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Isokinetic behaviors in the addition reactions of aniline nucleophiles with benzylic carbocations
Author(s) -
Lee Ikchoon
Publication year - 1996
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/(sici)1099-1395(199610)9:10<661::aid-poc842>3.0.co;2-m
Subject(s) - chemistry , nucleophile , carbocation , aniline , substituent , reaction rate constant , medicinal chemistry , reactivity (psychology) , computational chemistry , ring (chemistry) , kinetics , organic chemistry , quantum mechanics , catalysis , medicine , physics , alternative medicine , pathology
In the addition reactions of aniline nucleophiles with a benzylic carbocation, isokinetic behavior is often observed: ρ X (ρ nuc ) vanishes and the rate becomes constant irrespective of the substituent, X, in the nucleophile when α and Y substituents on the benzylic carbon and on the ring, respectively, are kept constant (σˆ α and σˆ Y ). This means that there is a region (σ Y < σˆ Y ) where thermodynamically more stable derivatives (δσ X > 0) are kinetically more labile (δ log k XY > 0) so that ρ X is positive, i.e. an inverse thermodynamic stability‐reactivity relationship holds. The observable isokinetic point at σˆ Y is rationalized by the compensation effect of the intrinsic barrier, Δ G 0 ≠ , and thermodynamic driving force, Δ G ° , for the specified α and Y substituents. Moreover, the two substituents, α and Y, that satisfy the condition of vanishing ρ X are found to be related in a compensating manner to preserve the condition of nearly complete cancellation between the two energy terms for the vanishing ρ X . The cancellation of the two energy terms is made possible by an imbalance or non‐synchronization of the expression of destabilizing polar and stabilizing resonance interactions in the transition state.