Effect of Solvent Changes

As has been suggested in the previous section, explanations of solvent effects on the basis of the macroscopic physical properties of the solvent are not very successful. The alternative approach is to make use of the microscopic or chemical properties of the solvent and to consider the detailed interaction of solvent molecules with their own kind and with solute molecules. If a configuration in which one or more solvent molecules interacts with a solute molecule has a particularly low free energy, it is feasible to describe at least that part of the solute-solvent interaction as the formation of a molecular complex and to speak of an equilibrium between solvated and non-solvated molecules. Such a stabilization of a particular solute by solvation will shift any equilibrium involving that solute. For example, in the case of formation of carbonium ions from triphenylcarbinol, the equilibrium is shifted in favor of the carbonium ion by an acidic solvent that reacts with hydroxide ion and with water. The carbonium ion concentration in sulfuric acid is greater than it is in methanol- 

It seems reasonable that the electron-releasing methyl group not only reduces the affinity of the Lewis acid XIV for bromide ion but also increases the reactivity of any complex XV towards carbonium ions. 

The extent of the ionization produced by a Lewis acid is dependent on the nature of the more inert solvent component as well as on the Lewis acid. A trityl bromide-stannic bromide complex of one to one stoichiometry exists in the form of orange-red crystals, obviously ionic. But as is. always the case with crystalline substances, lattice energy is a very important factor in determining the stability and no quantitative predictions can be made about the behaviour of the same substance in solution. Thus the trityl bromide-stannic bromide system dilute in benzene solution seems to consist largely of free trityl bromide, free stannic bromide, and only a small amount of ion pairs.187 There is not even any very considerable fraction of covalent tfityl bromide-stannic bromide complex in solution. The extent of ion pair and ion formation roughly parallels the dielectric constant of the solvents used (Table V). The more polar solvent either provides a 

Just as interaction of the negative ion with solvent or with a catalyst can promote the ionization, the same is true of the interactions of the carbonium ion. The triaryl carbonium ions discussed so far should all be colored. Reports of colorless but conducting solutions of triarylmethyl derivatives are fairly common and may represent covalently solvated carbonium ions which are not expected to be colorless. For example, a colored solution of the ion XVI in acetic acid slowly fades when it is diluted with methanol containing enough acid to prevent the formation of any color base. The fading is attributed to the formation of XVII.m 

No such fading in methanol is observed with the ion XVIII, in which the additional phenyl group extends the unsaturated system and stabilizes the ion XVIII, as compared with the ion XIX, in which the nitrogen atom insulates the two parts of the molecule. 

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Alkyl-Alkylidene Tautomerism. Some 2- or 3-(substituted alkyl)quinoxalines, like 3-ethoxycarbonylmethyl-2(177)-quinoxalinone (133), have long been known to exist in equilibrium with their (substituted methylene) tautomers, for example 3-ethoxycarbonylmethylene-3,4-dihydro-2( 1 /7)-quinoxalinone (133a).The effects of solvent change, protonation, and the like on such tautomeric systems have been examined as well as the kinetics thereof. In... 

Ground State (GS) TS Relative Charge Effect of Solvent Change AH Rate... 

Table II. Effect of solvent changes on elution of PMMA and Igepals...

Table III describes the effect of solvent change on the lignin model compounds. None of the model compounds exhibited evidence of association all had unimodal elution in the different solvents. The Kp of the fully derivatized model compounds tended to increase as solvent polarity was increased, as had that of the polymer standards. As these are fully derivatized, relatively small molecules, the possibility for size change through interaction with the solvents is small. Increasing affinity for the column gel as the solvent polarity increased is the most probable explanation for their greater retention.

Table III. Effect of Solvent Changes on Elution of Lignin Model Compounds and Polymers...

