Thermodynamics phenol hydrogenation

Hydrogen bonding and complexing properties TABLE 4. Thermodynamic data for formation of phenol adducts... 

The thermodynamic EM for the hydrolysis of the salicylate derivative is therefore favoured by a factor presumably similar to that which favours the hydrogen-bonded form of the salicylate anion. In terms of the dissociation constant of the phenol group, which is some 3 pAT units less acidic than expected for a phenol with an ortAo-carboxylate group (Eberson, 1969), a factor of 103, which is of the right order of magnitude to explain the observed effect, could be involved. 

When the binding energy of a hydrogen to a heteroatom is weak, heteroatom-centered radicals are readily produced by H-abstraction or one-electron oxidation followed by H+ loss. Typical examples are phenols (e.g vitamin E in non-aqueous media), tryptophan and related compounds and thiols. Deprotonation of radical cations is indeed often a source of heteroatom-centered radicals even if a deprotonation at carbon or OH addition upon reaction with water would be thermodynamically favored. The reason for this is the ready deprotonation at a heteroatom (Chap. 6.2). 

Of special interest for petrochemical and organic synthesis is the implementation of thermodynamically hindered reactions, among which incomplete benzene hydrogenation or incomplete cyclohexene and cyclohexadiene dehydrogenation should be mentioned. Cost-effective methods of cyclohexene production would stimulate the creation of new processes of phenol, cyclohexanol, cyclohexene oxide, pyrocatechol synthesis, cyclohexadiene application in synthetic rubber production, and a possibility for designing caprolactam synthesis from cyclohexene and cyclohexadiene via combined epoxidation. At present, the most... 

The study of chemical equilibrium can detect thermodynamic constraints on the achievable conversion and selectivity. In this section we make use of the Gibbs free-energy minimization method available in Aspen Plus [9], We assume that both cyclohexanone and cyclohexanol are products. The curves in Figure 5.2 show the evolution of the phenol equilibrium conversion, yield and selectivity with the ratio hydrogen/phenol at temperatures of 180, 200, 220 °C and a pressure of 3 bar. 

Hindered phenolates have low nucleophilicity and in aprotic solvent may act usefully as EGBs. 2,6-Di-t-butyl-/ -cresol = 16.8) was reduced directly with concomitant hydrogen evolution to give, ex situ, the corresponding tetraethylammonium phenolate [59,60], which was clearly capable of deprotonating aromatic ketones and in the presence of aromatic aldehydes promoted aldol reaction to a, /3-unsaturated ketones which underwent Michael addition. The initial proton transfer from the aromatic ketone ] K = 24.7) is thermodynamically very unfavorable. Even so, aldol reaction took place within a matter of hours upon addition of an aromatic ketone together with an aromatic aldehyde leading to or, /3-unsaturated ketones which subsequently underwent Michael addition with a sec-... 

A topic related to that of the gas-phase acidities of phenols is the quest for quantitative data on the thermodynamic stability of hydrogen-bonded complexes, or clusters , [ArO-H X ] and [ArO H-X] between phenols and various anions derived from other Brpnsted acids. The thermodynamics of cluster formation of the phenolate ion [1 — H] with water, ethanol and acetic acid have been determined by using a pulsed electron-beam mass spectrometer and their stability was found to increase with the gas-phase acidity of the Br0nsted acid. For example, association of [1 — H] with H2O is much weaker. A// = —15.4 kcalmor , than that of [1 — H] with CH3COOH, AHq = —27.4 kcalmor The association enthalpy of the complex of phenol and fluoride ion, [1 F ], has been measured to be A//° = —41.3 kcalmor , much stronger than... 

In these compounds, a strong, thermodynamically stable intramolecular hydrogen bond is formed between a phenolic OH group and an A,A-dialkylaminomethyl substituent in the 2-position (73). orf/jo-Mannich bases are excellent models for the investigation of... 

Various types of phenol complexes will be examined in Sections IV-VI. Dimers and multimers of self-associated phenols, (ArOH) , will be considered both in solution and in the solid state (Section IV). The existence and, subsequently, the geometry and energy of intramolecular hydrogen bonds in orf/zo-substituted phenols are discussed in Section V. The most recent thermodynamic, spectroscopic (mainly IR), geometrical and theoretical results on the heterodimers of phenols complexed to Lewis bases, ArOH- -B, will be presented in Section VI. 

FIGURE 2. Correlation between the thermodynamic pIsThb (towards 4-fluorophenol) and the spectroscopic Av(OH) (towards perfluoro-t-butyl alcohol) scales of hydrogen-bond basicity (n = 7, r = 0.998) allowing the calculation of p fuB for very weakly basic cohols and phenols... 

Most thermodynamic studies of the equilibria between hydrogen-bonded complexes of phenols and their free component molecules have been conducted in a diluting solvent. Binary solutions of phenols (phenol o-cresol ) in the pure base propionitrile have also been studied by means of Raman and IrW3,i44 spectrometry. Factor analysis of the v(C=N) band indicates the formation of a 1 1 complex over a large concentration range. However, this procedure is not recommended for the determination of equilibrium constants because these exhibit a strong concentration dependence. 

TABLE 11. Summary of thermodynamic results for hydrogen-bonded complexes of phenols with various bases at 25 °C in different solvents... 

For technical reasons, phenols are convenient reference hydrogen-bond donors for hydrogen-bonding studies. We present below their use for constructing thermodynamic and spectroscopic scales of hydrogen-bond basicity. These scales are either solute scales when the phenol and the base are dissolved in an inert solvent, or solvent scales when the phenol is studied in the pure base. In the latter case, methods such as the solvatochromic comparison method or the calorimetric pure base method have been developed to unravel the hydrogen-bond contribution to the overall solvent effect. 

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