Steric sorbates

This division is somewhat arbitrary siace it is really the pore size relative to the size of the sorbate molecule rather than the absolute pore size that governs the behavior. Nevertheless, the general concept is useful. In micropores (pores which are only slightly larger than the sorbate molecule) the molecule never escapes from the force field of the pore wall, even when ia the center of the pore. Such pores generally make a dominant contribution to the adsorptive capacity for molecules small enough to penetrate. Transport within these pores can be severely limited by steric effects, leading to molecular sieve behavior. 

Diethyl azodicarboxylate forms normal adducts with 2,3-dimethylbutadiene and with ethyl sorbate however, it is not a very good dienophile, presumably because it exists in the fraws-configuration. The sterically hindered diene 254 adds the ester to give mainly the ene product 255 (equation 137) and even cyclohexa-1,3-diene undergoes an analogous... 

An understanding of olefin chemoselectivity in CM is also crucial when homologating 1,3-dienes, which represent a particularly challenging substrate class. In 2005, Grubbs and co-workers demonstrated that, by employing either an electronic or steric barrier to reaction, one of the olefins in a conjugated diene could be deactivated relative to the other for CM. For example, in the reaction of ethyl sorbate with 5-hexenyl acetate in the presence of 5 (I0mol%),... 

Note that in Eq. 11-3, in contrast to the equation used for describing bulk phase partitioning, CA, is not the total surface area (TSA,) of the organic molecule. Rather it reflects the contact area between the molecule and the surface. But as a first approximation, we will assume that CA, is proportional to TSA,. By doing so, we neglect any special steric aspects that may affect the ability of a sorbate to closely approach the surface moieties with which it interacts. 

The increased ionic freedom between the propagating polymer ion and its gegen ion occurs concurrently with increased space separation between the two ion species. The studies of Schuerch and co-workers and of Yoshino and co-workers (98) with deuterated acrylates and by Natta and co-workers (99) with sorbic esters show that this increased separation allows trans addition to mono olefins and 1,4 trans addition to conjugated dienes before complete loss of isotactic steric control at the end of the chain. The increased freedom between the propagating ion and the less closely associated gegen ion appears to result in a distortion of the cyclic transition state which permits backside attack at the beta position of the incoming acrylate monomer and 1,4 attack on the incoming sorbate monomer. 

Micropore diffusion Diffusion within the small micropores of the adsorbent which are of a size comparable with the molecular diameter of the sorbate. Under these conditions the diffusing molecule never escapes from the force field of the solid surface and steric hindrance is important. For zeolites the terms micropore diffusion and intracrystalline diffusion are synonymous. Raffinate Product stream containing the less strongly adsorbed species. 

RCu BFj is also useful for the conjugate addition to the a,P unsaturated ketones and esters, whose double bonds are sterically crowded. Such conjugated additions occur selectively by a 1,4 manner. Certain a,P-unsaturated carboxylic acids also undergo a 1,4-addition through this reagent (Eq. 88) Methyl sorbate reacts with BuCu BFj to give predominantly the 1,4-adduct (Eq. 89). Two mechanistic... 

The loading due to physisorption decreases monotonicaUy with temperature, while that due to chemisorption can increase with temperature, especially considering rate effects. The net contribution of chemisorption may be nil at low temperatures and rise to levels much higher, even substantially above the boiling point, where physisorption would sharply diminish. Physisorption is practically fully reversible, even when multilayered, whereas chemisorption may or may not be reversible. To clarify, it must be reversible to exhibit catalytic behavior, but it tends not to be when it acts as a poison. Physisorption is not very specific, except in molecular sieves, which discriminate based on size and steric effects. In contrast, chemisorption is much more specific, depending on conditions and the nature of the sorbate and adsorbent as well as its previous treatment. 

In addition to size exclusion and steric inhibition, the intermolecular forces between the zeolite and sorbate molecules offer opportunities to achieve unique selectivity based on competitive sorption properties of various zeolites. Variables such as silica to alumina ratio, the nature of the cation species and the geometry of the channels have been shown to be important factors for consideration (13-14). They also can contribute to catalyst stability and reduced coking propensity, two important characteristics of commercially useful catalysts. 

Methyl sorbate added to a soln. of 1-chloro-l-nitrosocyclohexane in ether-ethanol, and kept 2 weeks at 0° c/s-3-methyl-6-methoxycarbonyl-3,6-dihydro-1,2-oxazine hydrochloride. Y 67%.—The use of a 6-membered ring intermediate provides a higher degree of steric control than open-chain analogs in the construction of contiguous asym. centers (s. a. 47). B. Belleau and Yum-Kin Au-Young, Am. Soc. 85, 64 (1963). 

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