Solvent ionizing power parameter

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... 

Since reaction rates can be strongly affected by solvent polarity cf. Chapter 5), the introduction of solvent scales using suitable solvent-sensitive chemical reactions was obvious [33, 34]. One of the most ambitious attempts to correlate reaction rates with empirical parameters of solvent polarity has been that of Winstein and his co-workers [35-37]. They found that the SnI solvolysis of 2-chloro-2-methylpropane (t-butyl chloride, t-BuCl) is strongly accelerated by polar, especially protic solvents cf. Eq. (5-13) in Section 5.3.1. Grunwald and Winstein [35] defined a solvent ionizing power parameter Y using Eq. (7-13),... 

Solvent coordination number, 134, 403 Solvent effects, 385, 418 initial and transition state, 418 kinetic measures of, 427 Solvent ionizing power parameter, 430 Solvent isotope effects, 272, 300 Solvent nucleophilicity, 431 Solvent participation, covalent, 429 Solvent polarity, 399, 425 Solvent polarity parameter, 436 Solvent properties, 389 Solvent-separated complex, 152 Solvent sorting, 404 Solvent structure, 402 Solvophobic interaction, 395 Solvophobicity parameter, 427 Sound absorption chemical, 145 classical, 145... 

For comparative purposes, the solvolytic rate constants for I-VI at 25 °C in hexafluoroisopropyl alcohol (HFIP) are given in Table I, together with the m values, which measure the dependence of the rates on the solvent ionizing power parameter OTs by the equation log (k/k0) = raY0Ts (3, 6). 

The observed absence of a correlation of the rates of XI with the OTs solvent ionizing power parameter (3, 6) or with the new Y0Tf parameters for triflates (29, 30) is also indicative of a significant kinetic effect of the solvent acting as a base in assisting the elimination according to equation 4. This process is an example of the E2C+ mechanism and has also been invoked to explain the results of solvolysis of some a-carbonyl- and a-phosphoryl-substituted mesylates (31-34). 

Solvent effects on the rate of solvolysis of 37 are also consistent with these conclusions. As Fig. 10 shows, a large scattering is obvious in the plot against the solvent ionizing power parameter, but the rates correlate with the solvent... 

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... 

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). 

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... 

Unfortunately, only five solvents appear to have been studied, so it is not surprising that the four parameters. (Ci, C2, C3 and C4) could be optimized to give a good fit to the solvolytic data. As would be expected, the nucleophilicity term (C2) was found to be small for solvolyses of t-butyl chloride, but, surprisingly, the value of (associated with solvent ionizing power) was found to be similar for solvolyses of both t-butyl chloride and methyl tosylate. 

Kinetic measurements also show that the solvolysis is of the SnI-type small solvent polarity effects were found in the correlation with the ionizing power parameter qts, with a small m value of 0.12, characteristic of a reaction of a cationic substrate to give a cationic product. Furthermore, the rate data show that the leaving group ability of the phenyliodonio group is about 10 times as great as triflate or 10 -fold higher than iodide. ... 

In Eq. (7-14), k and ko are the specific rate constants for the SnI solvolysis of RX (in this case t-BuCl) in a given solvent and in the standard solvent, respectively, m is the sensitivity of the specific rate of solvolysis of RX to changes in the solvent ionizing power (T), T is a parameter characteristic of the given solvent, and c is the intercept (zero for an ideally behaved solvolysis). Eq. (7-14) is expected to be applicable to reactions very similar to the standard reaction, that is, SnI substitutions. The similarity between Y and m of Eq. (7-14), and a and g of the Hammett equation (7-6) is obvious. Y values are known for some pure, mainly protic solvents and for various binary mixtures of organic solvents with water or a second organic solvent [35, 36]. Typical Y values are... 

Nucleophilicity has been discussed by many investigators as one of the properties of the reaction medium that determine the reactivities in carboca-tionic solvolysis reactions (6, 14-18). The nucleophilicity parameter of a solvent, N (15, 16), was determined for most systems by dissection of solvent effects into solvent ionizing power (described by the parameter Y) and nucleophilicity (N), by a correlation technique (19) that could raise doubts as to whether the relevant solvent properties were fully separated into the two parameters. 

From the deviations, a scale of nucleophilicity was derived. Halogenated acetic acids were included, on the basis of reactivities with halonium ions. Other scales appeared from the Schleyer group (5, 6) at about the same time. The various nucleophilicity scales were used to correlate solvolysis rates by now familiar four-parameter equations AG = N + mY or AG = sN + mY. (G — free energy N = solvent nucleophilicity Y = solvent ionizing power s = sensitivity m = sensitivity.) Previously, parameters for such equations had not been determined. 

Y in A and B. Kevill tabulated values of N and Y for hydroxy lie solvents. In common with Kevills treatment, these solvents and a few others that we designate as reaction solvents are the only ones for which we have interpreted the converted values in terms of independent nucleophilicity and ionizing power parameters. 

A two-parameter scale of solvent ionizing power is the Yots scale introduced by Schleyer and co-workers. This system is based on the solvolysis of 2-adamantyl tosylate (5). The free energy relationship is... 

The solvolyses of p-methoxybenzyl chloride in 39 solvents are well correlated by an extended Grunwald-Winstein equation involving terms in Tci (solvent ionizing power), At (solvent nucleophilicity) and I (Kevill s aromatic ring parameter). A claim by Liu that nucleophilic participation is greater than in the solvolyses of the corresponding bromide is discussed and said to be disproved. 

In 1948, Grunwald and Winstein2 4 attempted to define the ionizing power of a solvent by the Y parameter, based on the comparison of the rate for the solvolysis of t-butyl chloride. In 1956, Kosower5 made an attempt to define the polarity of a medium (solvent) by introducing the Z parameter based on the spectroscopic properties (in various solvents)... 

Z-value (or polarity parameter Z) — is an index of the ionizing power of a solvent based on the frequency of the longest wavelength electronic absorption maximum of l-ethyl-4-methoxycarbonylpyridinium iodide in the solvent. The Z-value is defined by Z = 2.859 x 104/A where Z is in kcalmol-1 and A is in nm, respectively. 

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