Thermodynamically controlled distribution

Thus, independently of thermodynamically controlled (equilibrium) distribution, exclusively linear polymer can be obtained if kp > kc. In such a system, the consumed monomer is converted practically quantitatively into linear chains, which, if the mechanism is available, i.e., if active species are living, will slowly undergo cyclization until thermodynamically controlled distribution is attained. If, however, polymerization is terminated before significant cyclization occurs, the purely linear polymer will be the product. 

We first present the thermodynamic theory of cyclization and then discuss possible deviations which prevent establishing thermodynamically controlled distributions and lead to kinetic control of cyclization. 

For the kinetically controlled formation of 1,3-disubstituted tetrahydro-P-carbolines, placing both substituents in equatorial positions to reduce 1,3-diaxial interactions resulted in the cw-selectivity usually observed in these reactions." Condensation reactions carried out at or below room temperature in the presence of an acid catalyst gave the kinetic product distribution with the cw-diastereomer being the major product observed, as illustrated by the condensation of L-tryptophan methyl ester 41 with benzaldehyde. At higher reaction temperatures, the condensation reaction was reversible and a thermodynamic product distribution was observed. Cis and trans diastereomers were often obtained in nearly equal amounts suggesting that they have similar energies."... 

Similar results are obtained from incineration of polymeric materials with octabromo- and pentabromodiphenyl ether (refs. 11,12). The temperature with the maximum PBDF-yield depends on the kind of polymeric matrix. All three bromo ethers 1-2 give the same isomer distribution pattern with preference for tetrabrominated dibenzofiirans. The overall yield of PBDF is lower for incineration of pentabromobiphenyl ether 2, 4 % at 700°C compared to 29 % for ether 1 at 500 °C (ref. 12). The preferred formation of tetrabrominated fiirans observed at all temperatures cannot be a result of thermodynamic control of the cyclisation reaction it is likely due to the special geometry of the furnaces. One explanation is that a spontaneous reaction occurs at approximately 400°C while the pyrolysis products are transferred to the cooler zones of the reactor details can be found elsewhere (ref. 12). 

The debate about the origin of the discrepancies in these results is ongoing (85,88). Very likely, the preparation procedures of TS-1 have a significant influence on the Ti site distribution, and it was argued that kinetics rather than thermodynamics controls the framework formation and stability (85,87) (Section V.C.3). 

The distribution of products is dependant on the concentrations demonstrating that this is a truly thermodynamically controlled reaction. 

It has therefore been established170 from the product distributions that, while the oxymercuration is reversible, unless a base (e.g. sodium acetate) is added to the reaction medium, and gives almost exclusively the more stable compound 199, the aminomercu-ration takes place to give the kinetically controlled adduct 200, or under thermodynamic control the aminomercurial 201. Reactions are kinetically controlled when the mercurating species is a mercury(II) salt deriving from a weak acid such as mercury(II) acetate. Conversely, they are thermodynamically controlled with the covalent mercury(II) chloride. In the latter case, the presence of a strong acid in the medium allows the thermodynamically controlled product to be obtained. 

The product distribution may depend on the reaction conditions if the nucleophilic attack is reversible (kinetic vs. thermodynamic control). An additional complication arises from... 

Another area to which the MM method can be advantageously applied is the prediction of product distribution under thermodynamic control, where the errors in energy calculations tend to cancel if structurally related products are compared (120). A remarkable example is the dodecahydrogenation of phenanthrene, in which 25 structural isomer products are possible, each having one to four stable... 

The influence of the classical anomeric effect and quasi-anomeric effect on the reactivity of various radicals has been probed. The isomer distribution for the deu-teriation of radical (48) was found to be selective whereas allylation was non-selective (Scheme 37). The results were explained by invoking a later transition state in the allylation, thus increasing the significance of thermodynamic control in the later reactions. Radical addition to a range of o -(arylsulfonyl)enones has been reported to give unexpected Pummerer rearrangement products (49) (Scheme 38).A mechanism has been postulated proceeding via the boron enolate followed by elimination of EtaBO anion. 

Boltzmann Equation. The equation governing the distribution of products in Thermodynamically-Controlled Reaction. 

For the synthesis of heterodimeric cystine peptides where two different peptide chains are cross-linked by a disulfide bridge random co-oxidation of the two chains besides producing the heterodimer leads in the optimal case to the additional two homodimers in statistical distribution. Therefore, chemical control of the disulfide bridging via site-directed disulfide formation techniques is required since a thermodynamic control for generation of heterodimers is extremely difficult to achieve (see Section 6.1.5). 

