Question

Following are the four molecules ``P``, ``Q``, ``R`` and ``S``. Which one among the four molecules will react with H-Br(aq) at the fastest rate? Molecules:

\[ P: \text{Cyclic compound with two O groups attached to the ring.} \] \[ Q: \text{Cyclic compound with one O group and one CH\(_3\) group attached to the ring.} \] \[ R: \text{Cyclic compound with one O group attached to the ring and one CH\(_3\) group attached to the ring.} \] \[ S: \text{Cyclic compound with one CH\(_3\) group attached to the ring.} \]

• A. S
• B. Q
• C. R
• D. P

Hint:

The speed of reaction with H-Br is influenced by the electron-donating or electron-withdrawing groups attached to the molecule. Electron-donating groups stabilize the carbocation intermediate, leading to faster reactions.

Answer 1

The Correct Option is B

The objective is to identify the cyclic compound exhibiting the highest reaction rate with aqueous H-Br. An analysis of each option is provided:

1. Molecule P:
   • This molecule features two oxygen atoms bonded to a ring structure.
   • While the oxygen atoms can contribute to resonance stabilization, their electron-withdrawing nature decreases ring electron density, diminishing reactivity towards nucleophilic attack by H-Br.
2. Molecule Q:
   • This molecule comprises a ring substituted with one oxygen atom and one methyl (CH\(_3\)) group.
   • The oxygen atom enhances reactivity through electron donation via resonance, and the methyl group offers inductive stabilization.
   • This combined effect significantly increases the ring's reactivity towards H-Br, leading to a faster reaction.
3. Molecule R:
   • Similar to molecule Q, this molecule contains an oxygen atom and a methyl group attached to the ring, but their arrangement differs.
   • The specific positional isomerism and resonance effects, when compared to Q, suggest a less favorable reaction due to steric and electronic considerations.
4. Molecule S:
   • This molecule consists of a benzene ring with two methyl (CH\(_3\)) groups.
   • The absence of an oxygen atom renders it the least reactive among the presented options when exposed to H-Br.

Based on this assessment, molecule Q is predicted to react most rapidly with H-Br(aq). This is attributed to the electron-donating capacity of the oxygen atom facilitating the reaction, and the stabilizing influence of the methyl group via hyperconjugation on the transition state.

Correct Answer: Q