Distinguishing primary vs. secondary alcohols by oxidation to aldehyde vs. ketone

Steps:

• Propan-1-ol (primary) undergoes mild oxidation to propanal (aldehyde).
• Propan-2-ol (secondary) undergoes mild oxidation to propanone (ketone).
• Fehling's reagent reacts with aldehydes to give a red precipitate but not with ketones.
• Thus, only propan-1-ol shows a positive Fehling's test after mild oxidation.

Why D is correct:

• Primary alcohols oxidize to aldehydes (RCH2OH → RCHO), which reduce Fehling's to Cu2O (red ppt); secondary alcohols form ketones (R2CHOH → R2C=O), which do not react.

Why the others are wrong:

• A: Both alcohols decolorize acidified KMnO4 via oxidation to carbonyl compounds.
• B: Dehydration of both yields propene, which adds Br2 to form dibromopropane.
• C: Both alcohols are oxidized by acidified K2Cr2O7 (primary to acid, secondary to ketone), with similar color changes.

Final answer: D