See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: SN1 reaction rate depends on the stability of the carbocation intermediate formed after the leaving group departs. More stable carbocations form faster, giving higher SN1 rates. The order of carbocation stability is: tertiary > secondary > primary, but allylic and benzylic carbocations are stabilized by resonance. Step 1: Analyze each option: Option (a): tert-butyl chloride (3° carbocation) vs allyl chloride (allylic 1° carbocation, resonance-stabilized). A tertiary carbocation is generally more stable than an allylic primary carbocation, so tert-butyl chloride > allyl chloride for SN1 rate seems plausible. However, allylic systems can be quite reactive. The claim is tert > allylic, which is generally correct (tert carbocation is more stable than allylic primary). But this option is presented as correct — we need to check the others. Step 2: Option (b): isobutyl chloride (2-methyl-1-chloropropane) forms a primary carbocation upon ionization — specifically a primary non-stabilized carbocation. Allyl chloride (3-chloro-1-propene) forms an allylic carbocation stabilized by resonance over two carbons. An allylic primary carbocation is more stable than a simple primary carbocation. Therefore, isobutyl chloride < allyl chloride in SN1 rate. This is correct. Step 3: Option (c): n-propyl chloride (primary, non-stabilized) vs tert-amyl chloride (tertiary). Tertiary carbocations are far more stable than primary ones, so tert-amyl chloride should have a higher SN1 rate. The claim that primary > tertiary is incorrect. Step 4: Option (d): benzyl chloride (benzylic primary, resonance-stabilized carbocation) vs chlorobenzene (aryl chloride, vinyl-type). Chlorobenzene undergoes SN1 extremely poorly because the aryl carbocation is highly unstable (sp2 carbon, no resonance stabilization in the cation sense, and the lone pair on Cl is delocalized into the ring). Benzyl chloride forms a stable benzylic carbocation. So benzyl chloride should have a much higher SN1 rate than chlorobenzene, meaning benzyl chloride > chlorobenzene, not <. Option (d) claims benzyl < chlorobenzene, which is incorrect. Step 5: Why other options fail: - Option (a): While tert-butyl > allylic primary is defensible, this is not the "correct" answer per the given answer key. - Option (c): Primary carbocation more stable than tertiary is wrong. - Option (d): Claims aryl chloride reacts faster via SN1 than benzylic chloride, which is wrong. Option (b) correctly states that isobutyl chloride (non-stabilized primary carbocation) reacts more slowly via SN1 than allyl chloride (resonance-stabilized allylic carbocation), making the inequality isobutyl < allyl correct. Therefore, the correct answer is B.