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An investigation into rate-determining factors in electrophilic aromatic halogenation
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0003-3832-2331
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0003-2673-075X
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We have studied the halogenations of monosubstituted benzenes in polar, protic solvents at the PCMM06-2X/6-311G(d,p) level. We verify that the reaction with Cl2 proceeds through a C-atom coordinated π-complex and a rate-determining transition state for the formation of the σ-complex.

The last step of this reaction proceeds in two steps – first the dissociation of Cl- and then the abstraction of the proton with a weak base. The use of the σ-complex as a model for the ratedetermining transition state is more accurate the later this transition state comes along the reaction coordinate, and thus, it is in general more accurate for halogenations than for nitrations, and for halogenations the more deactivated the substrate. The bromination of anisole with Br2 is shown to have a much later rate-determining transition state than the corresponding reaction with Cl2. The energy barrier for the abstraction of the proton in the iodination of anisole and phenol was several times higher than the corresponding step in chlorination and nitration, and the transition state structure obtained with ICl as electrophile is for both substrates consistent with a concerted reaction without the formation of a stable σ-complex. The computed hydrogen kinetic isotope effects are in good agreement with experiment.

National Category
Theoretical Chemistry
Research subject
Theoretical Chemistry and Biology
Identifiers
URN: urn:nbn:se:kth:diva-206963OAI: oai:DiVA.org:kth-206963DiVA, id: diva2:1094562
Note

QC 20170510

Available from: 2017-05-10 Created: 2017-05-10 Last updated: 2017-05-17Bibliographically approved
In thesis
1. Quantum Chemical Studies of Aromatic Substitution Reactions
Open this publication in new window or tab >>Quantum Chemical Studies of Aromatic Substitution Reactions
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, density functional theory (DFT) is used to investigate the mechanisms and reactivities of electrophilic and nucleophilic aromatic substitution reactions (SEAr and SNAr respectively). For SEAr, the σ-complex intermediate is preceded by one (halogenation) or two (nitration) π-complex intermediates. Whereas the rate-determining transition state (TS) for nitration resembles the second π-complex, the corresponding chlorination TS is much closer to the σ-complex. The last step, the expulsion of the proton, is modeled with an explicit solvent molecule in combination with PCM and confirmed to be a nearly barrierless process for nitration/chlorination and involves a substantial energy barrier for iodination. It is also shown for nitration that the gas phase structures and energetics are very different from those in polar solvent. The potential energy surface for SNAr reactions differs greatly depending on leaving group; the σ-complex intermediate exist for F-/HF, but for Cl-/HCl or Br-/HBr the calculations indicate a concerted mechanism. These mechanistic results form a basis for the investigations of predictive reactivity models for aromatic substitution reactions. For SEAr reactions, the free energy of the rate-determining TS reproduces both local (regioselectivity) and global reactivity (substrate selectivity) with good to excellent accuracy. For SNAr reactions good accuracies are obtained for Cl-/HCl or Br-/HBr as leaving group, using TS structures representing a one-step concerted mechanism. The σ-complex intermediate can be used as a reactivity indicator for the TS energy, and for SEAr the accuracy of this method varies in a way that can be rationalized with the Hammond postulate. It is more accurate the later the rate-determining TS, that is the more deactivated the reaction. For SNAr reactions with F-/HF as leaving group, the same method gives excellent accuracy for both local and global reactivity irrespective of the degree of activation.

Place, publisher, year, edition, pages
Stockholm: Universitetsservice US AB, Stockholm, 2017. p. 74
Series
TRITA-CHE-Report, ISSN 1654-1081
National Category
Physical Chemistry
Research subject
Theoretical Chemistry and Biology
Identifiers
urn:nbn:se:kth:diva-206964 (URN)978-91-7729-324-8 (ISBN)
Public defence
2017-06-07, Sal F3, Lindstedtsvägen 26, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20170510

Available from: 2017-05-10 Created: 2017-05-10 Last updated: 2017-05-10Bibliographically approved

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Halldin Stenlid, JoakimBrinck, Tore

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