Kinetics of Iodination of Benzenes and Their Reactivity

Abstract

The kinetics of iodination of benzenes have been investigated
with the iodinating sytems A: I₂—NaNO₂—HNO₃ and B: I₂—H₂SO₄—
HNO₃ separately. The data obtained from such studies
show the following common features with both the systems: (a) The
order of the reaction is 0.5 in [I₂]. (b) The reaction shows adominant
substituent effect. (c) The reactivity order is m-xylene > o-xylene
> p-xylene > toluene> benzene> iodobenzene > bromobenzene >
nitrobenzene > chlorobenzene > fluorobenzene. The activation
parameters have been evaluated for each substrate. Analysis of these
parameters indicates the involvement of a bulky transition state. A
mechanism involving the attack of the benzene by the electrophilic
iodine species has been suggested. The iodination involves the
initial formation of electrophile HNO₂I⁺ in system A and I⁺ in
system B.