SN1 Favored by Tertiary Halogenoalkanes

Steps:

• SN1 mechanism involves carbocation intermediate; stability determines pathway (tertiary > secondary > primary > methyl).
• Classify halogenoalkanes: tertiary form stable 3° carbocations and undergo SN1; primary and methyl favor SN2 due to unstable carbocations.
• Examine pairs: identify if both are tertiary for SN1.
• Select pair where both undergo SN1 based on structure.

Why A is correct:

• Tertiary halogenoalkanes form stable tertiary carbocations, enabling SN1 as per the mechanism's rate-determining step (R-LG → R⁺ + LG⁻).

Why the others are wrong:

• B: Both primary (ethyl); unstable 1° carbocations favor SN2.
• C: Both methyl; no carbocation possible, strictly SN2.
• D: Both methyl (same compound); strictly SN2.

Final answer: A