Nowe spojrzenie na efekt podstawnikowy
Journal Title: Wiadomości Chemiczne - Year 2017, Vol 71, Issue 7
Abstract
Classical view on the substituent effect (SE) is associated with an empiric approach presented 80-years ago by Hammett [1]. He proposed a simple formula to represent the effect of a substituent upon the rate or equilibrium constants of a reaction in which the reacting group is in a side chain attached to the ring and introduced quantitative descriptors of the SE named substituent constants σ, defined in terms of dissociation constants of meta- and para- substituted benzoic acids. Then the Hammett’s equation relied on using them to describe SE for various physicochemical properties, P(X), by means of linear regression like P(X)=ρ·σ, where ρ is so called reaction constants describing sensitivity of a system in question on the SE. Application of the quantum chemistry modeling allowed to find descriptors (independent of empirical approaches) which are characterized by clear physical meaning and are accessible by use of standard computational packages. The oldest descriptor is based on homodesmotic reaction [X-R-Y + R = R-X + R-Y] in which energy of products is subtracted from that of substrates [32]. The model is named as SESE (substituent effect stabilization energy) and its values are usually well correlated with empirical constants σ, or their modifications. Ten years ago Sadlej-Sosnowska introduced [23, 24] an effective descriptor of SE based on atomic charges of a substituent X and the ipso carbon atom named cSAR(X) (charge of the substituent active region). Unlike atomic charges at substituent, q(X), the cSAR(X) values correlate well with the Hammett substituent constants [25]. Recently as an interesting and showing new aspects descriptor of SE appeared a model making use of population of electrons at sigma and pi orbitals of planar pi-electron systems (or their fragments), named as sEDA and pEDA [33]. Again in particular cases these descriptors correlate with the Hammett σ. This descriptor allowed to reveal how strong is SE on population of pi-electron systems in substituted derivatives of benzene, and how much is this different for para and meta substituted species. Analysis of the relation of pEDA vs sEDA for meta and para substituted derivatives of nitrobenzene revealed that sEDA values increase with a decrease of electronegativity of the linking atom [47]. The above mentioned action of the sigma structure is modulated by the remaining part of the substituent as well as its pi-electron structure. This part of substituents (including also the linking atom) is responsible for an interplay of the sigma structure with the pi-electron one. Application of cSAR(X) for series of meta- and para- substituted phenol and phenolate derivatives [36] revealed that reverse substituent effect, i.e. the effect of impact of the functional group Y on the electron accepting/donating power of the substituent in systems like X-R-Y may be as large as the overall differences in these kind of properties between NO and NMe2! In the σ constants scale this is full range of σ for uncharged substituents, 1.73 units of σ. Application of cSAR for CH2 groups in 1-X-bicyclo[2.2.2]octane derivatives and using the regression of cSAR(CH2) against cSAR(X) values allowed to document that substituent effect in these systems is inductive in nature [39]. In summary, substituent effect descriptors based on quantum chemistry modeling are usually consistent with the empirical ones, but are able to present more detailed information on physical aspects of the problem.
Authors and Affiliations
Halina Szatyłowicz, Tadeusz M. Krygowski
Keywords
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