What is the correct order of o/p ratio when `E^+` attacks the following system ?
A- PhF , B-PhCl, C-PhBr, D-Phl

Options

A. A lt B lt C lt D
B. A = B = C=D
C. D lt C lt B lt A
D. D lt B lt A lt C

Answer

A

Explanation

That the steric factor is not the sole determinant is, however, seen in the figures for the nitration of the halobenzenes, which are o-/p-directing but on which overall attack is slightly slower than on benzene.

Despite the increase in size of the substituent Y from F `to` I, the proportion of o-isomer increases. An increasing steric effect will, as with the alkyl benzenes, be operating to inhibito-attack, but this must here be outweighed by the electron-withdrawing inductive/field effect exerted by the halogen atom (Y). This effect will tend to decrease with distance from Y, being exerted somewhat less strongly on the distant p-position compared with the adjacent o-position. Electronegative F, and relatively little o-attack thus takes place on `C_6H_5F` , despite the small size of F. The electron-withdrawing effect of the halogen (Y) decreases considerably from F to I (the biggest change being between F and Cl), resulting in increasing attack at the o-position despite the increasing bulk of Y.