See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Liebermann's nitroso reaction is a qualitative test for phenols. The mechanism and color changes proceed as follows: Step 1 - Concept: When phenol is heated with a small amount of potassium nitrite (KNO2) and concentrated sulfuric acid, a nitroso compound is formed which then condenses with excess phenol to give an indophenol-type dye. Step 2 - Color sequence reasoning: - Initially, the reaction mixture produces a deep red or brown-red colored melt/solution due to the formation of the nitroso-phenol intermediate and the indophenol in acidic medium. In concentrated H2SO4 (acidic conditions), the color observed is deep blue (the indophenol chromophore is protonated/in its acid form showing deep blue). - When the acid solution is diluted with water and then made alkaline (by adding NaOH or excess alkali), the color changes to green (the alkaline/deprotonated form of indophenol is green). - Therefore the sequential color change is: Red --> deep blue --> green. Step 3 - Why other options fail: - Option (a) states brown/red --> green --> deep blue, which reverses the last two colors; the deep blue in acid medium comes before the green in alkaline medium. - Option (c) red --> green --> white does not correctly describe the intermediate deep blue stage. - Option (d) white --> red --> green does not match the known observations of Liebermann's nitroso test. The correct sequential color change in Liebermann's nitroso reaction is: Red --> deep blue (in acid) --> green (in alkali). Therefore, the correct answer is B.