See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: These are solvolysis reactions of exo-norbornyl nosylates (sulfonate esters) with water. The key mechanistic feature is neighboring group participation (anchimeric assistance) by the double bond (pi-bond participation) in the norbornene system. The exo-ONS undergoes ionization assisted by the C=C pi electrons, forming a non-classical carbocation (a norbornyl cation stabilized by homoconjugation/pi participation). The rate of solvolysis depends on how well the double bond can stabilize the developing positive charge in the transition state. Step 1 - Identify the effect of methyl substitution on the double bond: - In structure (i): unsubstituted double bond (no methyl groups on the alkene carbons). Pi participation is present but unenhanced. - In structure (ii): one methyl group on the double bond carbon (C3-methyl norbornene nosylate). One methyl group increases electron density on the double bond, modestly enhancing pi participation and stabilizing the cationic transition state. - In structure (iii): two methyl groups on both double bond carbons (2,3-dimethyl norbornene nosylate). Two methyl groups provide maximum electron density to the double bond, giving the greatest pi participation and the most stabilized non-classical carbocation transition state. Step 2 - Predict relative rates: Greater alkyl substitution on the double bond → greater electron density available for pi participation → greater homoallylic/pi assistance → lower activation energy → faster rate of solvolysis. Therefore: (iii) > (ii) > (i). Step 3 - Eliminate wrong options: (a) (i) > (ii) > (iii): This would imply more substitution slows the reaction, the opposite of pi-participation logic. (b) (ii) > (i) > (iii): This is inconsistent; the disubstituted alkene should be fastest. (d) (iii) > (i) > (ii): This would mean one methyl group is worse than no methyl group, which contradicts the electron-donating effect. Only option (c) (iii) > (ii) > (i) is consistent with increasing pi-bond electron density facilitating homoallylic pi participation in the rate-determining ionization step. Therefore, the correct answer is C.