Oxidation of alcohols

The oxidising agents

Two are used, and you need both:

• Acidified permanganate, $MnO_4^-/H^+$ — usually potassium permanganate with dilute sulfuric acid.

• Acidified dichromate, $Cr_2O_7^{2-}/H^+$ — usually potassium dichromate with dilute sulfuric acid.

• Both are warmed with the alcohol.

• Each has its own colour change, and you must quote the right one for the reagent named in the question.

Oxidising agent	Colour before	Colour after
$MnO_4^-/H^+$	purple	colourless
$Cr_2O_7^{2-}/H^+$	orange	green

What forms depends on the classification

This is the whole of the topic in three lines:

• Primary (1°) alcohol → carboxylic acid
• Secondary (2°) alcohol → ketone
• Tertiary (3°) alcohol → no reaction

Primary alcohols

• A primary alcohol is oxidised all the way to a carboxylic acid.

$$CH_3CH_2OH \xrightarrow{MnO_4^-/H^+} CH_3COOH$$

• Ethanol → ethanoic acid. This is what happens to wine left open — it goes sour.
• The number of carbons does not change: the –OH carbon becomes the –COOH carbon.
  • butan-1-ol → butanoic acid (still four carbons).

Secondary alcohols

• A secondary alcohol is oxidised to a ketone.
• The –OH carbon keeps its two carbon neighbours and gains a double-bonded oxygen.
• You are not required to name ketones in this standard — recognising the product is a ketone is enough.

Tertiary alcohols

• A tertiary alcohol is not oxidised at all.
• Its –OH carbon is bonded to three carbons, so there is no hydrogen on that carbon to remove, and oxidation of an alcohol requires one.
• The colour does not change — the permanganate stays purple, the dichromate stays orange.
• This "no reaction" is genuinely useful: it is how a tertiary alcohol is identified.

The method for any oxidation question

• Classify the alcohol first: 1°, 2° or 3°.
• Predict the product from the classification.
• Quote the colour change for the reagent actually named.
• If the alcohol is tertiary, say there is no reaction and no colour change — that is the answer, not a failure to find one.