See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: In an SN1 reaction, the rate-determining step is the unimolecular ionization of the substrate to form a carbocation intermediate. The rate depends only on the concentration of the substrate (Rate = k[RX]), and is independent of the nucleophile concentration. Reasoning: The stability of the carbocation intermediate determines how fast ionization occurs. Carbocation stability increases with increasing substitution: - Methyl carbocation (CH3+): least stable, primary - Primary (1°) carbocation (Et-Br → primary): very unstable - Secondary (2°) carbocation (i-Pr-Br → secondary): more stable - Tertiary (3°) carbocation (t-Bu-Br → tertiary): most stable Since more substituted substrates form more stable carbocations, they ionize faster. Therefore, the rate of SN1 reaction increases in the order: CH3-Br < Et-Br < i-Pr-Br < t-Bu-Br This means log(rate) increases as we go from CH3-Br to t-Bu-Br, producing an increasing curve on the graph. Why other options fail: (a) Shows a decreasing curve — this would represent SN2 reactivity (where steric hindrance decreases rate with increasing substitution), not SN1. (b) Shows a flat line — this would imply all substrates react at the same rate in SN1, which is incorrect since carbocation stability differs significantly. (d) Shows a sinusoidal/wave pattern — this has no chemical basis for SN1 reactions. Option (c) correctly shows log(rate) increasing from CH3-Br to t-Bu-Br, consistent with increasing carbocation stability and thus increasing SN1 rate. Therefore, the correct answer is C.