Linear Taft correlation

Evidently, the approximately linear Taft correlation obtained for the elimination of / -substituted primary chlorides6 suggested an examination of a possible linear free-energy relationship for aliphatic secondary alkyl chlorides pyrolyses, that is, for a-substituted ethyl chlorides ZCH(C1)CH370 (Table 6). 

The F nmr spectra of the fluorine-substituted oxiranes have been reasonably well studied, for in the region of high chemical shifts, the fluorine nuclei readily yield spectra that can be interpreted directly. A further principle is that the long-range coupling constants can be measured well because of the large nature of the constants.In the case of a fluorinated substituent, the linear Taft correlation may be used to determine the inductive and resonance substituent constants. ... 

The kinetics of the reaction of fert-BuOK with aryl-substituted oxiranes follow a linear Taft correlation. Steroid 5a,6a- and 5(3,6/3-oxiranes react with pyridine to give 97 (Eq. 113). ... 

Linear Free-Energy Relationships Hammett and Taft Correlations... 

Estimation methods for the hydrolysis rates of several types of carboxylic acid esters, carbamates, aromatic nitriles and phosphoric acid esters have been reported. Hydrolysis rates are subject to substituent effects, and consequently linear free-energy relationships (LFERs), as represented by Hammett or Taft correlations, have hence been applied to their estimations. Reviews (e.g. Harris, 1990 Peijnenburg, 1991) reveal that QSARs are available for only a few compound classes (Table 4.8) and are mostly based on limited sets of experimental data. 

Here again it is possible to find a linear relationship between the log (k/feo) (ko = methyl) values of 2-alkyl- and 2,4-dialkylthiazoles and between the latter value and Tafts Eg parameter (256). The value of 5 for 2,4-dialkylthiazoles is 1.472 with a correlation coefficient of 0.9994. Thus the sensitivity to substituent effects is more marked than in the case of a single substituent in the 2-position. Furthermore, the 4-position is again more sensitive than the 2-position. 

The results of kinetic studies of the quatemization of 4-alkyl-2,5-dimethylthiazoles do not give a linear correlation between log(fc/ko) and Tafts Es parameter, such as is found for 2-alkyl-, 4-alkyl-, and 2,4-dialkylthiazoles (258). 

In the benzene series, an approximately linear relationship has been obtained between the chemical shifts of the para-hydrogen in substituted benzenes and Hammett s a-values of the substituents. Attempts have been made, especially by Taft, ° to use the chemical shifts as a quantitative characteristic of the substituent. It is more difficult to correlate the chemical shifts of thiophenes with chemical reactivity data since few quantitative chemical data are available (cf. Section VI,A). Comparing the chemical shifts of the 5-hydrogen in 2-substituted thiophenes and the parahydrogens in substituted benzenes, it is evident that although —I—M-substituents cause similar shifts, large differences are obtained for -j-M-substituents indicating that such substituents may have different effects on the reactivity of the two aromatic systems in question. Differences also... 

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

The second problem involves the measurement of pKa values for carbonyl and thiocarbonyl derivatives. Grieg and Johnson (157) have pointed out that the measurement of pKa values for very weak bases (11) is an inaccurate and arbitrary process. Of particular difficulty for our purposes is the fact that different carbonyl derivatives may require different acidity functions. As a result of this situation, no attempt was made to make correlations of pKa data for carbonyl and thiocarbonyl derivatives with eq. (2). Because accurate pKa values can be measured for imines, these values were correlated with eq. (2), although the conformational problem remains. The imine sets were first studied by Charton and Charton (73), who correlated them with eq. (2). No correlations of data for carbonyl or thiocarbonyl derivatives with eq. (2) are extant in the literature. Bhaskar, Gosavi, and Rao (158) have reported that AG values for complex formation of substituted thioureas with iodine are a linear function of the Taft a values. Drago, Wenz, and Carlson (159) have reported similar results for complex formation between iodine and substituted amides. Oloffson (160) has reported a linear relationship between -AH for the complex of substituted N,N-dimethylamides with SbCls and the ffj constants. 

