Propane, $CH X_{3} CH X_{2} CH X_{3}$ , is monochlorinated by free radicals at $2 5^{\circ} C$ . Take the relative rates of abstraction per hydrogen as primary : secondary $= 1 : 3.8$ . The percentage of 2-chloropropane in the monochloro product mixture is closest to

Answer: B

Propane has 6 primary and 2 secondary hydrogens. Weighted reactivity: primary

6 \times 1 = 6.0

, secondary

2 \times 3.8 = 7.6

, total

13.6

. Fraction of 2-chloropropane

= 7.6/13.6 = 55.9 %

(ignoring the number of hydrogens gives a wrong 79.2 %).