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Electrophilic aromatic substitutions: reactions of hydroxy‐ and methoxy‐substituted benzenes with 4,6‐dinitrobenzofuroxan: kinetics and mechanism
Author(s) -
Terrier François,
Pouet MarieJosé,
Halle JeanClaude,
Kizilian Elyane,
Buncel Erwin
Publication year - 1998
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(1998100)11:10<707::aid-poc37>3.0.co;2-v
Subject(s) - chemistry , electrophile , kinetics , mechanism (biology) , electrophilic aromatic substitution , electrophilic substitution , reaction mechanism , medicinal chemistry , computational chemistry , organic chemistry , stereochemistry , catalysis , physics , quantum mechanics , philosophy , epistemology
Rate constants have been determined in aqueous Me 2 SO mixtures for the reaction of super‐electrophilic 4,6‐dinitrobenzofuroxan (DNBF) with a series of hydroxy‐ and methoxy‐substituted benzenes whose p K a values range between ‐3 and ‐9. The study extends the reactivity range of weakly basic aromatics with DNBF, from the family of indoles previously studied with p K a values ranging from ‐1 to ‐6. The overall rate constants for the reactions of DNBF as the electrophile are at least one order of magnitude greater than for the reactions of H 3 O + with the same series of aromatics. This lends further credence to the notion that DNBF possesses super‐electrophilic properties. An LFER is observed between log k 50%Me 2 SO DNBFand p K a H 2 Owith slope 0.54. In the case of 1,3,5‐trimethoxybenzene a significant kinetic isotope effect (KIE) is observed ( k H / k D = 3.71 in 50% Me 2 SO). This system hence affords one of the few instances in which a KIE has been observed in S E Ar reactions. It follows from the observed KIE that the addition of DNBF to the aromatic is not rate‐limiting and that reversion to reactants and proton loss from the arenonium intermediate occur at comparable rates. Structures of the products of electrophilic substitution have been confirmed by 1 H NMR. In all cases the regiochemistry of the reactions was identical to that observed in protonation studies of the starting aromatics. © 1998 John Wiley & Sons, Ltd.