SO₄) — the dichromate provides the oxygen
• The reaction is written as:

CH₃CH₂OH(l) + [O] → CH₃COOH(aq) + H₂O(l)

• The square-bracketed [O] is shorthand for "an atom of oxygen supplied by the oxidising agent"
• Observation: the orange potassium dichromate solution turns green as the dichromate ion is reduced to chromium(III) (Cr³⁺) while the ethanol is oxidised
• The orange-to-green colour change is the test result: a positive result for an alcohol being oxidised
• Apparatus: the ethanol–dichromate mixture is heated under reflux (with a vertical condenser fitted to the flask) so that volatile ethanol and ethanoic acid do not escape as they boil over