Question

What is the correct order of o/p ratio when $E^{+}$ attacks the following system?

A	$PhF$
B	$PhCl$
C	$PhBr$
D	$PhI$

• A. A < B < C < D
• B. A = B = C = D
• C. D < C < B < A
• D. D < B < A < C

Answer 1

The correct option is A A < B < C < D

Since halogens show $-I$ effect, the o/p ratio depend upon the $-I$ effect of the halogens. The order of $-I$ effect is:
$-F>-Cl>-Br>-I$
Halogens decrease the electron density on benzene ring by $-I$ effect. Since, inductive effect involving delocalisation of sigma bond orbitals, higher the -I effect will decrease the electron density at ortho position more than at the para position. Therefore, if the halogen has higher $-I$ effect, the percentage of para product will be higher i.e. in case of fluorine $(-F)$ and if the halogen has lower $-I$ effect, the percentage of ortho product will be higher i.e. in case of iodine $(-I)$.
Hence, the order of o/p ratio is:
$A<B<C<D$