Substitution electrical effects

A number of correlations of ionization potentials for substituted benzenes (40-42), benzyl (43), phenoxy (44), and alkyl (45) radicals and substituted pyridines (46) with the simple Hammett equation have been reported. Charton (47) has studied the application of the extended Hammett equation to substituted ethylenes and carbonyl compounds. The sets studied here are reported in Table II (sets 2-10 and 2-11). Results of the correlations are set forth in Table 111. The results obtained are much improved by the exclusion of the values for X = C2 H3, Ac, F, H and OAc from set 2-10 (set 2-lOA) and the value for X = H from set 2-11 (set 2-11 A). The composition of the electrical effect corresponds to that found for the Op constants as is shown by the pR values reported in Table IV. 

With regard to the composition of the electrical effects, values of pj for the sets studied are reported in Table XIV. The sets in which the hydroxyl group is the reaction site generally exhibit a Pr value of about 39. The cis-3-substituted acrylic acids show a p value of 39 in aqueous solution. The value of 71 obtained in 50% aqueous ethanol seems too large. 

The composition of the electrical effect in the case of the 1,4-benzo-quinones seems to depend upon the degree of substitution. The association constants and charge transfer absorption energies of the 2-substituted-l,4-benzo-quinone-hexamethylbenzene complexes (sets 15-2 and 154) show values of pj of 55 and 44 respectively. