Steric hindrance/bulk

Steric hindrance can cause polymerization to predominate even if (5) is an aldehyde. With increasing bulk of substituents on one side of a double bond, epoxidation can compete with ozonolysis. 

It is interesting to note that the oxidation of sulphoxides by peracids is faster in alkaline than in acidic solution. This is in contrast to the oxidation of sulphides and amines with the same reagents " . The oxidation rate of ortho-substituted aryl alkyl sulphoxides with aromatic peracids is less than the corresponding meta- and para-substituted species due to steric hindrance of the incoming peracid anion nucleophiles . Steric bulk in the alkyl group also has some effect . Such hindrance is not nearly so important in the oxidation reaction carried out under acidic conditions . 

The other anomalous behavior was the smaller-than-expected permeability of highly branched compounds. This deviation has been explained on the basis that membrane lipids are subject to a more highly constrained orientation (probably a parallel configuration of hydrocarbon chains of fatty acids) than are those in a bulk lipid solvent. As a result, branched compounds must disrupt this local lipid structure of the membrane and will encounter greater steric hindrance than will a straight-chain molecule. This effect with branched compounds is not adequately reflected in simple aqueous-lipid partitioning studies (i.e., in the K0/w value). 

The first factor k. 1 = 35, is expected to be temperature dependent via an Arrhenius fjfpe relationship the second factor defines functionality dependence on molecular size the third factor indicates that smaller molecules are more likely to react than larger species, perhaps due to steric hindrance potentials and molecular mobility. The last term expresses a bulk diffusional effect on the inherent reactivity of all polymeric species. The specific constants were obtained by reducing a least squares objective function for the cure at 60°C. Representative data are presented by Figure 5. The fit was good. 

As an example, bulk modification by the organic reaction of unsaturated PHA with sodium permanganate resulted in the incorporation of dihydroxyl or carboxyl functional groups [106]. Due to the steric hindrance of the isotactic pendant chains, complete conversion could not be obtained. However, the solubility of the modified polymers was altered in such a way that they were now completely soluble in acetone/water and water/bicarbonate mixtures, respectively [106]. Solubility can play an important role in certain applications, for instance in hydrogels. Considering the biosynthetic pathways, the dihydroxyl or carboxyl functional groups are very difficult to incorporate by microbial synthesis and therefore organic chemistry actually has an added value to biochemistry. 

One of the key technologies used in combinatorial chemistry is solid-phase organic synthesis (SPOS) [2], originally developed by Merrifield in 1963 for the synthesis of peptides [3]. In SPOS, a molecule (scaffold) is attached to a solid support, for example a polymer resin (Fig. 7.1). In general, resins are insoluble base polymers with a linker molecule attached. Often, spacers are included to reduce steric hindrance by the bulk of the resin. Linkers, on the other hand, are functional moieties, which allow the attachment and cleavage of scaffolds under controlled conditions. Subsequent chemistry is then carried out on the molecule attached to the support until, at the end of the often multistep synthesis, the desired molecule is released from the support. 

Methacrylic acid also polymerizes in bulk under precipitating conditions. It forms molecular associations very similar to those of acrylic acid. However, the conversion curves were found to be linear under a variety of experimental conditions temperatures of 16.5 to 60°C and broad ranges of initiation rates and monomer concentration in numerous solvents (7). It was assumed that structures of type III do arise but owing to steric hindrance and to the rigidity of the poly(methacrylic acid) molecule the monomer cannot align to form a "pre-oriented" complex as in the case of acrylic acid and propagation is not favored. 

In a qualitative sense, the rate constants correlate with the change in degree of steric hindrance provided by the increasing bulk of the substituents. We can relate those changes,... 

Thermochemistry has been used to determine heats of formation of metal-phosphine adducts. When electronic effects are small, the heats measured are a measure of the steric hindrance in the complexes heats of formation decrease with increasing steric bulk of the ligand. Recent thermochemistry concerning ligand effects can be found in reference [18],... 

Acyclic compounds that have bulky substituents at the central positions on the diene-synthon. The substituents at these positions are relatively close to each other, and bulk leads to steric hindrance. 

Another effect of the high steric hindrance in dithiin 25 due to the 3,3 -dibornane skeleton is the fact that this dithiin can be oxidized readily (with 1 mol of t-chloroperbenzoic acid (MCPBA) at 0°C) to sulfoxide 26 <1995T13247, 1994TL1973> (Scheme 30), a stmcture which is remarkably stable in contrast with the usual characteristics of sulfoxides in the dithiin series. The six-membered ring interconversion of the sulfoxide 26, slowed obviously by the steric bulk hindrance, was investigated by dynamic NMR (AG ca. 10-11 kcalmoP ). With an excess of MCPBA at elevated temperature, the sulfoxide 26 is oxidized to the sulfone 27 parallel treatment with oxygen, however, failed to yield the same compound. 

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