2-Methylbutane undergoes free-radical monochlorination. Take relative reactivity per hydrogen as primary : secondary : tertiary $= 1 : 4 : 5$ . The percentage of 2-chloro-3-methylbutane among all monochloro constitutional isomers is nearest to

Answer: B

Hydrogens: 6 on the two methyls at C2 (primary), 3 on C4 (primary), 2 on C3 (secondary), 1 on C2 (tertiary). Weighted: 6, 3,

2 \times 4 = 8

1 \times 5 = 5

; total

= 22

. 2-Chloro-3-methylbutane comes from the secondary hydrogens:

8/22 = 0.364

, about 36.4%.