Elucidation of the Vicarious Nucleophilic Substitution of Hydrogen Mechanism via Studies of Competition between Substitution of Hydrogen, Deuterium, and Fluorine

• Published in: Journal of organic chemistry Vol. 67; no. 2; pp. 394 - 400
• Main Authors: Ma̧kosza, Mieczysław, Lemek, Tadeusz, Kwast, Andrzej, Terrier, François
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 25.01.2002; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Organic; Exact sciences and technology; Kinetics and mechanisms; Organic chemistry; Physical Sciences; Reactivity and mechanisms; Science & Technology; NITROARENES; AMINATION; ALKYL HYDROPEROXIDE ANIONS; PHENYL SULFONES; HYDROXYLATION; SULFENAMIDES; ORGANIC-ANIONS; CARBANIONS; Deuterium; Sulfone; Fluorine; Hydrogen; E2 mechanism; Carbanion; Chlorides Organic compounds; Experimental study; Dehydrochlorination; Concentration effect; Reaction mechanism; Basic medium; Aromatic compound; Kinetics; Aromatic substitution; Nucleophilic substitution; SNAr mechanism
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo010590z

Relations of rates of the vicarious nucleophilic substitution of hydrogen (VNS) and SNAr substitution of fluorine in 2-fluoronitrobenzenes with chloroalkyl aryl sulfone carbanions were determined from competitive experiments carried out at various concentrations of base. The observed dependence of the VNS/SNAr rate ratio on the base concentration confirmed the two-step mechanism of the VNS, 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 processes can be the rate-limiting steps:  the β-elimination at low base concentration and the nucleophilic addition at high base concentration. Consistent with that conclusion is the finding that the kinetic isotope effect in the VNS reaction decreases from 4.2 (a value typical of a primary KIE) to 0.8 (a value typical of a secondary KIE) with increasing base concentration. Also reported is our discovery that the SNAr substitution of the 2-fluoronitrobenzenes studied in this work was subject to base catalysis under some of the experimental conditions employed in our competitive experiments.