Yukawa-Tsuno treatment

Rate data for the Menshutkin reaction between strongly activated Z-substituted benzyl / -toluenesulfonates and Y-substituted lV,lV-dimethylanilines in MeCN at 35 °C fit the equation kohs = h +k2 [DMA], which is consistent with concurrent first- and second-order processes.The 5 nI constant ki is unaffected by changing the nucleophile and conforms to Yukawa-Tsuno treatment with p — -5.2 and r — 1.3. The 5 n2 constant k2 was increased by electron-donating substituents in the nucleophile and showed upward curvature when subjected to the Brown a + treatment. 

A series on substituent effects on the solvolysis of l,l-diphenyl-2,2,2-trifluoroethyl systems has continued in a study of the effects of electron-donating substituents in the fixed aryl moiety. " The fixed substituent in one benzene ring was p-MeO, p-PhO, or p-Me, and the substituent in the other ring was varied. The substrates were chlorides and bromides, and the reactions were carried out in 80% aqueous ethanol at 25.0 °C. The substituent effects were complex and required detailed analysis through Hammett-Brown and Yukawa-Tsuno treatments. The complexities arise largely from the twisting of the aryl groups out of coplanarity. 

Multiparameter treatments such as the Yukawa-Tsuno equation and the dual substituent-parameter equation have long been important and further treatments have been devised in recent years. A final section is devoted to some of these, with an indication of the place of NO2, NH2 and some other groups in these treatments. 

In considering quantitatively the response of these groups to high electron-demand there are certain caveats. In the first place it must be remembered that amino and related groups are liable to be protonated in the kind of media often used for studying electrophilic aromatic substitution. The observed substituent effect will then be that of the positive pole. Secondly, the straightforward application of the tr+ scale to electron-demanding reactions is not necessarily appropriate. It may well be that some form of multiparameter treatment is needed, perhaps the Yukawa-Tsuno equation (Section II.B). 

Earlier sections of this chapter contain accounts of the Yukawa-Tsuno equation85,86, the Dual Substituent-Parameter (DSP) equation91,92 and Extended Hammett (EH) equation95 (see Section II.B), with the particular intention of showing how these may be applied to data sets involving the substituents of particular interest for this chapter. These equations are not now the only possibilities for multiparameter treatment. In this section we shall give accounts of some of the other approaches. The accounts will necessarily be brief, but key references will be given, with indications as to how the substituents of interest for this chapter fit into the various treatments. 

It should be stressed that there is nothing exceptional about the benzo[f>] compounds in not providing constant literature data for practically every aromatic shows the ct+ values to be variable. For some molecules that are not very polarizable, the variation in ct+ is quite small, leading to the erroneous conclusion of their constancy. The presently described molecules are particularly polarizable and show that satisfactory treatment of reactivity data can generally be achieved only through the use of a multiparameter equation, notably the Yukawa-Tsuno equation (59BCJ971). 

The Swain-Lupton, Taft and Dewar-Grisdale equations all require that the substituent in the reaction under investigation remains at the same position relative to the reaction centre. Thus meta and para series have to be treated separately unlike the Yukawa-Tsuno method where both meta and para substituents may be taken together. The Swain-Lupton treatment may be illustrated by the reaction of chloride ion with substituted benzenediazonium chlorides (Figure 7, Equation 26). ... 

In a short section the electronic effect of the isocyano group is discussed. The possibility of including unipolar substituents in Hammett and similar treatments has long been a matter of controversy and this problem is discussed with special reference to the N2 substituent. The effects of acetylenic groups are examined in some detail and various unsatisfactory aspects are uncovered. Multiparameter treatments such as the Yukawa-Tsuno equation and the dual substituent-parameter equation have long been important and further treatments have been devised in recent years. A final section is devoted to some of these, with an indication of the place of CN, and to a lesser extent of the other groups of interest, in these treatments. 

The greater than unit value of Kf in pure solvents is the result expected by the field effect model (33) on a distance basis. The lower than unit value ofKj in the mixed aqueous organic solvents appears to be related to preferential hydration of the reaction center, which results in an increased effective dielectric constant from the m- compared to the p- position. The fact that values fall into two separate categories for pure organic and mixed aqueous organic solvents does not support the treatments of Exner (20) or Yukawa and Tsuno (16). 

Also in 1959, Yukawa and Tsuno undertook a detailed test of the application of the pah treatment to electrophilic side-chain reactions. They confirmed the failure of the Hammett equation for these reactions. More important, their study revealed important deficiencies in the application of the modified pa+ treatment to electrophilic side-chain reactions. The discrepancies were recognized to he in resonance interactions. Yukawa and Tsuno also realized the essential independence of the reaction constant and the extent of the deviation. The incorporation of such diverse behavior in a correlation equation requires four parameters (48). 

For the effects of ir-electron withdrawing substituents in nucleophilic reactions forming anionic reaction centres, the same Y-T treatment as with (2) of ir-electron donating substituents has also been developed (Yukawa and Tsuno, 1965 Yukawa et al., 1966) in the form of equation (2a),... 

Thus the p R values of symmetrically trisubstituted triarylmethanols [3] (see Scheme 1 Deno and Schriesheim, 1955 Deno and Evans, 1957) give a reasonably good correlation with rather than with a constants, but it is suggested that the Y-T equation results in a better correlation than the simple Brown p a" treatment (Yukawa and Tsuno, 1959). 

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