Hydrogen bonds nitroanilines

Control of Symmetry and Asymmetry in Hydrogen-Bonded Nitroaniline Materials... 

Etter, M. C., Huang, K. S., Frankenbach, G. M. and Adsmond, D. (1991). Control of symmetry and asymmetry in hydrogen-bonded nitroaniline materials. In Materials for nonlinear optics, chemical perspectives (ed. S. R. Marder, J. E. Sohn and G. D. Stuckty), ACS Symposium Series, 455,446-55. [207]... 

Later, Taft and his coworkers examined the H-bonding donor ability of the amino protons in 4-nitroaniline and 4-nitrophenol as the hydrogen bond donors. They also tried... 

The 7t scale of solvents (Kamlet-Taft scale) is intended to represent the solute-solvent interactions in the absence of strong forces such as hydrogen-bonding or dipole-dipole interactions. It is based on the n to n absorption of a series of seven solutes (six uitroaromatics, such as 4-uitroanisole, A, A -diethyl-3-nitroaniline, 1-ethyl-4-uitrobeuzeue, etc., aud 4-dimethylamiuobenzophenone) [51-53]. 

The extra base-weakening effect, when the substituent is in the o-position, is due in part to the short distance over which its inductive effect is operating, and also to direct interaction, both steric and by hydrogen bonding, with the NH2 group (cf. the case of o-substituted benzoic acids, p. 63). o-Nitroaniline is such a weak base that its salts... 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Figure 1.17 Hydrogen-bond formation between nitroanilines and ethanol, (a) V,V-diethyl-4-nitroaniline is a HBA only (b) 4-nitroaniline is a HBA and (c) a HBD...

As was the case for dinitrobenzene, the meta and para nitroaniline isomers have essentially the same gaseous enthalpy of formation. In the gaseous phase, it is surprising to find that despite the more attractive quinonoid resonance structures92 for the para isomer (58) than for the meta (59) the met a and para nitroaniline have essentially the same gas-phase enthalpy of formation. In the solid and liquid states the intermolecular stabilization lowers the enthalpy of formation of the para isomer relative to the meta. Interestingly, the gas-phase intramolecularly hydrogen-bonded ortho isomer is of comparable stability to its isomers. In contrast, it is considerably less stable than its isomers in the solid state because it can form fewer intermolecular hydrogen bonds. All isomers of nitroaniline are more stable than calculated by additivity. 

Nitroanilines were extensively investigated by infrared, Raman, UV/VIS and H NMR spectroscopic methods. In some cases the comparison with similar spectroscopic data of the corresponding tV,/V-dimethylanilines provides simple and consistent conclusions involving geometrical properties of molecules caused by intramolecular hydrogen bonding. 

In several examples including nitroanilines, the effect of twisting the chromophore from planarity decreases the absorption intensities. The reasons for the bathochromic effect as the angles of twist in the 4-aniline series increase is subject to discussion. When considering this (as well as in all attempts to obtain definitions of polarity of solvents by quantitative parameters) it is important to exclude or minimize the presence of hydrogen bonding overlapping158 other interactions. 

Recently195, the hydrogen bond basicity scale (p.K hb as logarithm of the formation constant of 4-fluorophenol/base complexes in carbon tetrachloride, equilibrium 21) has been measured for several nitro derivatives (nitromethane, nitrobenzene, IV-nitrocamphorimine, 2-nitropropane, 4-nitro-o-xylene, 4-nitroanisole, lV,./V-diethyl-4-nitroaniline, l-dimethylamino-2-nitroethylene, l-piperidino-2-nitroethylene) ... 

Complex 1 1 is considered the only complex present, but the hydrogen bond may be either two (76) or three center (77). Nitroenamines are more prone to complex with 4-fluorophenol than the nitroanilines and they form the strongest hydrogen bond presently known for nitro-bases. In particular, l-piperidino-2-nitroethylene (78) and 1-dimethylamino-2-nitroethylene (79) (both in E form) present a hydrogen bond basicity comparable to that of tributylamine. 

We might have thought that the o-nitro group would provide stabilization for the hydrox-ylamine and the phenol by two mechanisms (a) enhanced (i.e. zwitterionic, dipolar, quinonoid-like) resonance as is often discussed for o- and p-nitroaniline, cf. Reference 12, and (b) intermolecular H—O hydrogen bonding between the nitro and hydroxyl groups. 

Fig. 15. Structure of the [18]crown-6 3-nitroaniline complex 27) Upper picture host/guest units I and II lower picture as viewed perpendicular to the plane of the crown (hydrogen bonds drawn dotted)...

Incidentally, it seems that weak acids, eg, hydrazoic, nitrophenols and nitroanilines (note their large values of the specific heats of sublimation in Table 6) tend to form intermolecular hydrogen bonds, whereas strong acids, as discussed above, do not... 

Addition of a second proton donor, as in p-nitroaniline (PNA), serves to link acentric chains together (8-10). These chains form an acentric hydrogen-bonded array, as shown. PNA is a centric crystal because the hydrogen-bonded sheets pack with an inversion center between them. There are no inversion centers within the layer. 

Ortho substituted primary nitroanilines usually form two-center intermolecular hydrogen bonds, rather than three center. 

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