Assertion (A): Radical monobromination of 2-methylpropane gives 2-bromo-2-methylpropane almost exclusively (more than 99%, taking 1 deg : 3 deg = 1 : 1600 per H). Reason (R): Br^. is more reactive than Cl^., and therefore more selective.

Both A and R are true and R is the correct explanation of A.

Both A and R are true but R is not the correct explanation of A.

JEE Advanced levelSingle correct optionDifficulty 3 of 3g3156

Assertion (A): Radical monobromination of 2-methylpropane gives 2-bromo-2-methylpropane almost exclusively (more than 99%, taking $1^{\circ} : 3^{\circ} = 1 : 1600$ per H). Reason (R): $Br X^{∙}$ is more reactive than $Cl X^{∙}$ , and therefore more selective.

2-Methylpropane has 9 primary H and 1 tertiary H, so the tertiary product is

1600/ (9 + 1600) \times 100 = 99.4

per cent. A is true. R is false:

Br X^{∙}

is less reactive than

Cl X^{∙}

, and the high selectivity arises from this lower reactivity (a late transition state), not from a higher one.