Equilibration reverse

TABLE 3.1 Comparison of Reversible and Irreversible Gas Expansion with Respect to Equilibration, Reversibility, Rate, and Work Capacity... 

ScHLiTT and Klose [226] then worked intensively on the problem of moisture control and TLC. They showed amongst other things that during chromatography, that part of the layer which has not yet been moistened, can take up solvent vapours and release water. The uptake of water and numerous other equilibration reversals can take place within a few minutes. They developed a so-called GS-moisture chamber (Fig. 27) in order to eliminate the shortcomings known up to that time. It... 

Reversed-phase columns are used to separate polar substances. Although in LC the stationary phase is a solid, it is necessary to bear in mind that there may be a thin film of liquid (e.g water) held on its surface, and this film will modify the behavior of sample components equilibrating between the mobile and stationary phases. A textbook on LC should be consulted for deeper discussion on such aspects. 

For the other broad category of reaction conditions, the reaction proceeds under conditions of thermodynamic control. This can result from several factors. Aldol condensations can be effected for many compounds using less than a stoichiometric amount of base. Under these conditions, the aldol reaction is reversible, and the product ratio will be determined by the relative stability of the various possible products. Conditions of thermodynamic control also permit equilibration among all the enolates of the nucleophile. The conditions that permit equilibration include higher reaction temperatures, protic solvents, and the use of less tightly coordinating cations. 

The calculated results are demonstrated in Figs. 7a and 7b [25, 33). The system maintained at T—1.85 is subject to down quenching with a temperature step AT = 0.05 and isothermal aging is followed at each temperature to equilibrate the system. After reaching the equilibrium at T =1.5, the operation is reversed up to T =1.85. In Fig. 7a, one sees that the resultant relaxation time is shortened with decreasing temperature and... 

Mutarotation occurs by a reversible ring-opening of each anomer to the open-chain aldehyde, followed by reclosure. Although equilibration is slow at neutral pH, it is catalyzed by both acid and base. 

An interesting case of product-controlled simple diastereoselectivity has been reported103. [l-[Methyl(nitrosoamino)]-2-propenyl]lithium adds to benzaldehyde at — 78°C to give the amino alcohol with an anti/syn ratio of 65 35, but equilibration of the reversible reaction at room temperature leads exclusively to the more stable, vv -product. 

The classical aldol addition, which is usually run in protic solvents, is reversible. Most modern aldol methodologies, however, rely on highly reactive preformed metal enolates, whereby proton donors are rigorously excluded. As a consequence, the majority of recent stereoselective aldol additions are performed under kinetic control. Despite this, reversibility and, as a consequence, an equilibration of yrn-aldolates to a t/-aldolates by retro-aldol addition, should not be excluded a priori. 

Although lithium aldolates generally display a rather moderate preference for the u/f/z-isomer4, considerable degrees of diastereoselectivity have been observed in the reversible addition of doubly deprotonated carboxylic acids to aldehydes20. For example, the syn- and uw/z-alkox-ides, which form in a ratio of 1.9 1 in the kinctically controlled aldol addition, equilibrate in tetrahydrofuran at 25 C after several hours to a 1 49 mixture in favor of the anti-product20. 

Intermolecular reaction of the mannose-derived 2,3-0-isopropylideiie-a-D-/y.vc>-pentodialdo-l,4-furanoside 13 affords a diastereomeric mixture of nitroalcohols 14. Upon fluoride-catalyzed desilylation, a stereoisomerically pure nitrocyclitol 15 was obtained from a successive intramolecular nitroaldol reaction as a consequence of the reversibility of the nitroaldol reaction which, in this case, allows the equilibration of isomers through open-chain intermediates33. 

The reversal of the stereoselectivity is attributed to the ability of chlorotrimethylsilane to trap the initially formed cuprate-enone complex, thereby suppressing equilibration of the diastereomeric complexes. The copper-catalyzed 1,4-addition of Grignard reagents to 5-substituted 2-cyclo-hexenone also proceeded with very high trans diastereoselectivity22. 

The initial addition step is reversible allowing isomerization of the ( )- and (Z)-nitroalkenes and equilibration between the initially formed syn- and ann -imminium ion adducts. The spn-ad-duct is identical to that obtained from the lithium enolate of cyclohexanone and ( >(2-nitro-cthenyl)benzenc. The preference for the. yyu-adduct can be rationalized by inferring the transition state 1 which is similar to that proposed for the reaction of (-E)-nitroalkcnes with ( )-eno-lates11, l2. 

The reversible chain transfer process (c) is different in that ideally radicals are neither destroyed nor formed in the activation-deactivation equilibrium. This is simply a process for equilibrating living and dormant species. Radicals to maintain the process must be generated by an added initiator. 

According to Eq. (3-7), a plot of In [A], - [AL will be linear. The plot has, as the negative of its slope, the sum k + k-. The implication that this data treatment yields a sum is at first surprising, because this rate constant characteristic of the equilibration is clearly larger than the forward rate constant alone. The net rate itself, on the other hand, is smaller than the forward rate, since the reverse rate is subtracted from it, as in Eq. (3-2). These statements are not contradictory, and they illustrate the need to distinguish between a rate and a rate constant. 

One useful means of studying reversible reactions is to effect a sudden change that perturbs a previously attained equilibrium. One might do this in several ways by injecting one component, by suddenly diluting with solvent (if An = 0), or by rapidly changing the temperature. The perturbation must be made rapidly compared with the rate of re-equilibration. This procedure is referred to as relaxation kinetics. 

The ratio of the different isomeric products was found to vary with time, temperature, and initial concentration. This suggested that some kind of equilibration was occurring between isomers. I3C NMR spectroscopy of a reaction mixture showed, upon cooling, the reversible formation of a pair of signals in the anomeric region. These signals were ascribed to the anomeric carbon atoms of fructofuranosyl fluorides (10), which were presumed to be in equilibrium with the reactive fructofuranosyl cation, 11. 

Although the conversion of an aldehyde or a ketone to its enol tautomer is not generally a preparative procedure, the reactions do have their preparative aspects. If a full mole of base per mole of ketone is used, the enolate ion (10) is formed and can be isolated (see, e.g., 10-105). When enol ethers or esters are hydrolyzed, the enols initially formed immediately tautomerize to the aldehydes or ketones. In addition, the overall processes (forward plus reverse reactions) are often used for equilibration purposes. When an optically active compound in which the chirality is due to an asymmetric carbon a to a carbonyl group (as in 11) is treated with acid or base, racemization results. If there is another asymmetric center in the molecule. 

The model predicts the behavior of the active state LRG to be analogous to cell activation itself. LRG rises in seconds, disappears in minutes as binding equilibrates, and, when binding is interrupted, disappears in a few seconds as this state disappears, transduction also "collapses" and cell responses decay. The model should not be viewed as complete, however. For example, amplification steps, which permit the activation of multiple G proteins by a single receptor, would be built into the model by adding a reverse rate from LR to LRG. Such amplification would have to be verified experimentally. 

The popularity of reversed-phase liquid chromatography (RPC) is easily explained by its unmatched simplicity, versatility and scope [15,22,50,52,71,149,288-290]. Neutral and ionic solutes can be separated simultaneously and the rapid equilibration of the stationary phase with changes in mobile phase composition allows gradient elution techniques to be used routinely. Secondary chemical equilibria, such as ion suppression, ion-pair formation, metal complexatlon, and micelle formation are easily exploited in RPC to optimize separation selectivity and to augment changes availaple from varying the mobile phase solvent composition. Retention in RPC, at least in the accepted ideal sense, occurs by non-specific hydrophobic interactions of the solute with the... 

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