Rates of isomerism

Nitration in acetic acid, in sulpholan and in carbon tetrachloride showed kinetic phenomena similar to those shown in nitromethane this is significant for it denies nitromethane a chemical involvement in the slow step. (Originally the rate of isomerization of nitromethane to its aci-form was believed to be a factor in the reaction. )... 

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

The composition of the products of reactions involving intermediates formed by metaHation depends on whether the measured composition results from kinetic control or from thermodynamic control. Thus the addition of diborane to 2-butene initially yields tri-j iAbutylboraneTri-j -butylborane. If heated and allowed to react further, this product isomerizes about 93% to the tributylborane, the product initially obtained from 1-butene (15). Similar effects are observed during hydroformylation reactions however, interpretation is more compHcated because the relative rates of isomerization and of carbonylation of the reaction intermediate depend on temperature and on hydrogen and carbon monoxide pressures (16). 

The above method is unsatisfactory when hydration takes place at two alternative sites in the molecule, although one hydrate is usually present in only a very small proportion, at equilibrium. Which oxo compound is preferentially formed in such a case depends on the rates of oxidation at the different sites and on the rate of isomerization of the water molecule from one position to the other, hence this method does not indicate which is the thermodynamically more stable hydrate. 

Decomposition of frarcs-[Co(pn)2Cl2](H502)Cl2 [1138] is accompanied by some solid state isomerization to the cis form. Reported values of E, based on first-order behaviour, were 138 and 500 kJ mole 1 for decomposition and isomerization, respectively. The rate of isomerization in the... 

The lack of any difference in the rate of isomerization between fluoro-sulfonic acid solutions of 34 which had been thoroughly degassed, and those which were saturated with oxygen, suggests that the reaction does not proceed via a triplet mechanism. In fluorosulfonic acid no unproton-ated acid is detected, ruling out the possibility of n,7r excitation. Thus, there is little doubt in this case that it is the Tr,Tr singlet state which is the reactive species. Experiments carried out with a variety of methyl-substituted protonated cydohexadienones have likewise ruled out the... 

A molecular view of the isomerization of ds-2-butene illustrates that the rate of isomerization decreases as the number of ds-2-butene molecules decreases. 

The effects of changing Jt-conjugation at the 4-position on both the rate of isomerization of the initially formed o-quinones to QMs and the reactivity of the quinoids formed from 4-propylcatechol, 2,3-dihydroxy-5,6,7,8-tetrahydronaphtha-lene (2-THNC), hydroxychavicol, and 4-cinnamylcatechol were studied (Fig. 10.6).9 These catechols were selectively oxidized to the corresponding o-quinones or QMs and trapped with GSH. Microsomal incubations with the parent catechols produced only o-quinone-GSH conjugates. However, if GSH was added after an initial incubation period both o-quinone- and QM-GSH conjugates were observed. The results indicate that the extended Jt-conjugation at the para position enhances the rate... 

The rate of isomerization increases with increasing number of hydroaromatic rings. (Previous work [9] has indicated that the rate of rearrangement of hydroaromatics is greatly increased when free radicals are present). 

Our conclusion regarding the importance of reaction via an adsorbed carbocyclic intermediate, together with the data in Fig. 12, lead to the conclusion that over ultrathin film catalysts, the specific rate of isomerization via the carbocyclic pathway decreases as the average platinum particle... 

Recently, the hydroxy derivatives of furan, thiophene, and selenophene have been studied with regard to their physical properties and reactions. These compounds are tautomeric and if the oxygen function is placed in the 2-position they exist as unsaturated lactones and undergo carbon-carbon rearrangement, whereas the 3-hydroxy derivatives form oxo-enol tautomeric systems. By NMR the structures of the different tautomeric forms have been determined as well as the position of the tautomeric equilibrium and the rate of isomerization. 

