Correlations nucleophilicity scale

The nucleophilicity scale based on the stretching frequency of the hydrogen bond between a nucleophile and an acid (Nu-HX) 97 was tested against experiment using the hard acids HF, HCN, and BF3 and the soft acid BH3.98 The correlation with the hard acids is excellent but fails when a soft acid is used. A new nucleophilicity index, or = 7 (/xa - /t. )2/I M + b)2] 7a, where //A, and //B, are the chemical potentials of the nucleophilic and electrophilic molecules, respectively, and ijA and i B are their respective hardnesses, has been proposed. This gives the relative nucleophilicity that is... 

The final chapter of this section is by Rappoport and is concerned with nucleophilic reactions at vinylic carbon. Two reaction types are considered, those of neutral vinyl derivatives and those of vinyl cations. Correlation of rates for these reactions with both Ritchie and Swain-Scott equations was attempted without success. Rappoport concludes that these reactions are subject to a complex blend of polar, steric, and symbiotic effects and that a quantitative nucleophilicity scale toward vinylic carbon cannot be constructed . This conclusion is reminiscent of the earlier observation of Pearson (see the introduction to the section on the Brpnsted equation) and the later observation of Ritchie (Chapter 11) regarding the difficulty of correlating nucleophilic reactivity with a single equation. Rappoport finds another familiar situation when he explores the relationship between reactivity and selectivity for the vinyl substrates sometimes the RSP is obeyed and sometimes it is not. 

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. 

From this discussion, clearly, a quantitative nucleophilicity scale toward vinylic carbon cannot be constructed. Neither Ritchies nor Swain-Scott s correlations are applicable. Different blends of contribution of polar, steric, and symbiotic effect can change the reactivity order. Whether a qualitative order prevails could be inferred by comparing the three substitution reactions of chloro olefins, which are the only processes for which a relatively extensive change in the type of nucleophile was conducted (Table X). 

The solvolysis rate constants of 2,2,2-trichloroethyl chloroformate CClgCHjCXZICXZ in 30 different solvents were well correlated with the extended GW equation, using the NT solvent nucleophilicity scale and the YCl solvent ionizing scale, with sensitivity values of 1.28 0.06 and 0.46 0.03 for I and m, respectively. The kinetic solvent isotope effect ( MeOH/ MeOD) of 2.39 was in accordance with an 5 2 mechanism probably assisted by general base catalysis. " The specific rates of solvolysis of 2-butyn-l-yl... 

These indices have been used to study the reactivity for a series of chlorobenzenes and a good correlation is observed, for example, between W and toxicity of chlorobenzene [41]. For a detail discussion of this concept and its applications, we refer the readers to a recent review [41,42]. For studying intramolecular reactivity, these philicity indices and local softness contain the same information as obtained from the Fukui functions, because they simply scale the Fukui functions. In some cases the relative electrophilicity and relative nucleophilicity may be used although they provide similar trends as s(r) and co(r) in most cases [43]. In the same vein, the spin-donicity and spin-philicity, which refer to the philicity of open-shell systems [44], could also be utilized to unravel the reactivity of high-spin species, such as the carbenes, nitrenes, and phosphinidenes [45]. 

The development of these various solvent parameters and scales has been accompanied by the realization that there are uncertainties in the physical property of the solvent that is correlated by a particular parameter in cases where systematic changes in solvent structure affect several solvent properties. Consider a reaction that shows no rate dependence on the basicity of hydroxylic solvents, and a second reaction that proceeds through a transition state in which there is a small transition state stabilization from a nucleophilic interaction with the hydroxyl group. The rate constants for the latter reaction will increase more sharply with changing solvent nucleophilicity than those for the former, and they should show a correlation with some solvent nucleophilicity parameter. This trend was observed in a comparison of the effects of solvent on the rate constants for solvolysis of 1-adamantyl and ferf-butyl halides, and is consistent with a greater stabilization of the transition state for reaction of the latter by interaction with nucleophilic solvents. ... 

A point of key importance in study of solvolysis is the nucleophilicity of the solvent. Whereas the Y and other scales have been available for measuring ionizing power for some years, there has been no satisfactory scale for nucleophilicity. Swain, Mosely, and Bown attempted to set up an equation for correlation of solvolysis rates that included both nucleophilicity and ionizing power 112 their system did not prove particularly helpful for understanding mechanism.113 The Swain-Scott equation, discussed in Chapter 4 (p. 185), was not evaluated for solvents. 

The results are appreciably different from those based on the analysis of the reactions of monosubstituted triphenylmethyl cations (McClelland et al., 1989). Obviously, this is due to the difficulties of correlating the substituent effects in a simple manner. The preferred results on the trisarylmethyl cation lead to the conclusion that both the forward process of the k ionization and the reverse process of the solvent-recombination step of the carbocation with various nucleophiles can be described to a good approximation by a Y-T a scale with an r value of the transition state which is essentially identical with the intrinsic r value of the thermodynamic stabilities. However, we will consider the triarylmethyl system further following similar analysis of a-arylethyl cations. 

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