HYDROGEN BONDING ABILITIES OF PHENOLS

V. HYDROGEN BONDING ABILITIES OF PHENOLS A. Introductory Survey... 

TABLE 34. References for some experimental and theoretical data on the hydrogen-bonding ability of phenol and its derivatives. The piTj value of phenol and its derivatives is indicated in parentheses... 

Shindo studied the hydrogen-bonding ability of a fairly long series of substituted pyridine 1-oxides with methanol in chloroform solution and found that the OH frequency of the hydrogen-bonded OH group in methanol is well correlated with the <7-values. For four compounds, the intensity of the same band is also well correlated. In a similar study the OH frequencies of phenol vary monotonically with the CT-values, but not in a linear fashion. 

The same effect is typical of the reactions of alkoxyl radicals with phenols, that is, these reactions are much slower in solvents capable of forming hydrogen bonds with O—H and N—H groups [50]. MacFaul et al. [50] proposed a universal scale for correlating the reactivities of phenols and the hydrogen-bonding abilities of solvents. 

The hydrogen-bonding abilities of the ring nitrogen atom have also been the subject of investigation using IR spectroscopy, with proton donors such as phenol, pyrrole, and acetylenes in dilute carbon tetrachloride solution. Good correlations are observed 128-131 thus Eq. 

Despite this voluminous literature on hydrogen bonding, there have been very few discussions on the hydrogen-bond basicity of phenols. The ability of phenols to act as hydrogen-bond acceptors is considered in Section II. 

The potential for hydrogen bonding between a phenolic resin and wood depends upon (1) The number of reactive groups on the resin and wood molecules at which hydrogen bonding may occur (2) The ability of the adhesive to wet and penetrate the capillary structure of the wood ... 

Zaror [497] compared the uptakes of four phenols on a commercial activated carbon at a pH of 2 and reported both relatively low uptakes (<0.1 mmol/g at 0.3 mmol/L) and relatively small differences, in spite of the substantial differences in the nature of the substituents. Wang et al. [439] reported that the uptakes of p-nitrophenol, p-chlorophenol, and phenol were consistent with the notion that materials of high molecular weight are adsorbed to a more considerable extent than those of low molecular weight for compounds of similar chemical constitution Traube would be happy to learn that a century later [498] his rule lives on and that, under certain (perhaps very limited) conditions, the complexities discussed elsewhere in this review can be ignored. A clear example of the inapplicability of Traube s rule, however, is the study of Mostafa et al. [416] the authors did not attribute this to electrostatic effects or to changes in 7t- electron density (see below) but to the difference in the ability of hydrogen bond formation of the different phenols. ... 

During the 160 years since the discovery of phenol, thousands of studies were conducted on halophenols, partly due to their significance in the theory of hydrogen bonding indeed their hydrogen bonding abilities can be varied nearly continuously over a wide pATj range from 10.2 to 0.4 - ". 

The main purpose of Section III is to establish the position of phenols on the scales of hydrogen-bond acidity, either solute (log X", a" logXa) or solvent (Ej(30)) scales. Here, the ability of phenols to act as hydrogen-bond donors will be compared to that of other O—H (water, alcohols, carboxylic acids), N—H, S—H and C—H hydrogen-bond... 

It is well known that substituents have a profound effect on the hydrogen atom donating ability of phenols. Indeed, only those phenols bearing electron donating substituents, particularly at the ortho and/or para positions, are active as antioxidants. In general, this is as expected since such groups are expected to lower the phenolic O—H bond dissociation enthalpy and increase the reaction rates with peroxyl radicals. 

The dipole moments wctc related to hydrogen bond strength and to the ability of the proton acceptors (compounds with phosphoryl and carbonyl groups) to associate with phenol. Hydrogen bonded complexes of pyridines, quinolines and acridine with phenol are studied, in carbon tetrachloride, by dipole moment measures . Dipole moments of the complexes were related to the ability of the acceptors to associate with phenol and with their basicity. Hydrogen bonds may either reinforce the dipole moments of the resultant of the group moments or may oppose the dipole moment direction, depending on the nature of the acceptor. 

As a p-hydroxy derivative of phenylalanine, tyrosine has properties similar to it plus the ability to form hydrogen bonds involving its phenolic —OH group. 

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