JEE Chemistry 2025 Question with Solution

Given: Four molecules $P$, $Q$, $R$ and $S$ are compared for their rate of reaction with H-Br(aq).

Find: Which molecule reacts at the fastest rate.

The solution states that the speed of reaction with H-Br depends on how well the substituents stabilize the intermediate formed during reaction. Electron-donating groups increase reactivity by stabilizing the intermediate.

From the extracted working:

1. Molecule P has two oxygen atoms attached to the ring. The solution notes that although resonance is possible, the overall effect is less favorable for the fastest reaction.
2. Molecule Q has one oxygen atom and one $CH_3$ group attached to the ring. The oxygen donates electron density by resonance, and the $CH_3$ group provides inductive or hyperconjugative stabilization.
3. Molecule R also has one oxygen atom and one $CH_3$ group, but the arrangement is stated to be less favorable than $Q$.
4. Molecule S lacks oxygen support and is therefore less reactive than $Q$.

The second extracted approach also concludes that $Q$ forms the most stable intermediate because it is resonance-stabilized.

Therefore, the molecule that reacts fastest with H-Br(aq) is Q. Hence, the correct option is B.

Given: Reaction rate with H-Br(aq) is to be compared for four cyclic compounds.

Find: The most reactive molecule.

Hint used in the source: The reaction is faster when the intermediate formed is better stabilized by electron-donating groups.

• In $P$, the two oxygen atoms do not make it the fastest according to the provided solution.
• In $Q$, the oxygen atom can participate through resonance and the $CH_3$ group helps stabilize the developing positive character.
• In $R$, a similar set of groups is present, but the source solution says the arrangement is less favorable than in $Q$.
• In $S$, the ring has alkyl substitution but lacks oxygen participation, so it is less reactive than $Q$.

Thus, based on the extracted solution text, $Q$ has the most favorable electronic stabilization and reacts at the fastest rate. The correct option is B.