Steric stability, solvent role

Others113 have highlighted the role of the solvent in enabling the steric stabilizer to function in the colloid-forming role. At room temperature, they showed that poly(vinylmethylether), which does not function as a steric stabilizer in water, does so in ethanol. However, the colloids produced in an ethanol-containing solution were much lower in conductivity. 

The results of Clayfield and Lumb relate entirely to the loss of configurational entropy of the polymer chains on close approach of the particles, due either to the presence of the impenetrable surface of the opposite particle or the polymer chains that are attached to that particle. In the early papers, the effect of the solvent on the conformation of the macromolecules was ignored but an attempt was made to include the role of solvency in some of the later publications. Notwithstanding this, essentially what Clayfield and Lumb calculated was the elastic contribution to Ae repulsive free energy of interaction between sterically stabilized particles. As such, their results are manifestly unable to explain the observed flocculation of sterically stabilized particles that is induced by decreasing the solvency of the dispersion medium. Even if only for this reason, the assertion by Osmond et al. (1975) that the Clayfield and Lumb theory was the best available at that time is clearly untenable. 

In the early 1960s it became evident that the reaction environment had an important role in dictating the course of photochemical conversions acting on the course of the relaxation processes and stabilizing photoproducts.17 A constrained medium such as that of a porous matrix or a micelle provides the restricted environment to stop any bimolecular processes that could lead to degradation of products. These effects, however, are subtle. For instance, confinement of a molecule within a host instead of leading to inhibition of reactions of the trapped substrate often results in enhanced reactivity and selectivity because confinement does not mean steric inhibition of all motions of the entrapped host molecule which may eventually enjoy less restriction of some motions than in common solvents. 

The effect of a-silyl substitution on the stability of a carbenium ion was qualitatively unclear for a long time. Early solvolytic studies by the groups of liaborn36 and Cartledge37 suggest a destabilizing effect of a-silyl substitution compared with alkyl. The measurement and interpretation of the kinetic a-silicon effect in solvolysis reactions is, however, often complicated by the fact that steric and ground state effects may play an important role and that, in addition, the rates of ionization often involve a contribution from nucleophilic solvent assistance. 

The role of the apolar amino acid side chains in cholinesterase is still a neglected field of study, in spite of the development of solvent variation (B34, Yl), which provides new opportunities for experimentation. The resultant alteration in the configuration of the enzyme produced by hydrophobic solvents because of solvation of apolar side chains may expose changes in the configuration or stability of the steric structure of some of the enzyme variants, changes which could be of real value in their recognition and identification. 

In addition to enantiocontrol, the problem of regiocontrol arises in these reactions. There are various factors that influence the regioselectivity of allylic substitutions [3,4,13, 36, 37, 38, 39]. Electronic effects exerted by the catalyst and the allylic substituents, steric interactions between the nucleophile, the allyl system and the catalyst, and the relative stabilities of the Ti-olefin complexes formed after nucleophilic addition, can all play a role. The relative importance of these factors varies with the catalyst, the substrate, the nucleophile, the solvent and other reaction parameters and is difficult to predict. 

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