Which of the following compounds would not turn acidified potassium dichromate green?

Question

BlackTom AI · Accepted Answer

Acidified potassium dichromate (Cr2O7^2- / H+) is a strong oxidizing agent that is reduced from orange Cr(VI) to green Cr(III) when it oxidizes an organic substrate. Key mechanistic requirement: oxidation of an alcohol requires removal of a hydrogen from the carbon bearing the OH (an α‑hydrogen). Primary alcohols → aldehydes → carboxylic acids; secondary alcohols → ketones. Tertiary alcohols lack an α‑hydrogen on the carbon bearing OH, so they do not undergo oxidation under these mild conditions.

Step-by-step reasoning for each choice:
1) Butanal — this is an aldehyde. Aldehydes are readily oxidized to carboxylic acids by dichromate, reducing Cr(VI) to Cr(III). Result: orange → green. So butanal will turn dichromate green.
2) 2‑Pentanol — a secondary alcohol (the OH-bearing carbon has one hydrogen). It can be oxidized to a ketone (2‑pentanone) by dichromate, so it will reduce Cr(VI) and give the green color.
3) 1‑Methylcyclohexanol — the OH is on a carbon that also bears a methyl group and two ring carbon bonds, so that carbon is bonded to three carbons and has no hydrogen (a tertiary alcohol). Because there is no α‑hydrogen available for dehydrogenation, acidified dichromate cannot oxidize it under normal conditions; therefore it will not reduce Cr(VI) to Cr(III) and will not turn the solution green.
4) 4‑Penten‑2‑one (an α,β‑unsaturated ketone) — already a carbonyl compound and contains a C=C; while simple ketones are generally resistant to oxidation, the presence of an alkene or conjugation can allow oxidative cleavage or further oxidation under strong conditions; in typical dichromate tests supplied as an oxidizing reagent in solution, substrates with oxidizable functionalities (aldehydes, alcohols, certain alkenes) can reduce dichromate, so this option is less likely to be the uniquely unreactive one.

Why the correct answer is correct: 1‑methylcyclohexanol is a tertiary alcohol and lacks the C–H bond required for oxidation by Cr(VI); thus it does not cause the orange→green color change. Confirming final answer: 1‑methylcyclohexanol.

Which of the following compounds would not turn acidified potassium dichromate green?Single choice

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