Noncatalytic Bromination of Benzene: A Combined Computational and Experimental Study

Noncatalytic Bromination of Benzene: A Combined Computational and Experimental Study Full article

Journal

Journal of Computational Chemistry
ISSN: 0192-8651 , E-ISSN: 1096-987X

Output data

Year: 2016, Volume: 37, Number: 2, Pages: 210-225 Pages count : 16 DOI: 10.1002/jcc.23985

Tags

bromination; benzene; polybromide anions

Authors

Shernyukov Andrey V. ¹ , Genaev Alexander M. ¹ , Salnikov George E. ^1,2 , Rzepa Henry S. ³ , Shubin Vyacheslav G. ¹

Affiliations

1	(Данные Web of science) Russian Acad Sci, Siberian Branch, NN Vorozhtsov Novosibirsk Inst Organ Chem, Novosibirsk 630090, Russia
2	(Данные Web of science) Novosibirsk State Univ, Novosibirsk 630090, Russia
3	(Данные Web of science) Univ London Imperial Coll Sci Technol & Med, Dept Chem, London SW7 2AZ, England

The noncatalytic bromination of benzene is shown experimentally to require high 5-14 M concentrations of bromine to proceed at ambient temperatures to form predominantly bromobenzene, along with detectable (<2%) amounts of addition products such as tetra and hexabromocyclohexanes. The kinetic order in bromine at these high concentrations is 4.8 +/- 0.06 at 298 K and 5.6 +/- 0.11 at 273 K with a small measured inverse deuterium isotope effect using D-6-benzene of 0.97=0.03 at 298 K. These results are rationalized using computed transition states models at the B3LYP+D3/6-311++G(2d,2p) level with an essential continuum solvent field for benzene applied. The model with the lowest predicted activation free energies agrees with the high experimental kinetic order in bromine and involves formation of an ionic, concerted, and asynchronous transition state with a Br-8 cluster resembling the structure of the known Br-9(-). This cluster plays three roles; as a Br- donor, as a proton base, and as a stabilizing arm forming weak interactions with two adjacent benzene C-H hydrogens, these aspects together combining to overcome the lack of reactivity of benzene induced by its aromaticity. The computed inverse kinetic isotope effect of 0.95 agrees with experiment, and arises because C-Br bond formation is essentially complete, whereas C-H cleavage has not yet commenced. The computed free energy barriers for the reaction with 4Br(2) and 5Br(2) for a standard state of 14.3 M in bromine are reasonable for an ambient temperature reaction, unlike previously reported theoretical models involving only one or two bromines. (C) 2015 Wiley Periodicals, Inc.

Shernyukov A.V. , Genaev A.M. , Salnikov G.E. , Rzepa H.S. , Shubin V.G.
Noncatalytic Bromination of Benzene: A Combined Computational and Experimental Study
Journal of Computational Chemistry. 2016. V.37. N2. P.210-225. DOI: 10.1002/jcc.23985 WOS Scopus РИНЦ

Published online:	Jul 14, 2015
Published print:	Jan 15, 2016

Web of science	WOS:000367384500006
Scopus	2-s2.0-84950300692
Elibrary	26797676
OpenAlex	W1929514466