The effect of solvent change on an equilibrium involving non-ionic species is expected to be small. While specific effects do show up, this expectation is realized in the hydration of sym-dichloroacetone in dioxane-water mixtures (Bell and McDougall, 1960) and in the formation of acetone cyanohydrin in various solvents (Stewart and Fontana, 1940) (Table 7). 

Phenanthridine, like other monoazaaromatics which fluoresce only weakly in nonpolar solvents, is subject to marked fluorescence activation by hydroxylic solvents. Recent studies have shown this to result from the effects of solvent on vibronic interactions between re,77 and 77,77 electronic states, and the effect of solvent changes on the phosphorescence half-life has been similarly explained.237 Measurements of fluorescence and, more especially, phosphorescence characteristics have been proposed as analytical methods for mixtures containing phenanthridines238, 239 and detailed studies of the emission spectra of phenanthridine,240, 241 its cation,241 9-methylphenanthri-dine,242 and phenanthridine-iV-oxide243 have been reported. A... 

The rate constants for protonation of the excited singlet states of several compounds were determined by Weller (1961). Although the measurement of excited state equilibrium constants has become more common, there have been relatively few determinations of the rate constants involved. Trieff and Sundheim (1965) investigated the effects of solvent changes on the rates of protonation and deprotonation of 2-naphthol in the S) state. The dissociation rate constant decreased progressively with the addition of methanol or glycerol to the aqueous solution but the protonation rate constant varied in a more complex manner. As mentioned above, Stryer (1966) found both rate constants smaller in D20 than in H20. 

Another problem, often important when dealing with Marcus correlations may arise when the potentials are known in one solvent while the kinetic experiments are made in another one. Calculation of dG requires inclusion of the effect of solvent change on the electrochemical potentials. The usual procedure considers the difference in the solvation energies of the ions according to the Born equation. For instance, when using acetonitrile measurements to evaluate values in benzene, Eq. (8) may be used [97, 98],... 

Section XV.4, in which we treated the effect of solvent changes, can be subsumed under the present generalization if we consider the change from solvent Si to S2 as a change in external variable describing the mole fraction of Si and S2 ... 

Two recent determinations of activation volumes for Mel addition to rhodium(I) 8-diketonate complexes (197) could not discern which type operated. The values, along with those of AS - and the effects of solvent change, clearly indicated development of polar transition states (the charge separation involved in either 29 or 30 would fit) and the authors marginally favored 29. Interestingly, a complex of iridium(III) and Mel, 31, has been structurally characterized and reveals iodide-bonded Mel molecules (198). The Ir-I-C bond angles are 105.5° and 108.2° and although the interaction can be considered nucleophilic... 

What would be the effect of solvent changes on these reactions Would the reactions be accelerated or retarded by a change from a polar to a non-polar solvent ... 

GROUND STATE (GS) TS RELATIVE CHANGE EFFECT OF SOLVENT CHANGE RATE... 

Figure 5 Illustrates the effect of solvent changes on the rJ of the ionophore free acid and its anion. Kosower s Z values proved empirically an effective function for ranking solvents according to their Integrated polar and protic properties (18).

One can dissect the overall solvent effect for a reaction pathway in two different media into the effect of solvent change on the reactants, R, and on the transition state T. Consider an example of the reaction of a normal nucleophile with a substrate, in two solvents, such as H O (0) and DMSO-H O (S), as illustrated schematically in Figure 3.7. Equation 3.17, which simplifies to Equation 3.18, affords a link between the measured difference between the activation Gibbs energies, the transfer Gibbs energy of the reactant(s), SG (which can be obtained from... 

A further difficulty with the quantitative interpretation ofTartarelli s work [23] and of many other kinetic investigations lies in the use of mixed aqueous solvents. It is self-evident that the composition of the solvent in the resin phase may differ from that in the external phase owing to fractionation effects [27]. In such systems the effect of solvent change on the rate constant may be an additional factor influencing the value of the ratio //ch [15, 23]. The use of mixed solvents should therefore be avoided in kinetic investigations of heterogeneous catalysis. 

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