Isomeric reactions equilibrium state

Another example is the thermal and photochemical cis-trans isomerization of Cp2Fe2(CO)2( -CO)( -Sip-TolH).25 In this case, both cis(H) and trans isomers can be isolated at full purity by flash chromatography. Interconversion between these isomers occurs both thermally and photochemi-cally in cyclohexane-d12, and the composition in the thermal equilibrium state (cis(H) trans = 2 98 at 25°C) is extremely different from that in the photostationary state (cis(H) trans = 70 30). Kinetics of the thermal isomerization in decalin afforded the activation parameters shown in Eq. (58). The large negative activation entropies imply that this reaction also... 

The primary isomer distribution, which is the result of the disproportionation reaction, may deviate significantly from the thermodynamic equilibrium composition, for two reasons. First, the reaction may be controlled by the kinetics rather than the thermodynamics, i.e. mechanistic reasons may exist which cause the reaction to proceed along a certain path. Second, in the case that the reaction obeys a bimolecular mechanism, it may pass through a transition state which would presumably favor the (taller) para isomer. Hence, it is possible that the primary product contains an enhanced fraction of the para isomer. The departure from the equilibrium composition then gives the driving force for the subsequent (monomolecular) isomerization reaction. This will reduce the fraction of the para isomer, provided the formation of the bulky ortho and meta isomers are not inhibited by sterical effects, i.e. when the micropore diameter is sufficiently large or there is a chance for the isomerization reaction to take place at the outer surface of the crystallites. Thus, the secondary isomer distribution may approach the thermodynamic equilibrium composition, as a limiting case. 

The transalkylation and isomerization reactions can be satisfactorily explained by the Streitwieser mechanism( ). This mechanism proposes a 1,1-diphenylethane-type intermediate. For example, para-diethylbenzene. (Figure 3) Such an intermolecular mechanism is consistent with the experimental data and does not require the assumption of a sequence of intramolecular 1,2 shifts. The decay of the polyethylbenzenes towards equilibrium is consecutive and not concurrent. The catalyst seems to be associated with the most basic center and when it reaches steady-state, the catalyst transfers to the next most basic one. There is also a concurrent intramolecular isomerization such as 1,2,4 triethylbenzene going to 1,3,5 triethylbenzene. There is hence a movement towards isomer equilibrium as well as product equilibrium. 

Fig. 9.1 Reaction profile for peptide bond cis-trans isomerization. The kinetic constants for the trans to cis (k, J and cis to trans (kM>) isomerization are dependent on the Gibbs free energy of activation (AG ). The ground state energy difference AG° (G°as - C°mm) determines the population of the cis and trans isomers in the equilibrium state.

Eslami studied the solid-state thermal isomerization of [Co(NH3)5(ONO)]Cl2 (nitrito isomer) to [Co(NH3)5(N02)]Cl2 (nitro isomer) and the reverse reaction, using DSC [134], The isomerization was shown to be essentially an equilibrium process and the interconversions are accelerated at above 65 °C, to reach the equilibrium state at about 155 C. After establishment of the equilibrium, the relative amounts of the two isomers at any temperature are governed by a Gibbs energy relationship. 

On the other hand, it is expected for the crystalline products (Cl C2) that all variants for a pair of genuine supramolecular isomers in (l)-(3) are identical (Aj Vn s (soiv) = YynRT ln[fi ] = 0) in a given medium. Variation of reaction medium should have the same energetic effect on different isomers. Therefore, thermodynamic factors of the equilibrium states do not account for medium-dependent genuine supramolecular isomerism. The fact that variation of crystallization mediums can produce genuine supramolecular isomers reflects the kinetic nature of supramolecular isomerism. ... 

There exists a certain analogy between a mixture of quasicomponents and a mixture of chemically reacting species. For the sake of simplicity, we recall the case of isomerization reaction, treated in section 2.4. We have stressed there that the distinction between the two species A and B was based on an (arbitrary) classification of all the states of the molecules into two groups. If the classification is such that transitions between the two groups of states is very slow compared with transitions within each group, then it might be possible to isolate the species A and B as pure components. This normally involves the introduction of an inhibitor to the reaction A B. However, whether such an inhibitor actually exists or not, it is of no importance for the formal theory of chemical equilibrium. Therefore, we can use the classification into quasicomponents to view the one-component system as if it were a mixture of species in chemical equihbrium. 

The goals of microscopic theories of chemical rate processes are to define the range of validity of the macroscopic law, to provide microscopic expressions for the rate coefficients and to extend the kinetic description to smaller distance and time scales. Traditional approaches model the isomerization reaction by motion of a particle of mass m across a potential barrier in a thermal bath of temperature T. Transition state theory (TST) [2] assumes an equilibrium distribution at the barrier top the rate is given by the equilibrium one-way flux across the barrier. The rate thus obtained has the form ... 

Slobodin reported the photodimerization reaction of allene. Irradiation of 1,1-dimethylallene 49 affords dimers 63 and 64. The ring-opening reaction of cyclic bisallene 65 was reported by Kaupp to yield butatriene 66 on irradiation in a low-temperature matrix,and the reverse reaction from butatriene 67 to cyclic bisallene 68 in the solid state was also reported by Berkovitch-YeUin and co-workers. The photochemical racemization of optically active penta-2,3-diene 69 in hexane was reported by Rodriguez and Morrison. Kuhn and Schulz reported the photochemical ds,trans-isomerization reaction between cis-butatriene 70 and trans-butatriene 71 and also reported a 39.5% 60.5% cis-trans ratio at equilibrium upon irradiation at 313 nm. ... 

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