Phenol hydrogen-bond enthalpy

Third, the AH" and AS" valnes of many hydrogen-bonded complexes have been obtained from van t Hoff plots where the temperature range AT was nsnally too small. Enthalpies and entropies calcnlated with AT = 10° for the complexes of 4-nitrophenol with amines are inevitably less reliable than those calcnlated with AT = 78° for snbstitnted phenols hydrogen-bonded to dhnethylacetamide or with AT = 57° for snb-stimted phenols complexed with diphenyl snlfoxide , simply becanse the error in AH" is inversely related to AT. 

Drago and coworkers have also calculated the enthalpy values for the formation of many complexes or hydrogen bonds by NMR and calorimetric techniques. For example, in a series of phenols or t-BuOH, they observed the IR frequency shifts (Avqh) of the hydroxyl compounds and found that a linear relationship exists between bases and individual acids. In Table 7 shows some AH values calculated by equation 2, and Avqh values of t-BuOH" " while in Table 8 frequency data Avq of various substituted phenols and the AH values are given. 

Any analytical method [312] suitable for determining equilibrium compositions of a reaction mixture at several temperatures can be used to obtain the enthalpy and the entropy of that reaction. The first example we describe involves a common analytical technique (infrared absorption spectroscopy) and addresses the energetics of the hydrogen bond between phenol and acetonitrile. This careful study on the equilibrium 14.6 was made by Sousa Lopes and Thompson more than 30 years ago [313]. 

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". ... 

The favourable intra-molecular hydrogen-bonding between the oxime proton and the phenol oxygen atom will reduce or overcome liganddigand repulsion enthalpy terms normally involved in the formation of bis-chelate complexes. The integrity of this head-to-tail macrocyclic assembly is preserved in c/.v-octahcdral complexes which are formed in the presence of a, co-diamines. [66, 67]... 

The amino acid tyrosine is related to the amino acid phenylalanine in the same way as phenol is related to benzene. The enthalpy difference between the amino acids is —219 1.8 kJ moH, somewhat larger than the 5 (OH/H) increment of —203 kJ mol . However, in the solid phase, tyrosine may be stabilized by additional hydrogen bonding sites unavailable to phenylalanine. 

The o- and p-nitrosophenols enjoy the possibility of resonance stabilization by jt-electron donation from the phenolic hydroxyl group to the nitroso group, and the o-isomer could also be stabilized by an intramolecular hydrogen bond. These species are also tautomeric with benzoquinone oximes. All of this could confound interpretation of enthalpy of formation values if only they were available—there are seemingly no measured enthalpy of formation values for o-nitrosophenol. The value for p-nitrosophenol will be discussed later in Section VI because of tautomeric ambiguity. The m-species lacks the stabilizing conjugate NO/OH interaction, and so the monomer-dimer equilibrium as found in other nitroso compounds becomes problematic—should the measurement of enthalpy of combustion be available. 

From archival enthalpies of formation and of fusion, the estimated enthalpy of formation of solid 3,5-dibromotoluene is 28 9 kJ moF. This value, combined with 5 (OH/H), gives a predicted enthalpy of formation of the corresponding phenol of —175 kJ moF. A bromine atom and methyl group crowd the intervening OH, which could account for at least some of the ca 16 kJ moF difference between the predicted and experimental values, and we don t expect 2,4-dibromo-6-methylphenol to participate in intermolecular hydrogen bonding. The remainder of the difference is accounted for by the error bars. Altogether, the value is plausible. 

From the archival enthalpies of formation and of fusion for benzene and bromobenzene, the estimated enthalpies of formation of 1,3,5-tribromobenzene are (s) 72 kJ mol and (g) 151 kJ moH. From these values and the appropriate OH/H exchange increments, we would predict enthalpies of formation for 2,4,6-tribromophenol of —131 kJ mol for the solid and —28 kJ moC for the gas phase species. The predicted results are both ca 30 kJ moC more exothermic than the experimental ones. Since we don t expect the solid tribromophenol to participate in intermolecnlar hydrogen bonding in the same way as solid phenol does, on that basis the estimated valnes are seemingly too negative. 

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