Taft a* 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. 

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>. 

Schuchmann MN, Schuchmann H-P, von Sonntag C (1989) The pJCa value of the O2CH2CO2H radical the Taft a constant of the -CH2O2 group. J Phys Chem 93 5320-5323 Schuchmann MN, Scholes ML, Zegota H, von Sonntag C (1995) Reaction of hydroxyl radicals with alkyl phosphates and the oxidation of phosphatoalkyl radicals by nitrocompounds. Int J Radiat Biol 68 121-131... 

The most complete measurements of the acidity of alcohols in water were made some time ago by Long and Ballinger (1,2 3) using conductivity methods. The pKa values for substituted methanols (RCH OH) (2 ) are a linear function of the Taft a constants (4, 5 for the R substituents, allowing the prediction of the actual pKa by using the formula pKa = 15.9 - 1.42 a. ... 

Taft polar constant Taft a constant -> electronic substituent constants (O inductive electronic constants)... 

TABLE 12. Taft a constants, activation energies and first-order frequency factors (A) for the depolymerization of substituted polysiloxanes. Major depolymerization products formed in the thermal degradation of substituted polysiloxanes are D 3 and D 4 (from Reference 156 reproduced by permission of John Wiley Sons, Inc)... 

Oxidation in wet nitromethane, 0.1 M in tetraethylammonium perchlorate, gave up to four products (Scheme 16), the relative amounts of which depended on the temperature and the presence or absence of sodium carbonate.52-55 At 15°C the 2//-pyrrole 50 was the major product, but as the temperature was increased more pyridine (52, R = H) was formed, and at 90°C 50 could not be detected. Compounds 51 and 53 were also found at higher temperatures. Measurement of half-wave potentials relative to Ag/ AgCl showed that the oxidation proceeded in two steps, values for each being 0.93 and 1.37 V for 47a,541.08 and 1.69 V for 47b,5 5 and 1.17 and 1.77 V for 23.53 These reduction potentials correlate well with Taft a+ constants.55... 

Ge(CH3)3COCeH5 (Table 24, No. 41). In mixtures with CeHgOH in CCI4, the compound causes a shift of the OH vibration due to hydrogen bonding to the carbonyl group, Av(OH) = 175 + 5cm this value has been related to the Taft a constant of the substituents in M(CH3)3COCeH5 compounds where M = C, Si, and Ge [13,39]. 

A new structure-activity relationship, Xjj =yS+l, where y is a negative constant, S is the total steric effect, and 4 is the total inductive effect, correlated strongly with available measurements of ozonolysis. New rate coefficients were measured for ozonolysis of a number of unsaturated heteroatomic compounds and it has been emphasized that the inductive effect rather than the steric effect is important in predicting their reactivity %, the inductive effect index, was compared with the Taft a constant and rates of reaction of hydroxyl radical with a given species it correlated strongly in both cases (which should be unaffected by steric factors) suggesting a universal response by olefinic species towards electrophilic addition. ... 

Linear Free Energy Relationships The observation that familiar polar effects influence the orientation ratios for radical addition to olefins prompts the application of the well established free-energy relationships. The orientation data can be accommodated in a Hammett plot by assigning a polar constant, a, to each radical (14). This is defined as the sum of the Hammett sigma constants for the groups attached to the radical centre. In practice, Taft a constants have been used (45). 

---