Q.Which one of the following nitro compounds does NOT react with nitrous acid?

Step 1. Reaction of a nitroalkane with nitrous acid (giving a coloured nitrolic acid from a primary nitroalkane, or a coloured pseudonitrol from a secondary one) depends on the alpha-carbon still carrying at least one removable hydrogen, exactly as required for the nitro-aci tautomerism and acidic character covered earlier in this unit.

Step 2. Options (a), CH3-CH2-CH2-NO2 (1-nitropropane, primary, alpha-CH2 has two H), (b), (CH3)2CH-CH2-NO2 (a primary nitroalkane with the alpha-CH2 again bearing two H), and (d), a secondary nitro compound alpha to a ketone carbonyl (alpha-CH bearing one H), all retain an alpha-hydrogen and so can react. …

Check each option's nitro-bearing carbon for a remaining hydrogen -- only a tertiary nitroalkane, with three carbon substituents an …

• Assuming reactivity with nitrous acid depends on chain length or branching pattern generally -- the actual requirement is narrowly structural (an alpha-H must be present), which is why …