In the above reaction `o//p` ratio will be highest when :

Options

A. `R = - CH_(3)`
B. `R = - CH_(2) - CH_(3)`
C. `R = - CHMe_(2)`
D. `R = - Cme_(3)`

Answer

A

Explanation

The steric demand of `H^(Θ)` is, however, extremely small, and when attack on `C_(6)H_(5)Y` is by any other electrophile, `E^(oplus)`, which will necessarily be larger, there will be increasing interaction between E and Y in the transition state for attack at the position `o-` to Y (57 b, `R = E`) as attacking electrophile and substituent increase in size, there can be no such interaction in the transition state for `p-` attack `(57 a, R = E)`. This will be reflected in an increasing `Delta G^(+)` for `o-` attack, a consequently slower reaction, and the relative propotion of `o-` product will thus fall as the size of E and / or Y increas. This is illustrated by the falling `f_(o)-//f_(p)-` ratios which are observed for the nitration of alkylbenzenes `(Y - CH_(3) rarr CMe_(3))` under comparable conditions :
`"Increase in size of Y"{:(Y,%o-,%p-,),(CH_(3),58,37,),(CH_(2)Me,45,49,),(CHMe_(2),30,62,),(CMe_(3),16,73,):}`