Nitroanilines intramolecular hydrogen bonds

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. 

Problem 24.2 In hich of the following compounds would you expect intramolecular hydrogen bonding to occur u-nitroaniline, o-cresol, o-hydroxybenzoic acid (salicylic acid), o-hydroxybenzaldchyde (salicylaldehyde), o-fluorophenol, o-hydroxy-benzonitrile. 

A number of authors [78-81 ] examined the intramolecular hydrogen bonds between NO2 and NH in o-nitroaniline. 

Hexakis(2,6-di-0-methyl)-a-CyD complexes with a small guest molecule, such as iodine and 1-propanol, crystallize with the cage-type packing structure [185]. Compared with the structure of the corresponding -CyD complex, the guest molecules in the both complexes are shifted to the secondary hydroxyl side from the center of the cavity. 3-Iodopropionic acid [186], m-nitroaniline [187], and acetonitrile [188] are also fully accommodated in the host cage . A 3-0 acetylated host, hexakis(2,6-di-0-methyl-3-0-acetyl)-a-CyD, was crystallized from butylacetate [189]. In spite of the disruption of intramolecular hydrogen bonds, the host molecule is in a round shape because of the inclusion of butylacetate. 

Greater electronic frequency shifts are observed when intramolecular charge transfer occurs upon excitation in push-pull compounds such as 4-nitroaniline and 4-nitrophenol. For example, in cyclohexane, free 4-nitrophenol absorbs at 285 nm and 4-nitrophenol hydrogen bonded to triethylamine at 307 nm. The shift amounts to 2515 cm (30 kJ mol ). 

Laurence et al. [23, 24, 135] measured two solvatochromic scales for about 200 bases, each based on one reference hydrogen-bond donor, 4-nitrophenol or 4-nitroaniline, one standard state, the pure base, and one physical property, the enhanced bathochromic shift of the longest wavelength n n transition (with intramolecular charge-transfer character) upon hydrogen bonding. The solvatochromic comparison method was improved and inert solvents were carefully chosen [135] to fix the comparison lines of Equations 4.37 and 4.38 ... 

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