2-Nitrophenol, hydrogen bonds

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 ). 

Some ortho-substituted phenols, such as o-nitrophenol, have significantly lower-boiling points than those of the rneta and para isomers. This is because the intramolecular hydrogen bond that forms between the hydroxyl group and the substituent partially compensates for the energy required to go from the liquid state to the vapor. 

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... 

What about jr-electron donor substituents on nitrosoarenes other than dimethylamino Pedley gives us the enthalpies of formation for three hydroxy derivatives the isomeric 4-nitroso-l-naphthol, 2-nitroso-l-naphthol and l-nitroso-2-naphthol, species 45-47 respectively. Of the three species, only the first cannot have an intramolecular hydrogen bond. By analogy to nitrophenols75 — there being no thermochemical data for the more related and hence relevant nitronaphthols—we expect that species 46 would be less stable than 45. After all, gaseous 2-nitrophenol is ca 20 kJ mol 1 less stable than its 4-isomer. We recall from the discussion of the isomeric naphthylamines that 1- and 2-naphthol are of almost identical stability. This suggests that species 46 and 47 should be of comparable stability. Both expectations are sorely violated by the literature results the enthalpies of formation of species 45, 46 and 47 increase in the order —20.3 4.9, —5.4 6.2 and 36.1 4.7 kJmol-1 respectively. If there is experimental error, where does the error lie ... 

Fig. 9 Proposed potential well for the hydrogen-bonded complex between 4-nitrophenol and 4-nitrophenolate (Kreevoy and Liang, 1980).

The ortho and para isomers can be separated by steam distillation. o-Nitrophenoi is steam voiatile due to intramolecular hydrogen bonding whiie p-nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules. 

This resulted in an enthalpy of reaction 39 of ca 70 klmol". While this is much larger than what would be predicted for the correction due to nitroso-phenol conjugation and the energies of most hydrogen bonds, the value for the related reaction for p-nitrophenol (equation 40) is the rather comparable 80 klmol". ... 

Among the nitrophenols, meta- and para-nitrophenols have high boiling points because of intermolecular hydrogen bonding, as shown below. 

These two nitrophenols are also soluble in water due to intermolecular hydrogen bonding with water molecules as outlined below. 

Intermolecular hydrogen bonding of para-nitrophenol with water... 

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. 

Slightly greater effects are observed for the B-bands of m-hydroxy-benzaldehyde and ra-nitrophenol (D, H-bond +68 and +96 A), although the nitro and formyl groups may not be terminal. Greater displacements should have been expected for the B-bands of o-hydroxy benzaldehyde and o-nitrophenol, but, owing to the formation of intramolecular hydrogen bonds by these derivatives (cf. below), an experimental confirmation is, unfortunately, not possible. 

Tills is illustrated in Table 7 by the B- and K-bands of benzene-azo-p-eresoi and c-nitrophenol, which contain intramolecular hydrogen bonds, and of their methyl ethers. The displacements of these bands on... 

The bis 2,4-dinitro- and 2,4,6-trinitro-phenol adducts of the 8-hydroxyquinolinate, T QTUNOU ROH, are obtained when ThtQHftNO -CsHvNO is heated with the phenol. The frequencies of the asymmetric and symmetric v(NO) features due to the nitrophenols are almost identical in their IR spectra to those of the free phenols, which may therefore be hydrogen bonded to oxygen atoms of the QTUNO groups. 

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... 

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 solvatochromic effects on UV/visible spectra of certain solutes are so large, that they can conveniently be employed as probes for certain solvating properties of the solvents. Those that have enjoyed widespread application in this capacity are discussed in Chapter 4. They include 2,6-diphenyl -4-(2,4,6-triphenyl-l-pyridino)-phenoxide, 4-methoxynitrobenzene, 4-(dimethylamino)-nitrobenzene, for the estimation of the polarity of solvents, acetylacetonato-N,N,N, N -tetramethylethylenediamino-copper(II) perchlorate, 4-nitrophenol, and 4-nitroaniline, for the estimation of the electron pair donicity of solvents, 4-carboxymethyl-l-ethylpyridinium iodide, 4-cyano-l-ethylpyridinium iodide, and bis-c/.v-1, lO-phenanthrolinodicyano-iron(II) for the estimation of the hydrogen bond donation abilities of solvents (Marcus 1993). 

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