Secondary isotope effects on pi-complex formation

Abstract

Abstract

Secondary Isotope Effects Part IV Complex Formation Between Aromatic Hydrocarbons and HCl,

“…The fact that the pi-complexes between toluene and HCI and between toluene and chloranil are affected differently by methyl substitution, has been rationalized elsewhere in some detail [2]. Briefly, it is consistent with the interpretation that hyperconjugation, and hyperconjugative isotope effects in particular, are quantum polarizability phenomena that depend on the extent to which the relevant moleculr orbitals have been altered by the interaction, whereas inductive effects are more simply electrostatic [11 (bj], It is not surprising that the complex with chlor-aml, the electron deficient orbitals of which strongly overlap those of the donor, should be sensitive to hyperconjugation, whereas HCI, which can merely imbed the positive end of its dipole into regions of high electron density with only local distortion of the electron cloud, should call forth inductive substituent and isotope effects.…”

Electrophilic and Nucleophilic Aromatic Substitutions are Mechanistically Similar with Opposite Polarity

“…Electrophilic substitution proceeds in general via addition of electrophiles to the ring to form cationic σ H adducts followed by fast departure of a proton. Thus in general, the addition, not the carbon–hydrogen bond breaking, is the rate‐limiting step, therefore usually secondary KIE is observed [1c, 22] . On the other hand, in nucleophilic substitution of hydrogen, two variants are possible: conversion of the σ H adducts can be a fast or slow process, therefore the addition or removal of hydrogen can be the rate‐determining step and secondary or primary KIE can be observed.…”