Thomas and Orville-Thomas (209) report a linear correlation of the quantity ( cN,x-FcN,Me) with the Taft a constants. Their data have been correlated with eq. (2). Of the five sets studied (sets 36-2 through 36-6), three gave significant results. The value of for these sets is not significant. 

Kamlet, M. J., Doherty, R. M., Carr, P., Abraham, M. H., Marcus, Y Taft, R. W. Linear solvation energy relationships. 46. An improved equation for correlation and prediction of octanol-water partition coefficients of organic non-electrolytes (including strong hydrogen bond donor solutes)./. Phys. Chem. 1988, 92, 5244-5255. 

Nuclear Quadrupolar Resonance. - A variety of dichlorophosphanes have n.q.r. 35C1 resonances whose average for each compound correlate linearly uith the total Taft constants.103 The spectra of a number of po1yf1uoroary1 hexachlorophosphazenes have been studied.104 A n.q.r. study of pentafluorophenyl-... 

Kamlet, M. J., M. E. Jones, J.-L. M. Abboud, and R. W. Taft. 1979. Linear Solvation Energy Relationships. Part 2. Correlations of Electronic Spectral Data for Aniline Indicators with Solvent tt and 3 Values. J. Chem. Soc., Perkins Trans. 2, 342. 

The relationship between the structure of 1,2,4-thiadiazolidines and their H NMR spectral solvent effects has been studied by measurement of the NMR chemical shift differences (Av) of 39 derivatives in various solvents (CgDg, CCU) for methyl or methylene groups attached to an sp2-hybridized nitrogen, Av correlates linearly with Hammett a constants and for those attached to an sp3 4-hybridized nitrogen, with Taft a° constants <1982AHC285>. 

Linear correlations proposed by Hammett [6] and Taft [7] in the form... 

Linear correlation was observed between log k and Taft a function [24] ... 

A linear correlation of the oxidation potentials Bp of sulfides with Taft s a values of fluoromethy] groups is obtained as shown in Fig. 1 [42]. This dearly indicates that the polar effect of the fluoroalkyl group plays a significant role in the electron-transfer step from the sulfides to anode. Namely, the oxidation potential increases linearly as the number of fluorine atoms of the fluoroalkyl group increases. However, interestingly the oxidation potential was not appreciably affected by the length of the perfluoroalkyl group (Table 4). 

Xrx is a parameter characterizing the homologous series RX. The values of /j,r are direct measures of the polar inductive effects of alkyl groups relative to that of methyl and correlate well with Taft s a values. Substituent-induced IP shifts can thus be handled by linear free energy relationships (LFER) of the Hammett pcr-type. 

Obviously the n ionization energy is a function of the type, number and position of the substituents. The IP( n) values of 4-substituted quinuclidines correlate linearly with Taft s a values51 (Figure 3). 

A plot of the pKa values of anilines in DMSO versus the pKa values of anilinium ions in water is linear with a slope of 1.8. This allows the extrapolation of 41 1 for the pKa of ammonia in DMSO from that of NH4+ in water (9.27)127. Alternatively, the pKa of ammonia in DMSO has been extrapolated105 from the intersystem correlation between the DMSO acidities of NH2X and PhNHX as a value of 35.8. Extrapolation of the pKa value of ammonia from the Taft-like dual substituent parameter (DSP) of NH2X DMSO acidities gave a similar value of 36.6. 

The correlation analysis of spectroscopic properties in terms of o/ and a -type parameters has been very important. Substituent effects on 19F NMR shielding in fluorobenzenes have been studied in great detail by Taft and colleagues90,96 97. For <5 linear regression on a/ is on the whole satisfactory, but a term in a with a small coefficient is sometimes introduced. The correlation analysis of <5°, however, requires terms in both o/ and os-type parameters, with rrR being widely applicable. Many new values of these parameters have been assigned from fluorine chemical shifts. In recent years there has also been extensive use of correlation analysis of 13C NMR data98,99. 

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