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| - Elucidation of the Vicarious Nucleophilic Substitution ofHydrogen Mechanism via Studies of Competition betweenSubstitution of Hydrogen, Deuterium, and Fluorine
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| - Relations of rates of the vicarious nucleophilic substitution of hydrogen (VNS) and SNAr substitutionof fluorine in 2-fluoronitrobenzenes with chloroalkyl aryl sulfone carbanions were determined fromcompetitive experiments carried out at various concentrations of base. The observed dependenceof the VNS/SNAr rate ratio on the base concentration confirmed the two-step mechanism of theVNS, which consists of reversible formation of σH adducts of the α-chlorocarbanion to nitroarene,followed by base-induced β-elimination of HCl. It was also evidenced that both of these processescan be the rate-limiting steps: the β-elimination at low base concentration and the nucleophilicaddition at high base concentration. Consistent with that conclusion is the finding that the kineticisotope effect in the VNS reaction decreases from 4.2 (a value typical of a primary KIE) to 0.8 (avalue typical of a secondary KIE) with increasing base concentration. Also reported is our discoverythat the SNAr substitution of the 2-fluoronitrobenzenes studied in this work was subject to basecatalysis under some of the experimental conditions employed in our competitive experiments.
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