On differences in substituent effects in substituted ethene and acetylene derivatives and their boranyl analogs

Mirosław Jabłoński,Tadeusz M Krygowski

doi:10.1007/s11224-020-01666-x

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.

Highlights

Quantitative approaches to the problems of substituent effects (SE) in organic chemistry originate from the fundamental works by L
The effects of substituent on CC bond lengths are significantly smaller for substituted acetylene than ethene derivatives and in both cases the boranyl functional group increases clearly the observed effects, in particular in the case of ethenes
It is important to indicate that any relationships are not observed between the CC bond lengths and the characteristics of substituents described by the substituent constants σp, F, and R

Summary

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). 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:. Where ρ is the so-called reaction constant that describes the sensitivity of the reaction under study (physicochemical process) to the effect of X. Hammett substituent constants represent the numerical characteristics of electron accepting/donating properties of substituents and have become a very convenient way for describing most of substituent effects encountered in physical organic chemistry. An avalanche of papers has appeared that have undertaken these problems presented and summarized in a numerous reviews and monographs [3,4,5,6,7,8,9,10,11]

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