An alcohol is oxidised to a carboxylic acid by an acidified oxidising agent: C2H5OH(aq) + 2[O] → CH3COOH(aq) + H2O(l). Acidified potassium dichromate(VI) turns orange to green; acidified potassium manganate(VII) turns purple to colourless.
The -OH group of an alcohol can be oxidised, and in the Form 5 Carbon Compounds chapter this is examined mainly for ethanol, which is oxidised to ethanoic acid (a carboxylic acid). Two things are tested: the equation using [O] to represent the oxygen supplied by the oxidising agent, and the sharp colour change that confirms the reaction has taken place.
The balanced equation
Chemists write oxidation by an oxidising agent using [O] for the oxygen atoms it provides. Ethanol is oxidised to ethanoic acid:
C2H5OH(aq) + 2[O] → CH3COOH(aq) + H2O(l)
Count the atoms to check the balance: the left side has C2, H6 and O1 from the ethanol plus O2 from the two [O], giving C2 H6 O3; the right side has C2 H4 O2 in ethanoic acid plus H2 O1 in water, giving C2 H6 O3 as well. The equation balances. The same pattern applies to other primary alcohols, for example methanol is oxidised to methanoic acid.
The oxidising agents and their colour changes
Two acidified oxidising agents are used, and each gives a colour change you must know:
- Acidified potassium dichromate(VI), K2Cr2O7, changes from orange to green as the orange dichromate(VI) ion is reduced to the green chromium(III) ion.
- Acidified potassium manganate(VII), KMnO4, changes from purple to colourless as the purple manganate(VII) ion is reduced to the colourless manganese(II) ion.
Either colour change is the evidence that the alcohol has been oxidised, so quoting the correct “from… to…” colours is worth full marks.
Conditions required
The mixture of ethanol and the acidified oxidising agent is heated, usually gently under reflux so that the volatile ethanol is not lost before it reacts. The oxidising agent must be acidified (with dilute sulfuric acid) to work. No other catalyst is needed.
Observations
- The reaction mixture shows the colour change above (orange to green, or purple to colourless).
- A liquid with the sharp, sour smell of vinegar is produced, this is ethanoic acid.
- The product is acidic: it turns blue litmus red and reacts with carbonates to give carbon dioxide, confirming the carboxylic acid.
Why oxidation matters in the carbon cycle of reactions
Oxidation links the alcohol family to the carboxylic acid family, so it sits between two reactions you also need. The ethanoic acid produced can then be used in esterification with another alcohol to make a sweet-smelling ester, while the ethanol itself can be made by fermentation or by the hydration of ethene. Seeing oxidation as the bridge from alcohol to acid helps you follow a whole sequence of organic conversions, which is a common structured-question theme.
Common mistakes to avoid
Use [O] for the oxidising agent’s oxygen and balance it, a frequent error is to forget the water product or to write only one [O]. Quote both parts of the colour change; “turns green” alone is incomplete without “from orange”. Do not say the product is an aldehyde only, at SPM level the expected product from the full oxidation of ethanol is ethanoic acid. State that the oxidising agent must be acidified and the mixture heated.
How it appears in the SPM exam
In Paper 2 (4541/2) you may be asked to write the equation with [O], name the oxidising agent, and give the colour change and the product. In Paper 1 (4541/1), objective items test the colour changes and the identity of the acid formed. In Paper 3 (4541/3), the orange-to-green or purple-to-colourless change is a practical observation you should be able to record and interpret. Anchor your answer on the alcohol becoming a carboxylic acid, and the whole question falls into place.
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