The rate of the reaction,

is influenced by the hyperconjugation effect of group R. If R sequentially is
I. `CH_3-` II. `CH_3 - CH_2 -` III. `H_3C - underset(CH_3)underset(|)(CH) -` (IV) `H_3 C - underset(CH_3)underset(|)overset(CH_3)overset(|)C-`
the increasing order of speed of the above reaction is

Options

A. `IV , III, II, I`
B. `I, II, III, IV`
C. `I, IV , III, II`
D. `III, II, I, IV`

Answer

B

Explanation

More the number of Hatoms on the `alpha`-carbon of the group R, more are the hyperconjugation structures, weaker is the C-Br bond and hence higher is the rate of reaction.
Since `CH_3` has three, ethyl has two, isopropyl has one and tert-butyl group has no `alpha`-hydrogen, hyperconjugation effect decreases and hence the rate decreases in the same order.