Chemistry - Hydrocarbon Question with Solution | TestHub

Question Explanation:

We are asked to compare the rate of acid-catalyzed hydration of different styrene derivatives.

Concept :

This question is based on Electrophilic addition of H₃O⁺ to alkenes, Carbocation stability, Resonance effects, Electron-donating vs electron-withdrawing substituents.

Solution :

Alkene + H₃O⁺ → carbocation intermediate → OH⁻ adds → alcohol.

▪ Rate depends on the stability of carbocation formed.

▪ Electron-donating groups increase rate.

▪ Electron-withdrawing groups decrease rate.

(I) Styrene ( $\mathrm{Ph}-\mathrm{CH}={\mathrm{CH}}_2$ ): carbocation formed is benzylic → resonance stabilized.

(II) p -Hydroxy styrene $\left(\mathrm{p}-\mathrm{OH}-{\mathrm{C}}_6{\mathrm{H}}_4-\mathrm{CH}={\mathrm{CH}}_2\right):-\mathrm{OH}$ is electron-donating via resonance → stabilizes carbocation more → rate increases.

(III) p-Nitro styrene $\left(\mathrm{p}-{\mathrm{NO}}_2-{\mathrm{C}}_6{\mathrm{H}}_4-\mathrm{CH}={\mathrm{CH}}_2\right):-{\mathrm{NO}}_2$ is strongly electron-withdrawing → destabilizes carbocation ⟶ rate decreases.

(IV) p-Methyl styrene ( $\mathrm{p}-{\mathrm{CH}}_3-{\mathrm{C}}_6{\mathrm{H}}_4-\mathrm{CH}=\mathrm{CH}2$ ): $-{\mathrm{CH}}_3$ is weakly electron-donating → slightly stabilizes carbocation ⟶ rate slightly higher than styrene.

Final answer : Option(C)