Which of the following compounds will form the precipitate with aq. AgNO₃ solution most readily?

Solution

Chemistry ma’am · 3 steps

Silver nitrate pulls off bromide, so ask which leftover CH₂⁺ gets the best lone-pair help.

1. What precipitates. Ag⁺ pulls the bromide away as AgBr, leaving a carbocation behind. This is an S_N1-type step, so the compound that forms the most stable carbocation gives the precipitate fastest.
2. Lone pair donation. When O or N sits directly on the CH₂⁺ carbon, its lone pair shares the positive charge. The more freely that lone pair is available, the more stable the cation. N is less electronegative than O, so it donates better, giving an iminium ion R₂N+=CH₂.

   Option	Atom next to CH₂⁺	Lone pair availability	Cation stability
   (a)	O of an ether	good, but O holds it tightly	stabilised, less than N
   (b)	N of piperidine, alkyl groups only	fully free	most stable iminium ion
   (c)	N attached to benzene	partly delocalised into the ring	weaker iminium
   (d)	none: Br is on the second carbon from N	cannot reach the cation	plain primary cation, poor

3. So. In (b) the nitrogen lone pair is not shared with any ring, so it stabilises the CH₂⁺ best. Option (b) gives the AgBr precipitate most readily.

The idea to keep. For AgNO₃ tests, rank carbocation stability. A heteroatom directly on the cationic carbon helps most when its lone pair is free: alkyl N beats aryl N, and N beats O.

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