Selectivity kinetics

A degree of control over the kinetic selectivity of molecular sieve adsorbents can be achieved by controUed adjustment of the pore si2e. In a carbon sieve this may be accompHshed by adjusting the bum-out conditions or by controUed deposition of an easily crackable hydrocarbon. In a 2eoHte, ion... 

Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

Such enantioselective deprotonations depend upon kinetic selection between prochiral or enantiomeric hydrogens and the chiral base, resulting from differences in diastere-omeric TSs.27 For example, transition structure E has been proposed for deprotonation of 4-substituted cyclohexanones by base D.28 This structure includes a chloride generated from trimethylsilyl chloride. 

It is concluded that the selectivities of electrophilic additions are not directly related to the reactivities but to the transition-state positions. Extensive comparison with similar data on the bromination and hydration of other ethylenic compounds bearing a conjugated group shows that this unexpected reactivity-selectivity behaviour can arise from an imbalance between polar and resonance effects (Ruasse, 1985). Increasing resonance in the ground state would make the transition state earlier and attenuate the kinetic selectivity more strongly than it enhances the reactivity. Hydration and halogenation probably respond differently to this imbalance. 

Van Der Laan, G.P. 1999. Kinetics, selectivity and scale up of the Fischer Tropsch process. PhD dissertation, Rijksunivertiteit Groningen. 

Another relevant example is found with the addition of 3-aminopropan-l-ol to 5-bromo-5-deoxy-D-xylose. The formation of the least stable stereomer 58 is 20 times as fast as that of 59 (at equilibrium [59]/[58] = 7.3).49 This kinetic selectivity was interpreted in terms of transition structures 60 and 61 which imply IV-alkylation of a tetrahydrooxazine intermediate as the discriminating step. The faster formation of the least stable product 58 arises from transition state 60 in which IV-alkylation corresponds to an axial attack of the oxazine intermediate. This is easier than equatorial attack in transition state 61 (Fig. 20). 

TABLE 13.5. Intrinsic kinetic constant (mol/s molsutface Pd ) and kinetic selectivity rfor different catalysts after reduction at 200°C. 

From the kinetic and adsorption constants, the kinetic selectivity r is defined as... 

In the results presented in Table 13.5, the addition of tin affects the kinetic selectivity r differently, depending on the catalyst preparation method. When compared to the monometallic PdO catalyst, r slightly decreases for the coimpregnated PdSn catalyst, but it sharply increases for the PdOSn catalyst prepared via the colloidal oxide synthesis. As the intrinsic kinetic constant rates k do not show significant discrepancies between the different catalysts, the main contribution of the variation of the kinetic selectivity is ascribed to the adsorption constant ratio fBo/ Butenes- In the case of the PdOSn catalyst, formation of but-l-ene is favored compared to its consumption because the X Bo/ Butenes ratio increases, indicating that olefin adsorption is much more destabilized than diene adsorption. Thus, the olefin easily desorbs before being hydrogenated into butane. 

In the case of isolated particles, the kinetic selectivity increases with the reduction temperature (Fig. 13.35). In this case, the TfBD. K Butenes ratio remains almost constant, but the intrinsic kinetic constants relative to the hydrogenation and double bond... 

Dynamic combinatorial nitroaldol libraries were also used to illustrate iDCR [5,6], In this case, one of the library components was selected for its possibility to undergo an irreversible tandem cyctizafion reaction following equilibrium formation. This provided an internal kinetic selection pressure on the library, subsequently forcing the library to complete amplification of this novel reaction product. Furthermore, interesting crystalline properties were observed for one of the diastereoisomers of this isoindolinone-type product, providing a route to demonstrate consecutive resolution processes resolving coupled DCLs in a one-pot experiment. 

This selectivity presumably reflects several circumstances. Both carbonyl oxygens are presumably complexed by aluminum. The allylic stabilization of the y-deprotonation product can then lead to kinetic selectivity in the deprotonation. Selectivity for / -attack by the dienolate is accentuated by the steric bulk near the a. position. 

Independent of crystallization conditions, whether from solutions or melt, the polymer molecules crystallize into thin lamellae. The lamellar thickness is about 10 nm, about two orders of magnitude smaller than values allowed by existing equilibrium considerations. This is in contrast to the case of crystallized short alkanes, where the lamellar thickness is proportional to the length of the molecules. Clearly the chains in the case of polymers should fold back and forth in the lamellae to support the experimentally observed lamellar thickness. It is believed in the literature [3-9] that the lamellar thickness is kinetically selected and that if enough time is permissible, the lamella would thicken to extended chain crystal dimension. What determines the spontaneous selection of lamellar thickness ... 

Cyclocondensation of 5-halovaleraldehydes (186) and 1,3-amino alcohols (187) gave equilibrium mixtures of tram- and c s-pyrido[2,l-h][l,3]oxazines (188 and 189), with a predominance of the frans-fused bicycle both diastere-omers contained the substituent in the equatorial position (90TL4281 94JA2617,94JOC6904). However, kinetic selectivity for the formation of cis-pyrido[2,l-h][l,3]oxazine (189) was exhibited versus the tram compound 188 in the case of the dimethyl derivative (R = R1 = Me) (90TL4281). 

Selectivity. Selectivity in a physical adsorption system may depend on differences in either equilibrium or kinetics, but the great majority of adsorption separation processes depend on equilibrium-based selectivity. Significant kinetic selectivity is. in general, restricted to molecular sieve adsorbents—carbon molecular sieves, zeolites, or zeolite analogues. 

It is rare that a catalyst can be chosen for a reaction such that it is entirely specific or unique in its behaviour. More often than not products additional to the main desired product are generated concomitantly. The ratio of the specific chemical rate constant of a desired reaction to that for an undesired reaction is termed the kinetic selectivity factor (which we shall designate by 5) and is of central importance in catalysis. Its magnitude is determined by the relative rates at which adsorption, surface reaction and desorption occur in the overall process and, for consecutive reactions, whether or not the intermediate product forms a localised or mobile adsorbed complex with the surface. In the case of two parallel competing catalytic reactions a second factor, the thermodynamic factor, is also of importance. This latter factor depends exponentially on the difference in free energy changes associated with the adsorption-desorption equilibria of the two competing reactants. The thermodynamic factor also influences the course of a consecutive reaction where it is enhanced by the ability of the intermediate product to desorb rapidly and also the reluctance of the catalyst to re-adsorb the intermediate product after it has vacated the surface. 

If the reactions were not influenced by in-pore diffusion effects, the intrinsic kinetic selectivity would be kjk2(= S). When mass transfer is important, the rate of reaction of both A and X must be calculated with this in mind. From equation 3.9, the rate of reaction for the slab model is ... 

Kinetic data are reported as reaction rates (mol g l s 1) calculated assuming first order kinetics. Selectivities are calculated based on the carbon number of the original hydrocarbon. 

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