Ionizing solvent

A scale for solvent ionizing power, K+, applicable in solvolysis reactions of cationic substrates, has been developed. For example,... 

The numerical values of F+ are found to be related to Y, the measure of solvent ionizing power for neutral substrates, by the equation... 

Application of Hammond s postulate indicates that the transition state should resemble the product of the first step, the carbocation intermediate. Ionization is facilitated by factors that either lower the energy of the carbocation or raise the energy of the reactant. The rate of ionization depends primarily on how reactant structure and solvent ionizing power affect these energies. 

Now, it can be postulated that solvolysis rate should be a function of two properties of the solvent one is its ionizing power, and the other is its nucleo-philicity. An SnI process should be promoted by high ionizing power, and an Sn2 process by high solvent nucleophilicity. At this point, we are ready to bring the extrathermodynamic approach to bear on this problem. This was initiated by Grun-wald and Winstein, who defined a solvent ionizing power parameter Y by... 

Table 8-12. The Solvent Ionizing Power Measure for Aqueous Mixed Solvents at 25°C...

If Y is to be a valid measure of solvent ionizing power, presumably the defining reaction should proceed via the Lim (pure SnI) process. This was the basis for the original choice of r-butyl chloride. It is now believed that /-butyl chloride sol-volyzes with some solvent participation, and modern versions of Y are based on other compounds, of which 2-adamantyl tosylate (p-toluenesulfonate, OTs), 6, is the most favored." ... 

Our next concern is the solvent nucleophilicity. Schadt et al. " chose the solvolysis of methyl tosylate, which should be an Sn2 process, as the defining process. For this standard reaction the parameter / in Eq. (8-69) was set at 1.00. An empirical estimate of m, describing the sensitivity of methyl tosylate to solvent ionizing power, was obtained as the slope of the two-point line for methyl tosylate solvolysis in... 

Table 8-13. Solvent Ionizing Power (Yor,) and Nucleophilicity (Non) Based...

C3H5)2Sn, 13 (CjHs) CISn, 21 Cl2Sn, 28 (Me5C5)2Sn, 3 monomeric monomeric monomeric monomeric monomeric, polymerizes on standing monomeric only in coordinat. solvents ionized or strongly coordinated monomeric polymeric polymeric, one-dimensional polymeric monomeric 104,105 >106) 93,94,107) 20) 29,83)... 

The second series of data on protic solvent effects in bromination that are related to transition states comprises the m-values of solvent-reactivity correlations. First, it is important to underline that 7-parameters, the solvent ionizing powers, established from solvolytic displacements, work fairly well in this electrophilic addition. This is expected since bromination, like SN1 reactions, leads to a cation-anion pair by heterolytic dissociation of the bromine-olefin CTC, a process similar to the ionization of halogenated or ether derivatives (Scheme 14). 

This reaction represents a neutralization reaction in liquid sulfur dioxide. It makes no difference that the solvent does not ionize or that SOCl2 is a covalent molecule. The utility of the solvent concept is not that it correctly predicts that solvents undergo some autoionization. The value of the solvent concept is that it allows us to correctly predict how reactions would take place if the solvent ionized. Note that in this case SOCl2 does not ionize, but if it did it would produce S02+ (the acidic species characteristic of the solvent) and Cl-. 

In general, MS performance should not be compromised by a static magnetic field. Many factors such as the design of ion optics, selection of interface, and type of MS analyzer should be considered with regard to the construction and configuration of the double hyphenated system. The optimum design should overcome some of the mutual incompatibilities of LC-NMR and LC-MS systems. For LC-MS, ionization propensities vary considerably depending upon solvent, ionization source type, and complex matrix effects. Most NMR analyses, however, are not affected by variations... 

