The reactivity order for the $\mathrm{S_N 1}$ mechanism among haloalkanes is

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: syllabus Chemistry Part 2, Ch 6, §6.5.1 (SN1 mechanism — carbocation stability), p. 12._