Which test changes color from orange to green upon oxidation of primary or secondary alcohol?

Which test changes color from orange to green upon oxidation of primary or secondary alcohol?

• A. Fehling's solution
• B. Benedict's solution
• C. Acidified potassium dichromate
• D. Tollen's reagent

Answer: (C)

The key idea is that this test uses an oxidizing agent whose color changes as it is reduced. Acidified potassium dichromate contains orange Cr(VI). When a primary or secondary alcohol is oxidized, Cr(VI) is reduced to Cr(III), which is green, so the solution shifts from orange to green. This specifically signals oxidation of primary alcohols to aldehydes (and further to carboxylic acids with strong oxidants) or secondary alcohols to ketones. The other tests rely on different chemistry: Fehling’s and Benedict’s detect reducing species via copper(II) reduction (blue to brick red/green), and Tollen’s detects aldehydes via silver reduction (blue to silver). They don’t show the orange-to-green change, so they don’t indicate oxidation of primary or secondary alcohols in the same way.

The key idea is that this test uses an oxidizing agent whose color changes as it is reduced. Acidified potassium dichromate contains orange Cr(VI). When a primary or secondary alcohol is oxidized, Cr(VI) is reduced to Cr(III), which is green, so the solution shifts from orange to green. This specifically signals oxidation of primary alcohols to aldehydes (and further to carboxylic acids with strong oxidants) or secondary alcohols to ketones. The other tests rely on different chemistry: Fehling’s and Benedict’s detect reducing species via copper(II) reduction (blue to brick red/green), and Tollen’s detects aldehydes via silver reduction (blue to silver). They don’t show the orange-to-green change, so they don’t indicate oxidation of primary or secondary alcohols in the same way.