Specific solvent effects

The third type of deviation is due to solvent effects specific to the transition state. Either the transition state is more polar than the reactants or products and so has a higher solvation energy, or steric effects are present which selectively hinder solvation in the transition state. Sj 2 halide exchange... 

It is only the contribution of AH to AG that we are discussing here, but we see the effect of this contribution-in the systems for which the approximation is valid-is that a solvent becomes less suitable to dissolve a polymer the greater the difference is between their 6 values. At best, when 61 = 62, the solvent effect is neutral. Cases for which a favorable specific interaction between solvent and polymer actually promotes solution are characterized by negative values of AH and are therefore beyond the capabilities of this model. 

A decisive solvent effect is also observed with other a,/ -epoxy ketones. Specifically, 3jS-hydroxy-16a,17a-epoxypregn-5-en-20-one and its acetate do not react with thiocyanic acid in ether or chloroform. However, the corresponding thiocyanatohydrins are formed by heating an acetic acid solution of the epoxide and potassium thiocyanate. As expected, the ring opening reaction is subject to steric hindrance. For example, 3j6-acetoxy-14f ,15f5-epoxy-5) -card-20(22)-enoIide is inert to thiocyanic acid in chloroform, whereas the 14a,15a-epoxide reacts readily under these conditions.Reactions of 14a,15a-epoxides in the cardenolide series yields isothiocyanatohydrins, e.g., (135), in addition to the normal thiocyanatohydrin, e.g., (134). 

The followmg types of studies will not be presented individually but may have contnbuted supportmg data to coverage by compound type conformational analyses [23 24, 25, 26 27], fluoropolymers [28, 29, 30 31, 32], solid-state NMR [ii], and solvent effects [34 35, 36, 37] Many excellent articles with m-depth NMR interpretation of one specific compound or of a small, structurally related group of compounds can be found in the chemical hterature A few of these, not incorporated elsewhere in this secUon are referenced here carbonyl fluondes [JS 39 40], fluoropropanes [41 42, 43], fluorocyclopropanes [44, 45 46], fluorobutanes [47], perfluorocyclobutanone [48], fluorohexanes [49], and vinyl fluondes [50, 51 52, 53, 54]... 

Because the key operation in studying solvent effects on rates is to vary the solvent, evidently the nature of the solvation shell will vary as the solvent is changed. A distinction is often made between general and specific solvent effects, general effects being associated (by hypothesis) with some appropriate physical property such as dielectric constant, and specific effects with particular solute-solvent interactions in the solvation shell. In this context the idea of preferential solvation (or selective solvation) is often invoked. If a reaction is studied in a mixed solvent. 

In connection with electronic strucmre metlrods (i.e. a quantal description of M), the term SCRF is quite generic, and it does not by itself indicate a specific model. Typically, however, the term is used for models where the cavity is either spherical or ellipsoidal, the charge distribution is represented as a multipole expansion, often terminated at quite low orders (for example only including the charge and dipole terms), and the cavity/ dispersion contributions are neglected. Such a treatment can only be used for a qualitative estimate of the solvent effect, although relative values may be reasonably accurate if the molecules are fairly polar (dominance of the dipole electrostatic term) and sufficiently similar in size and shape (cancellation of the cavity/dispersion terms). 

Smoluchowski see von Smoluchowski) Solvent cage, 198, 202 Solvent effects. 197-199, 204—206 Specific acid-base catalysis,... 

FIGURE 2.3. The energetics of a heterolytic bond cleavage reaction in a polar solvent. The specific example shown corresponds to the CH3OCH3— CH3 + CH30 reaction in water. The energy of the covalent state does not include the effect of the solvent on this state, but a more consistent treatment (e.g., eq. (2.21) should account for the polarization of the solvent toward the charges of the ionic state. This would result in destabilization of H31. 

As demonstrated in the two previous sections, TRIR spectroscopy can be used to provide direct structural information concerning organic reactive intermediates in solution as well as kinetic insight into mechanisms of prodnct formation. TRIR spectroscopy can also be used to examine solvent effects by revealing the inflnence of solvent on IR band positions and intensities. For example, TRIR spectroscopy has been used to examine the solvent dependence of some carbonylcarbene singlet-triplet energy gaps. Here, we will focns on TRIR stndies of specific solvation of carbenes. 

For polar solutes and solvents, particularly those capable of hydrogen bonding, secondary solvent effects due to the specific nature of solute-solvent interactions may also have to be included in the model, since the ass imption that they are identical in the adsorbed and mobile phases, and therefore self-canceling, is no longer necessarily true. The addition of a secondary solvent term... 

When we perform experiment in such way that there is no interference of H-bonds or these bonds are stable and structure of solvent also does not varies essentially, solvatochromic plot demonstrates very good linearity as shown, for example, for some naphthylamine derivatives in ethanol-water mixtures. The linearity of solvatochromic plots is often regarded as an evidence for the dominant importance of nonspecific universal intermolecular interaction in the spectral shifts. Specific solvent effects lead to essential deviation of measured points from this linear plot. 

The method of revealing of H-bonds is very simple an addition of low concentration, 1-3% of molar fraction, of alcohols (ethanol, methanol) to the solution in neutral solvent (CH, for example) results in a substantial spectral shift. Further addition of alcohols, up to 100%, gives much smaller shifts. A small percentage of alcohol may cause 50-80% of total spectral shift. Upon addition of the trace quantities of alcohol, one sees that the intensity of the initial spectrum is decreased, and new red-shifted spectrum appears. The appearance of new spectral component is a characteristic of specific solvent effects. Because the specific spectral shifts occur only at low concentration of alcohol, this effect is probably attributed to H-bonding to electronegative group in the molecule. The next experiment, which can support this conclusion, is an addition of aprotic solvent, for example,... 

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