10.4 - Alcohols
10.4.1: Describe, using equations, the complete combustion of alcohols.
Alcohols burn in air with a pale blue flame forming carbon dioxide and water. This reaction is used as an energy source, for example, in gasohol (petrol/ethanol mixture) vehicles in Brazil.
When constructing the equation for combustion it is important to remember that the alcohol already contains an oxygen atom.
Complete combustion of ethanol:
10.4.2: Describe, using equations, the oxidation reactions of alcohols.
Primary and secondary alcohols are oxidised by heating with an acidic solution of potassium dichromate(VI) to aldehydes or ketones, respectively.
Tertiary alcohols cannot be oxidised in this context.
Primary alcohols are oxidised to aldehydes, with the alcohol group becoming a carbonyl group:
However, aldehydes themselves are oxidised under the same conditions to carboxylic acids:
To prevent the oxidation going all the way to a carboxylic acid, the reaction can be carried out in a distillation flask attached to a water jacket condensor. As soon as the aldehyde is formed, it distills off and escapes the oxidising agent (acidified potassium dichromate(VI))
Secondary alcohols are oxidised by heating with acidified potassium dichromate(VI) to ketones (alkanones)
Ketones cannot be further oxidised.
These cannot be oxidised as the carbon that holds the alcohol group has no hydrogen atoms to replace.
10.4.3: Determine the products formed by the oxidation of primary and secondary alcohols.
As stated above, primary alcohols give aldehydes and then carboxylic acids. Secondary alcohols give ketones.
Example: Ethanol gives ethanal followed by ethanoic acid.
Example: Propan-2-ol gives propanone