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Nucleophilic Substitution at Tetracoordinate Sulfur. Kinetics and Mechanism of the Chloride-Chloride Exchange Reaction in Arenesulfonyl Chlorides: Counterintuitive Acceleration of Substitution at Sulfonyl Sulfur by ortho-Alkyl Groups and Its Origin

1
Department of Organic Chemistry, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 Lodz, 112 Sienkiewicza, Poland
2
Department of Structural Chemistry, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 Lodz, 112 Sienkiewicza, Poland
*
Authors to whom correspondence should be addressed.
Molecules 2020, 25(6), 1428; https://doi.org/10.3390/molecules25061428 (registering DOI)
Received: 27 February 2020 / Revised: 16 March 2020 / Accepted: 18 March 2020 / Published: 20 March 2020
(This article belongs to the Section Organic Chemistry)
The chloride-chloride exchange reaction in arenesulfonyl chlorides was investigated experimentally and theoretically by density functional theory (DFT) calculations. The second order rate constants and activation parameters of this identity reaction were determined for 22 variously substituted arenesulfonyl chlorides using radio-labeled Et4N36Cl. The chloride exchange rates of 11 sulfonyl chlorides bearing para-and meta-substituents (σ constants from −0.66 to +0.43) in the aromatic ring followed the Hammett equation with a ρ-value of +2.02. The mono- and di-ortho-alkyl substituted sulfonyl chlorides exhibit an enhanced reactivity although both inductive and steric effects lower the reaction rate. The DFT calculations of their structures together with X-ray data showed that an increased reactivity is mainly due to a peculiar, rigid, strongly compressed and sterically congested structure. The DFT studies of the title reaction revealed that it proceeds via a single transition state according to the SN2 mechanism. The analogous fluoride exchange reaction occurs according to the addition–elimination mechanism (A–E) and formation of a difluorosulfurandioxide intermediate. The reliability of the calculations performed was supported by the fact that the calculated relative rate constants and activation parameters correlate well with the experimental kinetic data. View Full-Text
Keywords: sulfonyl chlorides; isotopic chloride exchange; density functional calculations; nucleophilic substitution; reaction mechanisms sulfonyl chlorides; isotopic chloride exchange; density functional calculations; nucleophilic substitution; reaction mechanisms
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Mikołajczyk, M.; Gajl, M.; Błaszczyk, J.; Cypryk, M.; Gostyński, B. Nucleophilic Substitution at Tetracoordinate Sulfur. Kinetics and Mechanism of the Chloride-Chloride Exchange Reaction in Arenesulfonyl Chlorides: Counterintuitive Acceleration of Substitution at Sulfonyl Sulfur by ortho-Alkyl Groups and Its Origin. Molecules 2020, 25, 1428.

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