Abstract
Electron-accepting properties of the nitro group were studied in a series of meta- and para-X-substituted nitrobenzene derivatives (X = NMe2, NH2, OH, OMe, CH3, H, F, Cl, CF3, CN, CHO, COMe, CONH2, COOH, COCl, NO2, NO). For this purpose Hammett-like approaches were applied based on quantum chemistry modeling; the B3LYP/6-311++G(d,p) method was used. The substituent effect (SE) was characterized by the mutually interrelated descriptors: the charge of the substituent active region, cSAR(X), and substituent effect stabilization energy, SESE, as well as substituent constants, σ. Classical SE is realized by dependences of the structural parameters of the nitro group (ONO angle and NO bond lengths) and cSAR(NO2) on the above mentioned SE descriptors. The reverse substituent effect was clearly documented by a comparison of cSAR(X) values for monosubstituted benzenes, meta- and para-substituted nitrobenzenes as well as, additionally, for meta- and para-X-substituted anilines. For para-substituted systems the electron-accepting ability of the nitro group increases from cSAR(NO2) = −0.170 up to −0.284 in dinitrobenzene and nitroaniline, respectively.
Highlights
The nitro group belongs to one of the most electron accepting (EA) substituents and it attains an unusual interest as a substituent or a functional group
The substituent effect in meta- and para-substituted nitrobenzene derivatives may be considered by means of three ways of understanding this term
When we look at these data by means of canonical structures we find that for para-substituted systems the quinoid form requires the structure with a single charge separation between the donating substituent and the nitro group, whereas for the meta-one a double excitation is needed, which is energetically much less favorable [35]
Summary
The nitro group belongs to one of the most electron accepting (EA) substituents and it attains an unusual interest as a substituent or a functional group. The nitro group is very electronegative (the group electronegativity in the Pauling scale, χNO2, is equal to 4.00 for a coplanar and 4.19 for a perpendicular orientation with respect to the benzene ring) [1] and as a consequence its strongly inductive effect influences the rest of the substituted molecule. This group exhibits a great range of variability of its EA properties [2, 3] with σp = 0.78 and σp− = 1.27 which dramatically depends on the kind of a moiety to which the group is attached [4]. A rotation of NO2 group around CN bond changes σp− values from 1.27 for a coplanar conformation to 0.70 for the perpendicular one [6], and so the latter is very close to the value to the field parameter [3]
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