Elevated reaction order of 1,3,5-tri-tert-butylbenzene bromination as evidence of a clustered polybromide transition state: a combined kinetic and computational study

The kinetics and mechanism of concurrent bromo-de-protonation and bromo-de-tert-butylation of 1,3,5-tri-tert-butylbenzene at different bromine concentrations were studied experimentally and theoretically. Both reactions have high order in bromine (experimental kinetic orders ∼5 and ∼7, respectively)...

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Published in:Organic & biomolecular chemistry 2019-04, Vol.17 (15), p.3781-3789
Main Authors: Shernyukov, Andrey V, Genaev, Alexander M, Salnikov, George E, Shubin, Vyacheslav G, Rzepa, Henry S
Format: Article
Language:English
Subjects:
Anions
Bromination
Bromine
Computer applications
Deuterium
Isotope effect
Kinetics
Organic chemistry
Protonation
Quantum chemistry
Reaction kinetics
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Summary:The kinetics and mechanism of concurrent bromo-de-protonation and bromo-de-tert-butylation of 1,3,5-tri-tert-butylbenzene at different bromine concentrations were studied experimentally and theoretically. Both reactions have high order in bromine (experimental kinetic orders ∼5 and ∼7, respectively). According to quantum chemical DFT calculations, such high reaction orders are caused by participation of clustered polybromide anions Br2n-1- in transition states. Bromo-de-tert-butylation has a higher order due to its bigger reaction center demanding clusters of extended size. A significant primary deuterium kinetic isotope effect (KIE) for bromo-de-protonation is measured indicating proton removal is rate limiting, as confirmed by computed DFT models. The latter predict a larger value for the KIE than measured and possible explanations for this are discussed.
ISSN:1477-0520
1477-0539
DOI:10.1039/c9ob00607a