Solvent-induced steric effect

Fig. 26. Schematic design of field flow fractionation (FFF) analysis. A sample is transported along the flow channels by a carrier stream after injection and focusing into the injector zone. Depending on the type and strength of the perpendicular field, a separation of molecules or particles takes place the field drives the sample components towards the so-called accumulation wall. Diffusive forces counteract this field resulting in discrete layers of analyte components while the parabolic flow profile in the flow channels elutes the various analyte components according to their mean distance from the accumulation wall. This is called normal mode . Particles larger than approximately 1 pm elute in inverse order hydrodynamic lift forces induce steric effects the larger particles cannot get sufficiently close to the accumulation wall and, therefore, elute quicker than smaller ones this is called steric mode . In asymmetrical-flow FFF, the accumulation wall is a mechanically supported frit or filter which lets the solvent pass the carrier stream separates asymmetrically into the eluting flow and the permeate flow which creates the (asymmetrical) flow field...

Taking the solvation into account in such models is both very difficult and necessary for the best possible understanding of these exceedingly complex phenomena. The disolvation of the amides dimer (one solvent per lithium in THF, THF + HMPA or THF + DMPU)50 seems to be indicated, while trisolvated dimers appear relatively unstable. However, a very extensive semiempirical theoretical (MNDO) study on the various cyclic and open mixed aggregates formed by LDA and LiTMP with LiCl or three different enolates, solvated by discrete molecules of THF or HMPA, showed that general conclusions are almost impossible to draw48. A complex interplay of steric effects, induced by the partners of the aggregate and the solvent, seems to be the dominant influence on the relative stabilities of the species characterized. 

Fluoride-ion-induced reactions A similar polymer to that in Figure 7.87 is obtained upon anionic polymerisation of hexafluoro-2-butyne initiated by fluoride ion in a solvent [311-313] (Figure 7.89). This is a clear example of an anionic polymerisation of an unsaturated fluorocarbon, although the growing anion can be trapped by a sufficiently reactive system [291, 314], such as pentafluoropyridine [315] (Figure 7.90). There is little difference between the ultraviolet spectra of 7.90A and 7.90B, confirming that conjugation in the polyene system is inhibited by steric effects. 

In conclusion, although solvent effects as intrinsically bound to the cyclization phenomenon are generally small, they can arise from a number of situations including desolvation of a bifunctional short chain, interdependence of solvation and steric strains in the transition state of medium-ring formation, steric inhibition of solvation in special reaction series, and solvent-induced conformational changes. Some of the effects require further investigation for a more quantitative assessment of the energetic factors involved. 

Thus, various kmds of bases are effective in inducing the Henry reaction The choice of base and solvent is not crucial to carry out the Henry reaction of simple nitroalkanes v/ith aldehydes, as summarized in Table 3 1 In general, sterically hindered carbonyl or nitro compounds are less reactive not to give the desired ni tro-aldol products in good yield In such cases, self-condensation of the carbonyl compound is a serious side-reaction Several mochfied procedures for the Henry reaction have been developed... 

---