Thermodynamically controlled system

One more aspect of extraction should be mentioned. Extraction obeys the thermodynamics of partitioning including the effects of temperature. The efficiency of extraction is dramatically affected by temperature. As the temperature increases, the relative amount of extractant in the phases changes. This is not the case with activated charcoal which operates by a trapping technique. From the perspective of regeneration, a thermodynamically controlled system would appear to be superior. 

In the thermodynamically favored species, therefore, both ligand strands must bind with their N -termini to the A-configured metal center of the meso-type dinu-clear complex to adopt the sterically more favored right-handed helical conformation. Thus, under thermodynamically controlled conditions the dominating species in solution is isomer I (Figure 1.3.8). The results and considerations discussed show that stereochemical communication between a metal center and an amino acid residue can control the microstructure at the amino acid. In the example presented this leads, in a thermodynamically controlled system, after initial formation of a complex mixture, to only one final dominating species [20, 21]. 

Interestingly, Frey et al. [119] reached exactly the same conclusions, although their polycondensations were kinetically controlled, whereas Feast et al. had worked on a thermodynamically controlled system involving cyclization by back-biting . Frey et al. reinvestigated the proton-catalyzed polycondensation of 2,2-bishydroxymethyl propionic acid ((a) in Formula 11.4). It was found that Hult s polyesters (inch the commercial Perstorf products) possess extremely low... 

An interesting case are the a,/i-unsaturated ketones, which form carbanions, in which the negative charge is delocalized in a 5-centre-6-electron system. Alkylation, however, only occurs at the central, most nucleophilic position. This regioselectivity has been utilized by Woodward (R.B. Woodward, 1957 B.F. Mundy, 1972) in the synthesis of 4-dialkylated steroids. This reaction has been carried out at high temperature in a protic solvent. Therefore it yields the product, which is formed from the most stable anion (thermodynamic control). In conjugated enones a proton adjacent to the carbonyl group, however, is removed much faster than a y-proton. If the same alkylation, therefore, is carried out in an aprotic solvent, which does not catalyze tautomerizations, and if the temperature is kept low, the steroid is mono- or dimethylated at C-2 in comparable yield (L. Nedelec, 1974). 

There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques, in which the rate of a chemical reaction is measured radiochemical techniques, in which a radioactive element s rate of nuclear decay is measured and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

Plot of signal versus time for an analytical system that is under (a) thermodynamic control and (b) under kinetic control. 

Product composition may be governed by the equilibrium thermodynamics of the system. When this is true, the product composition is governed by thermodynamic control. Alternatively, product composition may be governed by competing rates of formation of products. This is called kinetic control. 

Entries 7, 8, and 10 describe so-called Idnetically controlled syntheses starting from activated substrates such as ethyl esters or lactose. In two reaction systems it was possible to demonstrate that ionic liquids can also be useful in a thermodynamically controlled synthesis starting with the single components (Entry 11) [39]. In both cases, as with the results presented in entry 6, the ionic liquids were used with addition of less than 1 % water, necessary to maintain the enzyme activity. The yields observed were similar or better than those obtained with conventional organic solvents. 

Protected cyanohydrins may be employed as acyl anion equivalents in 1,4-additions in the presence of HMPA129. For instance cyanohydrins prepared from arylaldehydes add in a 1,4-fashion under thermodynamic control (THF or THF/HMPA) to cyclohexenone, isophorone and decalone systems in the latter case c/.s-octahydro-2(l/f)-naphthalenones are exclusively obtained 130-131. 

Cycloaddition of 125 with buckminsterfullerene (Ceo) at 3 kbar allowed the adduct [48] to be obtained, preventing a retro Diels-Alder process (Scheme 5.19). Cycloadditions of tropone (125) with furans 134 gave mixtures of 1 1 endo-dcad exo-monocycloadducts 135 and 136, respectively [49a], together with some bisadducts. In this case furan reacts solely as the 27t component in spite of its diene system. Whereas 2-methoxy furan gave mainly the kinetically controlled product 135 (R= OMe Ri =R2 =H), under the same conditions 3,4-dimethoxy furan afforded the thermodynamically controlled cycloadduct 136 (R=H Ri =R2 =OMe) as the major product (Scheme 5.19). 