Several interesting observations have been made on this reaction. First, the rate of isomerization was found to be the same as the rate of dehydration. All attempts to dehydrate the starting complex by conventional techniques were found to lead to isomerization. On the basis of this and other evidence, the mechanism proposed involves the aquation in the complex followed by anation. In this process, water first displaces Cl- in the coordination sphere and then is displaced by the Cl-, possibly by an SN1 mechanism. A trigonal bipyramid transition state could account for the Cl- reentering the coordination sphere to give an cis product. The rate law for this reaction is of the form... 

During the study of the isomerization of diallyl ether with catalysts of this type, it was observed that the nature of the complex anion (Y) has a profound influence on the relative rate of isomerization with respect to cyclization. Diallyl ether can be isomerized to a mixture consisting mainly of allyl propenyl ethers (44) with a small amount of 2-methyl-3-methylene-tetrahydrofuran (45) and dipropenyl ether (46) the catalyst for this... 

In the model study by Tolman discussed earlier, the half-life of syn-to-anti isomerization measured by H NMR was found to be 0.36 hours at 30°C. This rate of isomerization is far too slow to affect the stereoselectivity of the hexadiene formed with the catalyst considered here. With the bimetallic catalyst, reaction rates frequently approach 4000 molecules of hexadiene/Ni atom/hour at 25°C (or ca. 1 hexadiene/Ni/second). The rate of insertion reaction d must be at least as fast as this, and the isomerization reaction would have to be even faster to affect the trans/ cis ratio of the product. 

The primary product will be rich in the para isomer if initial m-and o-xylene diffuse out of the zeolite crystal at a lower rate (Dm q/t2) than that of their conversion to p-xylene (kj) and the latter s diffusion (Dp/r2). Conversion of the para-rich primary product to secondary product low in p-xylene is minimized when the actual, observed rate of isomerization (kj g is lower than the rate of toluene disproportionation (kD). 

Pettyjohn, E.S. and Christiansen, E.B., Effect of particle shape on free-settling rates of isomeric particles, Chemical Engineering Progress, 44, pp. 157-72 (1948). 

The rate of isomerization of 2-methylbicyclo[2.2. l]hepta-2,5-diene [165] to 5-methylenebicyclo[2.2.1]hept-2-ene [166] by t-BuOK in DMSO (30) was not affected by the presence of 18-crown-6 at low base concentrations (Maskor-nick, 1972). At high concentrations of t-BuOK (>0.1 M), the reaction is zero... 

Substituent effects on the rates of isomerization of aryl-di-rerf-butylcarbinols have been studied. The results are shown in Table 17 (116). The substituent effect on the barrier is, as expected, small if the substituent is in either the 4-or the 3-position. However, a buttressing effect of the substituents in 2,3,4,5-tetramethylphenyl-di-rm-butylcarbinol is apparent. The barrier is raised by ca. 1 kcal/mol in this case. 

Effect of Butyllithium Concentration on the Rate of Isomerization and Equilibrium Constant of 2-Methylphenyl-di-/m-butylcarbinol (74) in Hexane at 25°C... 

Monitoring the polymerization of each substrate provided an informative picture of the effect of both substituents and of isomer distribution on the curing process. We first addressed the question of the relative rates of substrate isomerization and polymerization. We found that, for the parent monomer (PN and PX), the rate of isomerization greatly exceeds the rate of polymerization. Under conditions where PN and PX are fully equilibrated (195°C/15 hrs or 250 /l hr) there is still less than 20% polymer formation in the neat sample. We conclude that for PN or PX the composition of the mixture undergoing polymerization is essentially independent of the starting isomer. The observation that fully cured samples of either PN or PX show identical and C NMR spectra and indistinguishable SEC analyses, is consistent with this contention. 

This compound undergoes a first-order thermal isomerization to give methylenecyclobutane in the temperature range 360-410° C (Flowers and Frey, 1961c). The rates of isomerization fit the Arrhenius equation... 

The thermal isomerizations of a number of cyclobutenes have also been studied in the liquid phase (Criegee ei a ., 1965). The rates of isomerization of cia- and [Pg.187]

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