Elimination, solvent effects

For UV analyses, the sample and the calibration standard must be dissolved in the same solvent to eliminate solvent effects on the UV absorbance. The sample and standard must also have similar pH. For many organic compounds, the UV absorbance maximum undergoes major shifts depending on the solvent and pH conditions. When the UV spectrum shifts to longer wavelengths it is termed a... 

Bimolecular elimination of HX from (143) with KOBu is known to give cis-olefins by an anti-elimination and trans-products by a syn-elimination by adding dicyclohexyl-crown-6-ether the ratio of trans- to cis-products may be drastically reduced, indicating that contact ion-pairs are responsible for the syn-elimination and separated ion-pairs for the anti-elimination. Solvent effects may then be discussed in terms of contact-ion-separating ability. 

Having considered how solvents can affect the reactivities of molecules in solution, let us consider some of the special features that arise in the gas phase, where solvation effects are totally eliminated. Although the majority of organic preparative reactions and mechanistic studies have been conducted in solution, some important reactions are carried out in the gas phase. Also, because most theoretical calculations do not treat solvent effects, experimental data from the gas phase are the most appropriate basis for comparison with theoretical results. Frequently, quite different trends in substituent effects are seen when systems in the gas phase are compared to similar systems in solution. 

Laby21 demonstrated in 1930, with a photographic plate as detector, that copper or iron in zinc could be detected in concentrations approaching 1 part per million by weight. To be sure, he used electron excitation so that absorption effects were minimized (7.10). By contrast, attempts made in the authors laboratory to estimate alkaline-earth metals in brines were unsuccessful, primarily because of the high absorption effects that accompanied x-ray excitation. The use of dilution with a relatively transparent solvent can sometimes reduce or eliminate absorption effects (7.8), but this procedure will fail if the element to be determined is present at too low a concentration in the presence of another substance (the salt in brine in the example cited) primarily responsible for the absorption effect. A case in which dilution is helpful in connection with the absorption effect of the. element sought is that of tetraethyllead fluid in gasoline (7.13). 

There is no clear reason to prefer either of these mechanisms, since stereochemical and kinetic data are lacking. Solvent effects also give no suggestion about the problem. It is possible that the carbon-carbon bond is weakened by an increasing number of phenyl substituents, resulting in more carbon-carbon bond cleavage products, as is indeed found experimentally. All these reductive reactions of thiirane dioxides with metal hydrides are accompanied by the formation of the corresponding alkenes via the usual elimination of sulfur dioxide. 

Remarkable solvent effects on the selective bond cleavage are observed in the reductive elimination of cis-stilbene episulfone by complex metal hydrides. When diethyl ether or [bis(2-methoxyethyl)]ether is used as the solvent, dibenzyl sulfone is formed along with cis-stilbene. However, no dibenzyl sulfone is produced when cis-stilbene episulfone is treated with lithium aluminum hydride in tetrahydrofuran at room temperature (equation 42). Elimination of phenylsulfonyl group by tri-n-butyltin hydride proceeds by a radical chain mechanism (equations 43 and 44). 

S-N bond cleavage 159 S-O bond lengths 543 Solvated electrons 897, 905 Solvent effects 672 in elimination reactions 772 S-O stretching frequencies 543, 545, 546, 552-555, 560-562 Spiroconjugation 390 Stereoselectivity 779, 789 of cylcoaddition reactions 799 of sulphones 761 Steroids... 

An alternative procedure to effect elimination resolves this problem. Opening the oxaspiropentane 26 with selenide anion in a non-protic solvent effects a direct elimination via a merged substitution — elimination mechanism to give the vinyl-... 

The technique generally used to minimize the degree of change in crystallinity of the milled product is to eliminate the water or other solvents from the product, usually by packaging the material within a suitable barrier (for example, aluminum foil laminate). Other techniques include the production of a 100% crystalline material, which may eliminate the effects of moisture. This technique, however, may require a secondary production stage of armealing or a period to allow the product to equilibrate under controlled storage conditions. 

A second factor from which the solvent effect stems is associated with the insertion process. The reaction of species 6 with phenylacetylene revealed that the insertion took place into the H-Rh bond (Scheme 24). Although isolation of species 10 was not possible due to its high reactivity, 2D NMR techniques confirmed the structure. In CD2CI2, a polar solvent, the process took place smoothly even at room temperature to generate 10 (and 11 8 through reductive elimination from 10). However, the process was sluggish in toluene and more than 93% of 6 remained unchanged even after 24 h. 

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