See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Concept: An oxidising agent is required when a substrate undergoes oxidation, i.e., when there is a loss of hydrogen, gain of oxygen, or an increase in the oxidation state of carbon. Step 1: Analyze each option for changes in oxidation state. Option (a): CH3-CH=CH2 --> CH3-CH2-CH3 Propene is converted to propane. This involves addition of H2 (reduction of the double bond). The carbon atoms gain hydrogen, so this is a REDUCTION. No oxidising agent needed; a reducing agent (H2/catalyst) is required. Option (b): CH3-CH2OH --> CH3CHO Ethanol is converted to acetaldehyde (ethanal). The -CH2OH group loses 2 hydrogen atoms to become -CHO. Loss of hydrogen = oxidation of the substrate. Therefore, an OXIDISING AGENT is required for this transformation. Option (c): CH3-CH2Cl --> CH3-CH3 Chloroethane is converted to ethane. This involves replacement of Cl with H (reduction). A reducing agent is needed, not an oxidising agent. Option (d): CH3-CH2OH --> CH3-CH2Cl Ethanol is converted to chloroethane. This is a substitution reaction (OH replaced by Cl), not an oxidation-reduction process in the classical sense. It requires a reagent like HCl or PCl5, not an oxidising agent. Step 2: Conclude which reaction requires an oxidising agent. Only option (b) involves oxidation of the organic substrate (alcohol to aldehyde), requiring an oxidising agent such as PCC, K2Cr2O7/H2SO4, or Cu at high temperature. Therefore, the correct answer is B.