Taft plots

However, the effects on the rates in nonpolar solvent systems is dramatically larger and often of apparent contradiction. For example, the Hammett rho value for the oxidation of substituted methyl cinnamates and cinnamic acids by tetrabutylammonium permanganate in methylene chloride solutions is positive (33,49). See Figure lb. However, a rho value of converse sign (-0.6) is obtained from a Taft plot (Figure Ic) for the oxidation of vinyl ethers in aqueous tetrahydrofuran (33,50). For many other compounds the Hammett relationships are no longer linear, but concave upward ... 

Figure 1. Hammett and Taft plots for the oxidation of unsaturated compounds by permanganate. (a) aqueous solutions (b) methylene chloride and (c) aqueous THF,...

The Taft plot for the oxidation of substituted alkynes, as illustrated in Figure 2c, is also concave upward (17). Consequently, it appears as if this reaction may also have a dual mechanism. One possibility has been summarized in Equations 8 and 9. 

The results of the Taft plot for the alkyl substituents of tertiary alkyl chlorides84 [Table 7, Z = H to (CH3)3C] yielded a very good straight line with a slope p = -4.75 and r - 0.993 at 300 °C. In the case of the few electron-withdrawing substituents listed in Table 7 (Z =... 

There appears to be an appreciable resonance interaction between the phenyl group and the developing enol system in the transition state as evidenced by the large acceleration in rate in the comparison of phenylmalonic and malonic acid. A Taft plot °- for the alkyl-substituted malonic acids (exclusive of the allyl-substituted acids, but including benzylmalonic acid) gives a p value of about + 2.0 0.3 Since the plot covers a small range of a values, the p value is only... 

Figure 4 Taft plot for the dissociation of carboxylic acids (X COdf p X-CH -CO H p = -0.67) (E is negligible)...

Values of and a are tabulated in Appendix 3, Table 1, and Figure 4 illustrates Taft plots for the dissociation of substituted carboxylic acids (X-CO2H and X CH CO Jl). 

Using the data of Table 4 graph the Taft plots of log sec" for... 

Another method " for predicting the rates of cyclic secondary systems (17) has recently become available. The method is a refinement of the a-Me/H ratio method of estimating the magnitude of solvent and anchi-meric assistance. Since a linear or plot (Taft plot) is observed for the solvolysis of acyclic tertiary derivatives (18) in various solvents, by placing... 

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