Correct Answer - 4
Isotope effectg (or rate ratio `(K_(H))/(K_(D)) or (K_(H))/(K_(T))`) is besed on the more rapid breaking of C-H than C-D (or C-T) bond in the rate-controlling step. In most aromatc substitutions, there is an absence of a primary isotope affect, indicating that breaking the C-H bond in not a part of the rate-controlling step. The first step in which `E^(_)` bonds to the aromatic ring wighout the cleavage of a `C-H` bond is rate-controlling. The C-H or C-D or C-T bond is broken in the fast second step that restores the stable aromatic system. Hence, the overall rate is independent of the particular isotopic species used. The absence of isotopic effect also rules out a one step substitution of H by E, i.e., a simultaneous bond-forming and C-H bond breaking mechanism.
