Distinguishing carbonyl compounds using selective reagents

Steps:

• Both compounds react with 2,4-DNPH, forming orange precipitates, indicating both are carbonyls (aldehyde and ketone), so it cannot distinguish them.
• Acidified K2Cr2O7 oxidizes the aldehyde to a carboxylic acid (color change from orange to green), but not the ketone, distinguishing them.
• Alkaline I2 (iodoform test) gives a yellow precipitate with the methyl ketone, but not the aldehyde, distinguishing them.
• Thus, only reagents 1 and 3 differentiate the compounds.

Why B is correct:

• B selects 1 and 3, matching the tests that uniquely react with one carbonyl type per the oxidation and iodoform reaction definitions.

Why the others are wrong:

• A includes 2, which fails to distinguish as both form hydrazones.
• C excludes 1, missing the oxidation distinction for aldehydes.
• D selects only 2, which reacts with both carbonyls indiscriminately.

Final answer: B