In `S_(N)2` reactions, the correct order of reactivity for the following compounds:
`CH_(3)CI,CH_(3)CH_(2)CI,(CH_(3))_(2)CHCI` and `(CH_(3))_(2)C CI` is:

Options

A. `CH_(3)Cl gt (CH_(3))_(2)CHCl gt CH_(3)CH_(2)Cl gt (CH_(3))_(3)"CCl"`
B. `CH_(3)Cl gt CH_(3)CH_(2)Cl gt (CH_(3))_(2)CHCl gt (CH_(3))_(3)"CCl"`
C. `CH_(3)CH_(2)Cl gt CH_(3)Cl gt (CH_(3))_(2)CHCl gt (CH_(3))_(3)"CCl"`
D. `(CH_(3))_(2)CHCl gt CH_(3)CH_(2)Cl gt CH_(3)Cl gt (CH_(3))_(3)"CCl"`

Answer

B

Explanation

Steric hindrance (crowding) is the basis of `S_(N)2` reaction. Greater the steric hindrance, slower will be the rate of `S_(N)2` reaction, i.e. rate of `S_(N)2 prop (1)/("Steric crowding of 'C'")`
Hence, as steric hinderance (crowding) increases, rate of `S_(N)2` reaction decreases.
`underset("crowded)")underset("(Less")(CH_(3)Cl)gt underset(1^(@))(CH_(3)CH_(2)Cl)gt CH_(3)-underset(2^(@))underset(CH_(3))underset("| ")"CH"-Cl gt underset("crowded)")(underset("(More")underset(3^(@))(CH_(3)-overset(CH_(3))overset("| ")underset(CH_(3))underset("| ")"C "-Cl)`