Furthermore, the organic functionalization studies have indicated that multiple reaction products can form even for simple systems. Kinetic and thermodynamic influences must be considered in any analysis of the product distribution. Moreover, the studies have revealed differences in the dominance of kinetic vs. thermodynamic control between the silicon and germanium surfaces. The dissimilarity primarily stems from the fact that adsorbate bonds are usually weaker on Ge than on Si. This difference in energetics leads to observable differences in the degree of selectivity that can be achieved on the two surfaces. Another important motif is the significance of interdimer bonding in the products. Many molecules, even as small as ethylene, have been observed to form products that bridge across two dimers. Consequently, each analysis of adsorption products should include consideration of interdimer as well as intradimer species. 

In contrast, the meta isomer is formed under isomerizing conditions (A1C13, FeCl3) in yields up to 67%, which reflects the thermodynamically controlled isomer distribution.138 As a result of severe steric interaction, little or no ortho isomer is formed. 

Leibler (1988) also noted that the very low interfacial tension resulting from the addition of copolymer could lead to a thermodynamically controlled vesicle phase containing minority homopolymer particles surrounded by a copolymer film and suspended in the majority homopolymer matrix. The size distribution of the droplets was found to depend on the rigidity and spontaneous... 

Whereas step 1 is stoichiometric, steps 2 and 3 form a catalytic cycle involving the continuous generation of carbenium ions via hydride transfer from a new hydrocarbon molecule (step 3) and isomerization of the corresponding carbenium ion (step 2). This catalytic cycle is controlled by two kinetic and two thermodynamic parameters that can help orient the isomer distribution, depending on the reaction conditions. Step 2 is kinetically controlled by the relative rates of hydrogen shifts, alkyl shifts, and protonated cyclopropane formation, and it is thermodynamically controlled by the relative stabilities of the secondary and tertiary ions. (This area is thoroughly studied see Chapter 3.) Step 3, however, is kinetically controlled by the hydride transfer from excess of the starting hydrocarbon and by the relative thermodynamic stability of the various hydrocarbon isomers. 

The high acidity of the Nafion-H catalyst is further demonstrated by its ability to promote both polyalkylation and isomerization. In reaction between benzene and ethylene at 190°C, 20% of the alkylated products are diethylbenzenes.187 The isomer distribution of the diethylbenzenes is 1 % of the ortho, 75% of the meta, and 24% of the para isomers. This composition is very close to the equilibrium composition of diethylbenzenes determined in solution chemistry with AICI3 catalyst and indicates that the reaction is thermodynamically controlled. 

We shall consider reactions catalysed by two different types of pro-catalyst the first (type A) employs Pd-allyl cations ([Pd(a]lyl)(PCy3)]+/Et3SiH or [Pd(allyl)(MeCN)2] + ), and the second (type B) employs Pd-alkyl or chloro complexes ([(phen)Pd(Me)(MeCN)]+, where phen = phenanthroline, and [(RCN)2PdCl2]). These two types of catalysts give very different products in the cyclo-isomerisation of typical 1,6-dienes such as the diallyl-malonates (10), Scheme 12.6. Since there is known to be a clear order of thermodynamic stability 11 < 12 <13, with a difference of ca. 3-4 kcal mol 1 between successive pairs, any isomerisation of products under the reaction conditions will tend towards production of 12 and 13 from 11 and 13 from 12. Clearly, when 11 is the major product (as with pro-catalysts of type A), it must be the kinetic product (see Chapter 2 for a discussion of kinetic and thermodynamic control of product distributions). However, when 12 is generated selectively, as it is with pro-catalysts of type B, there is the possibility that this is either generated by rapid (and selective) isomerisation of 11 or generated directly from 10. 

Besides the special reactivity of the OH-2, OH-1, and OH-3 groups lies also the classical relative reactivity between the primary and secondary hydroxyl groups. Depending on the reaction conditions and the nature of the electrophilic species, it may be seen that these two types of possible reactivity can direct the reactivity of sucrose. Of course, the product distribution also depends on whether the transformations are kinetically or thermodynamically controlled. For those reactions under kinetic control, if there is enough difference in the rate of the first substitution at the most reactive hydroxyl group and the second one, then the regioselectivity also monitors the degree of substitution. 

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