Isomerization equilibrium

We will introduce the (Z)/(E) isomeric equilibria of diazohydroxides and diazoates in Section 5.2. In the present section the formulas ArN2OH and ArN20 in Schemes 5-1 to 5-12 refer only to the (Z) isomers that are formed initially. 

Shortly afterwards and independently, the group of Sterba in Pardubice (Czechoslovakia) published a series of papers on the kinetics of acid-base and isomerization equilibria of substituted benzenediazonium ions. Initially they used classical methods for rate measurements (Machackova and Sterba, 1972 a, 1972 b Jahelka et al., 1973a), then later stopped-flow techniques (Jahelka et al., 1973b). 

Here Z denotes the number of isomeric equilibria, which have to be summed from i = l to i = n. fj, denotes a proportionality constant relating the w-electron energy and free energy differences. The results of these calculations are summarized in Table 27. In contrast to the tables given by Ebrenson (1961), no account is taken of proton addition complexes in which the addition of the proton takes place at a C-atom which carries a methyl group. 

Diazabicyclo[3,3,0]octa-2,6-dienes of type 391 undergo a sequence of electrocyclic reactions upon thermolysis and photolysis (82JPC338) to yield 4,5-diazaocta-l,3,5,7-tetraenes (393), probably via 392 (79CB2620). The influence of the substitution pattern on the conditions necessary for initiation of the reaction and on the valence isomeric equilibria have been studied (86JPC5552). 

TABLE 4. Computed energy and free energy changes, AE and AG, and equilibrium constants Jileq, for isomerization equilibria at 298 K, at toe B3LYP/6-311G(3df,2p) level... 

Thermodynamic Equilibrium, Kinetics, Activation Barriers, and Reaction Mechanisms for Chemical Reactions in Karst Terrains (White, 1997) Solvent Effects On Isomerization Equilibria—an Energetic Analysis in the Framework of Density Functional Theory (Lelj and Adamo, 1995)... 

Alkane isomerization equilibria are temperature-dependent, with the formation of branched isomers tending to occur at lower temperatures (Table 4.1). The use of superacids exhibiting high activity allows to achieve isomerization at lower temperature (as discussed below). As a result, high branching and consequently higher octane numbers are attained. Also, thermodynamic equilibria of neutral hydrocarbons and those of derived carbocations are substantially different. Under appropriate conditions (usual acid catalysts, longer contact time) the thermodynamic... 

Table 4.1. Temperature-dependent isomerization equilibria of C4-Ce alkanes12...

More recent work applying modem analytic techniques for reliable identification of products revealed that the product distribution was almost independent of the temperature (the normal isopropyl ratio was 34 66 at — 18°C, and 30 70 at 80°C).116 In contrast, alkylation of p-xylene exhibited strong temperature-dependent regioselectivity117 [Eq. (5.24)] due to obvious isomerization equilibria ... 

Steric and electronic factors operating in valence isomerization equilibria of the 7-substituted compounds greatly affect product distribution. They react to yield only the corresponding compounds 53 and 56. The 56 53 ratio increases in the order Me < Et < isoPr < tert-Bu < Ph 423 Alkenylarenes, such as stilbenes,424 indenes 425 vinylnaphthalenes426 [Eq. (9.92)], and vinylphenanthrenes,427 react with singlet oxygen with the participation of the aromatic n electrons in the transformation 424,425... 

Influences of substituents on the positions of isomeric equilibria that have been calculated116 by the HMO route do not agree with experimental findings (see Section III,B,1). A correlation of the acidity of 3H-pyrrolizine(l)with the difference in HMO delocalization energies between the pyrrolizine anion and the neutral molecule has been attempted but showed limited success.115... 

Table 5.4. Thermodynamic Isomerization Equilibria of Butanes, Pentanes, and Hexanes at Various Temperatures85...

The combined use of IR and NMR spectroscopy provides a complete picture of the isomeric equilibria of these compounds134 193 194. Quantum-mechanical calculations of the structure, relative energies, solvation185 186 and vibrational spectra195 of a series of model 2-acyl-2-nitroenamines have confirmed the structural assignment of the isomers either in solution or in the pure state. 

P. R. Lawrence, T. R. Proctor, and P. Politzer, Int. ]. Quantum Chem., 26, 425 (1984). Reactive Properties of rrans-Dichloro-oxirane in Relation to Carcinogenicities of Vinyl Chloride and truns-Dichloro-ethylene. J. S. Murray and P. Politzer, Int. J. Quantum Chem., 31, 569 (1987). A Computational Study of Isomerization Equilibria Relation to Vinyl Chloride Carcinogenicity. P. Politzer and J. S. Murray, in Reviews in Computational Chemistry, Vol. 2, K. B. Lipkowitz and D. B. Boyd, Eds., VCH Publishers, New York, 1991, pp. 273—312. Molecular Electrostatic Potentials and Chemical Reactivity. 

Thiopyrylium ions can function as catalyst in the equilibrium isomerization between the corresponding IH- and 4//-thiopyrans, by abstracting a hydride ion as shown in Eq. (10) [77ACS(B)496 91JOC1674]. Such isomerization equilibria have also been studied theoretically by MNDO and AMI methods (91JOC4431). The kinetics of isomerization of 2,4,6-triphenyI-4//-thiopyran in the presence of 2,4,6-triphenylthiopyrylium ion has been investigated in DMF at various temperatures (81JHC1517). 

All those spectral changes which arise from alteration of the chemical nature of the chromophore-containing molecules by the medium, such as proton or electron transfer between solvent and solute, solvent-dependent aggregation, ionization, complexation, or isomerization equilibria lie outside the scope of this chapter. Theories of solvent effects on absorption spectra assume principally that the chemical states of the isolated and solvated chromophore-containing molecules are the same and treat these effects only as a physical perturbation of the relevant molecular states of the chromophores [435-437]. 

Sigel H, Kapinos LE (2000) Quantification of isomeric equilibria for metal ion complexes formed in solution by phosphate or phosphonate hgands with a weakly coordinating second site. Coord Chem Rev 200 563-594... 

In general chemistry, in particular in discussion of isomerization equilibria, we may well require accuracy a good deal higher than this. Our only information about heats of isomerization of 1 or 2 kcal rnnle t may well come from combustion experiments where the total heat quantities measured are perhaps some thousands of kcal mole b At present the greatest accuracy normally attained is about 0 02 per cent, but Parks has pointed out that in- .c uraries in thermal data have such a large effect on equilibrium ( instants that a tenfold improvement in accuracy is desirable. 

The knowledge of isomerization equilibria is particularly important in hydrocarbon chemistry, and this is reflected in the body of very accurate measurements of heats of combustion of hydrocarbons which has become available since 1939, The spate of results has been such that determinations of the heats of combustion of various compounds have been replicated in several laboratories, and it is of interest to compare their results. The heats of combustion of several r/oparaffins, for example, have I >cen deter-... 

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