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Equilibria and UV‐Spectral Characteristics of BrCl, BrCl 2 − , and Br 2 Cl − Species in 1,2‐Dichloroethane − Stereoselectivity and Kinetics of the Electrophilic Addition of these Species to Alkenes
Author(s) -
Chiappe Cinzia,
Del Moro Federica,
Raugi Marco
Publication year - 2001
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200109)2001:18<3501::aid-ejoc3501>3.0.co;2-f
Subject(s) - chemistry , halogen , electrophile , nucleophile , kinetics , reactivity (psychology) , olefin fiber , reaction rate constant , chloride , product distribution , absorption spectroscopy , adduct , ionization , photochemistry , absorption (acoustics) , ion , medicinal chemistry , computational chemistry , organic chemistry , catalysis , alkyl , physics , quantum mechanics , medicine , alternative medicine , pathology , acoustics
UV/Vis spectrophotometric measurements have been used to determine the equilibrium constants and the absorption spectra of Bu 4 N + Br 2 Cl − and Bu 4 N + BrCl 2 − species in 1,2‐dichloroethane (DCE) at 25 °C. The formation constants and the absorption spectra of BrCl from Br 2 and Cl 2 and of Bu 4 N + Cl 3 − species from Bu 4 N + Cl − and Cl 2 were also determined in this solvent at 25 °C. The kinetics and the products of bromochlorination of several aryl‐substituted olefins with BrCl, with Br 2 in the presence of an excess of Bu 4 N + Cl − (Br 2 /Cl − ), and with BrCl in the presence of an excess of Bu 4 N + Cl − (BrCl/Cl − ) were investigated in DCE at different temperatures. The kinetics and product distribution data for the reactions with Br 2 /Cl − or BrCl/Cl − are interpreted on the basis of a mechanism involving a product‐ and rate‐determining nucleophilic attack by chloride on the olefin‐halogen (Br 2 or BrCl) π complex. The data relating to the electrophilic addition of BrCl are interpreted in terms of the initial formation of a 1:1 olefin−BrCl π complex, the ionization of which, catalyzed by a second halogen molecule, gives a bromonium (or β‐bromocarbenium) BrCl 2 − ion pair able to collapse to the corresponding bromochloro adducts. A significant proportion of the ion pair intermediates may, however, actually be present as bromonium (or β‐bromocarbenium) Br 2 Cl − and Br 3 − ion pairs, and the collapse of these should be responsible for the formation of the dibromides in BrCl addition.