Which of the following compounds is formed by the oxidation of ethanol?

Analyze the question: You're asked which compound forms when ethanol (C₂H₅OH) is oxidised.

Concept: Primary alcohols oxidise in stages:

Primary alcohol → aldehyde → carboxylic acid

With a strong oxidising agent such as acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) or acidified potassium permanganate (KMnO₄/H⁺), the oxidation goes all the way to the carboxylic acid. Both the alkyl chain and the carbon count stay the same, so the product has 2 carbons.

Solution:

1. Ethanol: CH₃CH₂OH
2. First oxidation gives ethanal (an aldehyde): CH₃CHO
3. Further oxidation gives ethanoic acid (a carboxylic acid): CH₃COOH

Overall: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

Checking the options:

• A. C₂H₄CO₂H is propanoic acid (3 carbons). Oxidation does not add a carbon, so this is wrong.
• B. C₂H₅OH is ethanol itself, the starting material, so this is wrong.
• C. CH₃OH is methanol (1 carbon). Oxidation does not remove a carbon, so this is wrong.
• D. CH₃CO₂H is ethanoic (acetic) acid, which is correct.

Exam tip: Count the carbons. Oxidation of an alcohol keeps the same number of carbons, and only the functional group changes: –CH₂OH → –CHO → –COOH. This is also why vinegar (ethanoic acid) forms when palm wine or wine is left exposed to air. Bacteria oxidise the ethanol in it.