Rate constants and products have been reported for solvolysis of benzhydryl chloride and /7-methoxybenzyl chloride in 2,2,2-trifluoroethanol (TFE)-water and-ethanol, along with additional kinetic data for solvolysis of r-butyl and other alkyl halides in 97% TFE and 97% hexafluoropropan-2-ol. The results are discussed in terms of solvent ionizing power Y and nucleophilicity N, and contributions from other solvation effects are considered. Comparisons with other 3 nI reactions show that the solvolyses of benzhydryl chloride in TFE mixtures are unexpectedly fast an additional solvation effect influences solvolysis leading to delocalized cations. 

A calculated transition energy used to assess the polarity of a solvent. The solvent ionizing capability directly affects the position of a peak, easily measured, in the ultraviolet region of the spectrum of the complex of an iodide ion with 2-methyl- or l-ethyl-4-carbomethoxypyridinium ion. Water has a Z value of 94.6, ethanol has a value of 79.6, dimethyl sulfoxide s value is 71.1, and benzene has a value of 54. A similar polarity scale, known as x(30) values, is related to the Z value scale Z = 1.41 t(30) -E 6.92. See Solvent Effects... 

There is an ongoing controversy about whether there is any stabilization of the transition state for nucleophilic substitution at tertiary aliphatic carbon from interaction with nucleophilic solvent." ° This controversy has developed with the increasing sophistication of experiments to characterize solvent effects on the rate constants for solvolysis reactions. Grunwald and Winstein determined rate constants for solvolysis of tert-butyl chloride in a wide variety of solvents and used these data to define the solvent ionizing parameter T (Eq. 3). They next found that rate constants for solvolysis of primary and secondary aliphatic carbon show a smaller sensitivity (m) to changes in Y than those for the parent solvolysis reaction of tert-butyl chloride (for which m = 1 by definition). A second term was added ( N) to account for the effect of changes in solvent nucleophilicity on obsd that result from transition state stabilization by a nucleophilic interaction between solvent and substrate. It was first assumed that there is no significant stabilization of the transition state for solvolysis of tert-butyl chloride from such a nucleophilic interaction. However, a close examination of extensive rate data revealed, in some cases, a correlation between rate constants for solvolysis of fert-butyl derivatives and solvent nucleophicity. " ... 

Many other solvent parameters have been defined in an attempt to model as thoroughly as possible solvent effects on the rate constants for solvolysis. These include (a) Several scales of solvent ionizing power Tx developed for different substrates R—X that are thought to undergo limiting stepwise solvolysis. (b) Several different scales of solvent nucleophilicity developed for substrates of different charge type that undergo concerted bimolecular substitution by solvent. (c) An... 

A. Williams, Concerted Organic and Bioorganic Mechanisms, CRC Press, New York, 2000. W. P. Jencks, How Does a Reaction Choose Its Mechanism , Chem. Soc. Rev. 1981,10, 345. J. P. Richard, Simple Relationships between Carbocation Lifetime and the Mechanism for Nucleophilic Substitution at Saturated Carbon, Adv. Carbocation Chem. 1989, 1, 122. T. W. Bentley and G. Llewellyn, Scales of Solvent Ionizing Power, Prog. Phys. Org. Chem. 1990, 17, 121. 

One of the first extensions of this type of analysis to coordination complexes was made by Langford,10 who examined the solvolysis of Cl and Br in /ran.y-[Co(en)2Cl2]+ and /nms-[Co(en)2-(N02)Br]+ in aqueous methanol mixtures. He obtained good correlations of log/ vs. Y at 25 °C for water-rich ranges, with values for m of 0.25 and 0.18, respectively. These values of m, smaller than those obtained for the solvolysis of /-butyl chloride, showed that the organic substrate was much more sensitive to solvent ionizing power. 

Moreover, many hydrogen compounds, when dissolved in such solvents, ionize more or less completely lo give solvated protons and anions, In the case uf polybasic acids, ionization constants are reported for each step in this dissociation. 

Bis-4-hydroxy-3y5-dimethoxyphenylmethane Neutral Solvent Ionization Ac Curve, ... 

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