On differences in substituent effects in substituted ethene and acetylene derivatives and their boranyl analogs
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ORIGINAL RESEARCH
On differences in substituent effects in substituted ethene and acetylene derivatives and their boranyl analogs ´ 1 Mirosław Jabłonski
· Tadeusz M. Krygowski2
Received: 11 September 2020 / Accepted: 15 October 2020 © The Author(s) 2020
Abstract This article is the first attempt to present different influence of substituent effects on double and triple bonds and, conversely, to present the impact of these bonds on the electronic structure of substituents. For this purpose, quantum-mechanical calculations were made for X-substituted derivatives of ethene and acetylene with 27 diverse substituents representing a wide spectrum of electronic properties, from strongly electron-accepting to strongly electron-donating ones. In addition to these systems, their boranyl derivatives are also investigated. It turns out that the Hammett substituent constants do not correctly describe changes in the CC bond length in any of the considered family of systems. However, the relationships with the CB bond length are significantly better. It is shown that the triple bond in acetylene derivatives is much more resistant to external perturbations than the double bond in the analogs containing an ethene unit. As a consequence, in acetylene derivatives, the substituent effects on CC bond length are about half of the substituent effects in ethene derivatives. We suggest that the observed lack of a clear linear correlation between the length of the CC triple bond in acetylene derivatives and the value of electron density on this bond is due to the disturbing additional interaction between the structure of the X substituent in the xy plane and the π bond being in the same plane in the acetylene unit—on the contrary, this interaction is not possible in ethene analogs. Keywords Acetylene · Boranyl · Ethene · Substituent · Substituent effect
Introduction Quantitative approaches to the problems of substituent effects (SE) in organic chemistry originate from the fundamental works by L. P. Hammett [1, 2] by introduction of substituent constants: σp/m = lg
KX K
(1)
where KX and K are dissociation constants for para/metaX-substituted and unsubstituted benzoic acids, respectively, and the acid-base reaction reads as follows: KX
XPhCOOH + H2 O XPhCOO− + H3 O+
The relationship between the influence of substituents X on the physicochemical properties P (e.g., kinetic constants, equilibrium constants, spectral or electrochemical properties for benzene derivatives) and σp/m constants is described by the Hammett equation: P (X) = ρσp/m
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11224-020-01666-x) contains supplementary material, which is available to authorized users. Mirosław Jabło´nski
[email protected] 1
Faculty of Chemistry, Nicolaus Copernicus University in Toru´n, Gagarina 7, 87-100, Toru´n, Poland
2
Department of Chemistry, Warsaw University, Pasteura 1, 02-093, Warsaw, Poland
(2)
(3)
where ρ is the so-called reaction constant that describes the sensitivity of t
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