See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: S_N1 reactivity depends on the stability of the carbocation formed after ionization (loss of the leaving group). More stable carbocations form faster, giving higher S_N1 reactivity. Carbocation stability order: allylic/benzylic > tertiary > secondary > primary. Step 2 - Analyze each compound: • Compound (1): n-butyl chloride - primary alkyl chloride. Upon ionization it forms a primary carbocation, which is very unstable. S_N1 reactivity: very low. • Compound (2): CH2=CH-CHCl-CH3 - secondary allylic chloride. Upon ionization it forms a secondary allylic carbocation (stabilized by resonance with the double bond: CH2=CH-CH(+)-CH3 <-> (+)CH2-CH=CH-CH3). Allylic stabilization makes this carbocation significantly more stable. S_N1 reactivity: highest. • Compound (3): The structure shown is a secondary alkyl chloride (isobutyl/sec-butyl type, no allylic or benzylic stabilization). Upon ionization it forms a secondary carbocation, which is more stable than primary but less stable than allylic secondary. S_N1 reactivity: intermediate. Step 3 - Order: Compound (2) [secondary allylic carbocation] > Compound (3) [secondary carbocation] > Compound (1) [primary carbocation]. This gives the decreasing order: 2 > 3 > 1. Step 4 - Why other options fail: • (a) 1 > 2 > 3: Incorrect; primary carbocation is least stable. • (c) 2 > 1 > 3: Incorrect; primary (1) is less reactive than secondary (3). • (d) 3 > 2 > 1: Incorrect; allylic stabilization makes (2) more reactive than simple secondary (3). Therefore, the correct answer is B.