Compound from the following that will not produce precipitate on reaction with AgNO₃ is :

Solution

Chemistry ma’am · 3 steps

Silver nitrate pulls off bromide only if the carbocation left behind is stable, so judge each cation.

1. What AgNO3 needs. Ag⁺ takes away Br⁻ as a pale yellow AgBr precipitate. That only happens if the C–Br bond can break to give a carbocation, so the more stable the cation, the faster the precipitate.

   R-Br + Ag⁺ → R⁺ + AgBr v

   Option	Where Br sits	Cation after losing Br⁻	AgBr precipitate?
   (a) Ph-CH=CH-CH₂Br	allylic sp³ carbon	allylic, conjugated with the ring, resonance stabilised	yes
   (b) 1-bromocyclohexene	vinylic sp² carbon	vinyl cation, very unstable	no
   (c) Ph-CH₂Br	benzylic sp³ carbon	benzyl, resonance stabilised	yes
   (d) 3-bromocyclopropene	ring sp³ carbon	cyclopropenyl, aromatic (2π electrons)	yes, very fast

2. Why vinylic fails. In 1-bromocyclohexene, Br sits on a double-bonded carbon. Resonance with the C=C gives the C–Br bond partial double bond character, and the vinyl cation it would form is very unstable, so the bond does not ionise.
3. So. Only (b), the vinylic bromide, gives no precipitate with AgNO₃.

The idea to keep. For AgNO₃ tests, ask what carbocation forms when the halide leaves. Allylic, benzylic or aromatic cations precipitate fast; vinylic and aryl halides do not react.

Answer from the NTA final answer key. Solution by PrepWiser.