Double bond cis-trans isomerization occurs during hydrogenation with a relative rate dependent on structure. The less stable double bond isomerizes to the more stable one, but, of course, kinetics and thermodynamics control the extent of isomerization. In a linear carbon chain, one can expect the cis alkene to isomerize to trans and vice versa if the thermodynamics are favorable. However, in a strained cyclic system, trans will isomerize to cis (Fig. 2.13).117... 

Domingo and coworkers [11] have contributed an important theoretical input for the understanding of domino reactions. An interesting example is the domino Diels-Alder reaction of 4-33 and 4-34, in which the products 4-37 and 4-38 could be formed via 4-35 and 4-36, respectively (Scheme 4.7). Visnick and Battiste [12] had shown that, at room temperature, only cycloadduct 4-37 is formed, whereas with heat 4-38 is obtained quantitatively. This is in line with the calculations showing that TS5 is higher in energy than TS4 (74.5 and 55.3 kj mol"1, respectively) on the other hand, cycloadduct 4-38 is more stable (-92.9 kj mol"1) than cycloadduct 4-37 (-78.7 kj mol"1), which explains the formation of 4-38 under thermodynamic control. Calculations have also been performed for the bisfuran system 4-28a [13]. 

In considering catalyzed olefin-cyclopropane interconversions, an important question arises concerning thermodynamic control and the tendency (or lack thereof) to attain a state of equilibrium for the system. Mango (74) has recently estimated the expected relative amounts of ethylene and cyclopropane for various reaction conditions and concluded that the reported results were contrary to thermodynamic expectation. In particular, the vigorous formation of ethylene from cyclopropane (16) at -78°C was stated to be especially unfavored. On the basis of various reported observations and considerations, Mango concluded that a reaction scheme such as that in Eq. (26) above (assuming no influence of catalyst) was not appropriate, because the proper relative amounts of cyclopropanes and olefins just do not occur. However, it can be argued that the role of the catalyst is in fact an important element in the equilibration scheme, for the proposed metal-carbene and [M ] species in Eq. (26) are neither equivalent nor freely interconverted under normal reaction conditions. Consequently, all the reaction pathways are not simultaneously accessible with ease, as seen in the published literature, and the expected equilibria do not really have an opportunity for attainment. In such a case, absence of thermodynamic control should not a priori deny the validity of Eq. (26). 

Most helicates have linear axes, though a few helicates with circular axes are known - indeed the chiral (D4) molecular squares formed from Zn2+ and 2,5 -bis(2,2 -bipyridin- 6 -yl)pyrazine, 22, may be regarded as circular helicates (450). The formation of circular or linear forms seems to depend on balances between kinetic and thermodynamic control iron(II)-poly-2,2/-diimine systems with their substitutionally-inert metal centers provide useful systems for disentangling thermodynamic and kinetic contributions. The mechanism of formation of circular helicates of this type is believed to entail a kinetically favored triple helicate intermediate (484). Self-assembly of chiral-twisted iron(III)-porphyrin dimers into extended polynuclear species takes place through the intermediacy... 

Chloro-l-alkenesS A regioselective route to these chloroalkenes involves thermodynamically controlled addition of C6H5SeCl to a 1-alkene followed by chlorination to provide a (2-chloroalkyl)phenylselenium dichloride (2). These products undergo elimination when treated with NaHC03 in a two-phase system to provide 2-chloro-l-alkenes (3). 

Thermodynamically controlled self-assembly of an equilibrated ensemble of POMs with [AlVWnO40]6 as the main component could act as a catalyst for the selective delignification of wood (lignocellulose) fibers (Figure 13.2) [55], Equilibration reactions typical of POMs kept the pH of the system near 7 during the catalysis that avoided acid or base degradation of cellulose. 

Although C is the first isolable product to be formed it can pass to the more stable form D. C is called kinetically controlled product of the reaction of A. D is the product isolated after the system reaches equilibrium, it is called thermodynamically controlled product. 

The possibility of obtaining, under kinetic control, a selective transformation of only one of the double bonds present in a dienic system, as well as the formation of 1,4-adducts under thermodynamic control, may find interesting applications. These two adducts may indeed be transformed into attractive synthetic intermediates, as shown in equation 97118. 

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