Q.Reaction of acetone with one of the following reagents involves nucleophilic addition followed by elimination of water. The reagent is a) Grignard reagent b) Sn / HCl c) hydrazine in presence of slightly acidic solution d) hydrocyanic acid

Step 1. The question asks which reagent gives nucleophilic addition FOLLOWED BY elimination of water, i.e. an addition-elimination sequence, not a simple one-step addition.

Step 2. Grignard reagent (a) and hydrocyanic acid (d) both add to the carbonyl carbon and stop there (giving, after work-up, an alcohol or a cyanohydrin respectively) -- pure addition, no water is subsequently eliminated.

Step 3. Sn/HCl (b) is a REDUCING combination (used for nitro-to-amine reduction elsewhere in the syllabus), not a carbonyl addition reagent at all.

Step 4. Hydrazine, however, first adds its NH2 nitrogen to the carbonyl carbon (as in any ammonia-derivative addition, Section 12.5.A.4), and the resulting addition product then loses a molecule of water to form the C=N-NH2 double bond of the hydrazone -- exactly the addition-then-elimination two-step pattern the question describes.

Distinguish reagents that stop at simple addition (Grignard, HCN) from ammonia-derivative reagents (hydrazine and its relatives) that specifically continue on to eliminate water and form a C=N bond.

• Picking hydrocyanic acid because 'HCN also reacts with carbonyls' -- true, but HCN addition stops at the cyanohydrin stage; it never eliminates water afterward.