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The reactivity order for the $\mathrm{S_N 1}$ mechanism among haloalkanes is
A1° > 2° > 3°
B3° > 2° > 1° > methyl
Cmethyl > 1° > 2° > 3°
Dall classes react at the same rate
Answer & Solution
Correct answer: B. 3° > 2° > 1° > methyl
1. SN1 mechanism: TWO-step process. Step 1: loss of leaving group → carbocation. Step 2: nucleophile attacks the carbocation.
2. The RATE-DETERMINING step is step 1 (carbocation formation). So the rate depends ONLY on the alkyl halide concentration (first-order kinetics) and on the STABILITY of the carbocation intermediate.
3. Carbocation stability order: TERTIARY > SECONDARY > PRIMARY > methyl (alkyl groups STABILISE the positive charge by hyperconjugation and induction).
4. So SN1 reactivity follows the same order: 3° > 2° > 1° > methyl.
5. This is the OPPOSITE order from SN2 (steric vs cation-stability factors).
6. Option A reverses the order. Option C is the SN2 order. Option D ignores the carbocation stability.
_Source: NCERT Class 12 Chemistry Part 2, Ch 6, §6.5.1 (SN1 mechanism — carbocation stability), p. 12._
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