Hydrogen bonding in o-nitrophenol

Proximity effects between functional groups in the same molecule frequently render the properties of the molecule unusual. A well-known example concerns proximity interactions of acetylenic bonds. Thus, internally hydrogen-bonded o-hydroxy-phenylacetylene is more voiatile (b.p. 75 °C/15 mm) than its methoxy derivative (b.p. 90°C/15mm), which is reminiscent of the effect of the hydrogen bond in o-nitrophenol. 

FIGURE 11. Intramolecular hydrogen bonding in o-nitrophenol (top left), and intermolecular hydrogen bonding in m-nitrophenol (top right), /j-nitrophenol (bottom left) and /j-nitrophenol (bottom right) ... 

Reeves and co-workers [76] correlated the change in the chemical shift of NMR spectra caused by formation of an intramolecular hydrogen bond in o- ubstituted phenols including o-nitrophenol, 2,4-dinitrophenol, I-nitro-2-naphthol, 2,4-dinitro-l-naphthol etc. 

CST sometimes gives some insight into molecular structure. Thus o-nitro-phenol has a CST with hydrocarbons much lower and with water much higher than do m-nitrophenol and p-nitrophenol. This indicates some hydrogen bonding in the ortho isomer, so that it resembles an ester in its solubility relationships. 

The frequency of the nitro group in o- nitrophenol was markedly reduced, to 1320 cm"1, as the result of hydrogen bonding between the nitro and phenolic group ... 

NMR spectra of nitrophenols offered a great help in examination of intramolecular hydrogen bond in nitroaJcohols [68a] and o-nitrophenols [42]. The problem is discussed in the paragraph on hydrogen bond (p. 81). 

Dega-Szafran, Z., Kania, A., Grundwald-Wyspiahska, M., Szafran, M. and Tykarska, E., Differences between the N H O and O H O hydrogen bonds in conplexes of 2,6-dichloro-4-nitrophenol with pyridines and pyridine iV-oxides, J. Mol. Struct. 381 (1-3), 107-125 (1996). 

In some cases, the intramolecular hydrogen bonding stabilizes Q and shifts the first wave to the positive side. For example, the first wave of o-nitrophenol E1/2= -0.33 V) is much more positive than those of m- and p-nitrophenols ( 1/2=-0.58 V and -0.62 V) [58]. This is due to the intramolecular hydrogen bonding of the radical anion of o-nitrophenol. 

This structure accounts for certain properties of o- nitrophenol, and namely those by which it differs from its m- and p- isomers. For example, o- nitrophenol is volatile in steam. This may be explained by the fact that owing to the presence of an internal hydrogen bond, the intermolecular links which are responsible for molecular association are either weakened or non-existent. This is the reason why the... 

The solid o-nitrophenol was neither stabilized nor destabilized relative to prediction neither is ca 9 kJ mol a very large stabilization for two possible pairs of hydrogen-bonding hydroxy/nitro substituents in 4,6-dinitro-l,3-benzenediol. Accordingly, the reaction depicted in eqnation 17 is only 3.6 kJ moH endothermic. 

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