Substituent Effect on the σ- and π-Electron Structure of the Nitro Group and the Ring in Meta- and Para-Substituted Nitrobenzenes

Abstract

An application of quantum chemical modeling allowed us to investigate a substituent effect on a σ and π electron structure of a ring and the nitro group in a series of meta- and para-X-substituted nitrobenzene derivatives (X = NMe, NHMe, NH, OH, OMe, Me, H, F, Cl, CF, CN, CHO, COMe, CONH, COOH, NO, and NO). The obtained pEDA and sEDA parameters (the π- and σ-electron structure characteristics of a given planar fragment of the system obtained by the summation of π- and σ-orbital occupancies, respectively) of the NO group and the benzene ring allowed us to reveal the impact of the substituents on their mutual relations as well as to analyze them from the viewpoint of substituent characteristics. The decisive factor for dependence of pEDA on sEDA of the ring is electronegativity of the atom linking the substituent with the ring; in subgroups an increase of sEDA is associated with a decrease of pEDA. The obtained mutual relation between pEDA(NO) and pEDA(ring) characteristics documents strong resonance interactions for electron-donating substituents in the para position. The observed substituent effect on the σ-electron structure of the nitro group, sEDA(NO), is significantly greater (∼1.6 times) for meta derivatives than for the para ones.

On the relations between aromaticity and substituent effect

“…Slope values of linear equations indicate weaker transmission from the meta (3-) position than from the para (4-) position, but in CHD much stronger this effect is observed than in benzene series. When HOMA values of the substituted nitrobenzene derivatives for the meta are plotted against the para positions [117], then the slope is 0.56 (with high R 2 = 0.97).…”

“…Some light in the attempt to understand can come from the scatter plot of pEDA(Ring) vs sEDA(Ring) [117], presented in Fig. 11.…”

“…Molecular systems of the series X-R-Y have been investigated for 16 substituents and seven Breaction Scheme 1 Graphical presentation of q(X) (a) and cSAR(X) (b) definitions. sites^, Y = NO 2 [116,117], OH [118], COOH [119], NH 2 [120,121] as well anionic COO − [119] and O − [118] moieties, with substituents in three and four positions of R = benzene or cyclohexa-1,3-diene. In addition, the use of both traditional and quantum chemistry-based descriptors of SE allows us to answer the question of how far the substituent effect in disubstituted cyclohexa-1,3-diene derivatives differ from those in bicyclo[2,2,2]octane and benzene [122].…”

Substituent effects of nitro group in cyclic compounds

“…This approach was applied to determine the strength of SE on pi-electron population of the ring in mono-and disubstituted pentafulvene derivatives [78], in double bonded substituted pi-electron cyclic systems [79], and in meta and para series of nitrobenzene derivatives [49,67]. Unlike previous descriptors, the found correlations of sEDA values with Hammett σ constants are not so clear, while pEDA values usually exhibit good correlations with R constants [49,77].…”

“…Application of both traditional and quantum chemistry-based descriptors of SE allows to show the nature of substituent effect in various reactions. Numerous studies have appeared recently on interrelation between the electron properties of the fixed functional group Y ("reaction site") and the ED/EA properties of the varying substituents X in a typical X-R-Y reaction series, where R is a transmitting moiety [24,[47][48][49][50][51][52][53][54].…”