Take the relative reactivities per hydrogen for $Cl X_{2}$ at 25 $^{\circ}$ C as primary : secondary : tertiary = 1 : 3.8 : 5. The alkane shown is monochlorinated at 25 $^{\circ}$ C. Which structure is the single most abundant monochloro product?

Answer: A

The alkane is 2-methylbutane (isopentane). Weights: C-2 tertiary

1 \times 5 = 5

; C-3 secondary

2 \times 3.8 = 7.6

; the six equivalent H on the two C-2 methyls (as C-1)

6 \times 1 = 6

; C-4 primary

3 \times 1 = 3

. Total 21.6, so 2-chloro-3-methylbutane (35.2%) is the major product, ahead of the tertiary chloride (23.1%).