Question

The compound that gives precipitate on warming with aqueous AgNO3 (i.e. undergoes fastest SN1 solvolysis) is:

• A. I
• B. II
• C. III
• D. IV

Hint:

Work out what carbocation each C-Br bond gives on ionization and pick the most stable one.

Answer 1

The Correct Option is C

Step 1: Warming a bromo compound with aqueous AgNO3 tests SN1 solvolysis rate; faster carbocation formation gives a faster precipitate.

Step 2: Compound I, bromobenzene, has a highly unstable non-delocalized phenyl cation on ionization; essentially unreactive.

Step 3: Compound II, allylic bromide, gives an allylic cation with modest resonance stabilization.

Step 4: Compound III ionizes to give the tropylium cation, a fully aromatic 7-membered ring cation with 6 delocalized pi electrons, exceptionally stable.

Step 5: Compound IV, 4-bromopyridine, has an even more destabilized aryl-type cation due to the electron-poor ring.

Compound III solvolyzes fastest.

\[\boxed{\text{III}}\]