Alcohol

An alcohol is a carbon compound that contains the hydroxyl functional group –OH, with the general formula CₙH₂ₙ₊₁OH. Malay: alkohol · Chinese: 醇.

Ethanol (C₂H₅OH) is the alcohol the exam concentrates on. It can be made by the fermentation of glucose by yeast in the absence of air, or industrially by the addition of steam to ethene. Because of the –OH group, ethanol has a set of characteristic reactions: it burns in air to carbon dioxide and water; it is oxidised to a carboxylic acid by acidified potassium dichromate(VI), which turns from orange to green, or by acidified potassium manganate(VII), which turns from purple to colourless; it is dehydrated to an alkene by hot aluminium oxide or concentrated sulfuric acid; and it reacts with a carboxylic acid, in esterification, to form a sweet-smelling ester.

Example. When ethanol is warmed with acidified potassium dichromate(VI), the orange solution turns green as ethanol is oxidised to ethanoic acid. This colour change is a common way the exam signals that oxidation of an alcohol has taken place.

Confusion to avoid. Do not confuse an alcohol with a carboxylic acid. Both contain oxygen, but the alcohol carries –OH while the acid carries –COOH; the alcohol is neutral, whereas the carboxylic acid is a weak acid that turns litmus red. Oxidation converts the alcohol into the acid, so keep the direction of that change clear.

Ethanol also matters well beyond the exam, which makes the term easy to remember: it is the alcohol in alcoholic drinks, a common laboratory solvent, and increasingly a cleaner-burning fuel blended into petrol. In the laboratory the two oxidising mixtures act as reagents whose colour change reports the reaction as it happens, so holding on to orange-to-green for the dichromate and purple-to-colourless for the manganate(VII) gives you both the product and the observation an answer needs, which is often worth two separate marks.

The alcohols sit at the crossroads of the chapter, linking the alkenes that make them to the acids and esters they become, so mastering ethanol’s reactions pays off widely in SPM Chemistry.