Which of the following reagents can cause oxidation of an alcohol? 1. Hot concentrated H2SO4 2. Alkaline KMnO4 3. Acidified K2Cr2O7 4. Acidified KMnO4 Select the answer using the code given below.

Correct — C, 2 and 3 only. Alkaline potassium permanganate and acidified potassium dichromate are the two oxidising agents school chemistry names for converting an alcohol to a carboxylic acid — ethanol to ethanoic acid is the standard example, and the colour change of each reagent (purple fading, orange turning green) is what signals the reaction. Hot concentrated sulphuric acid is not an oxidising route here at all; it dehydrates ethanol to ethene, removing water rather than adding oxygen. That removes statement 1 and with it options (a) and (b), and since statements 2 and 3 are both genuine oxidants, option (d) with only statement 4 cannot stand either.

• (a)1, 2 and 3 — Two right and one wrong. Statements 2 and 3 are the textbook oxidising agents, but hot concentrated sulphuric acid acts on ethanol as a dehydrating agent, giving ethene and water at about 443 K.
• (b)1, 3 and 4 — Carries the same error about sulphuric acid and also drops statement 2, alkaline potassium permanganate, which is one of the two reagents the textbook names explicitly.
• (d)4 only — Far too narrow. It rejects alkaline permanganate and acidified dichromate, the two reagents most closely associated with the oxidation of ethanol, and keeps only one.

Oxidation of an alcohol means adding oxygen or removing hydrogen, and a primary alcohol goes first to an aldehyde and then to a carboxylic acid. The reagents used in the school laboratory are alkaline potassium permanganate, which fades from purple as it is consumed, and acidified potassium dichromate, which turns from orange to green. Because these reagents visibly change colour, they double as tests. Concentrated sulphuric acid belongs to a different family of reactions altogether: with ethanol at high temperature it removes a molecule of water and gives ethene, an elimination rather than an oxidation.

The safest way through this item is elimination rather than an attempt to judge all four reagents on their merits. Statement 1 is the one whose behaviour is textbook and unambiguous — sulphuric acid dehydrates ethanol — and rejecting it kills two options at once. Acidified permanganate is, in wider chemistry, a perfectly good oxidising agent as well, but there is no option that takes statements 2, 3 and 4 together, so the item is asking for the pair the syllabus names, and only one option offers exactly that pair.

• Ethanol is oxidised to ethanoic acid by alkaline potassium permanganate or by acidified potassium dichromate.
• Alkaline potassium permanganate loses its purple colour as it is used up; acidified dichromate goes from orange to green.
• Hot concentrated sulphuric acid at about 443 K dehydrates ethanol to ethene.
• A primary alcohol oxidises first to an aldehyde and then to a carboxylic acid.
• Oxidising agents used in this way are often written as [O] above the arrow in a school equation.

• Treating concentrated sulphuric acid as an all-purpose oxidising agent; with ethanol it dehydrates.
• Expecting an option containing statements 2, 3 and 4 to exist — the paper does not offer one.
• Confusing oxidation of ethanol with its combustion; combustion goes all the way to carbon dioxide and water.

A code item in which one clearly wrong statement carries two options with it, so identifying the dehydrating agent is worth more than judging the oxidants.

• practice — not a real PYQ

  Ethanol heated with excess concentrated sulphuric acid at about 443 K gives

  • (a)ethanoic acid
  • (b)ethene
  • (c)ethyl ethanoate
  • (d)methane

  Answer(b) ethene — the acid acts as a dehydrating agent, removing a molecule of water from ethanol.

• practice — not a real PYQ

  The colour change seen when acidified potassium dichromate oxidises an alcohol is

  • (a)purple to colourless
  • (b)orange to green
  • (c)green to orange
  • (d)colourless to pink

  Answer(b) orange to green — dichromate is reduced to chromium(III), which is green in solution.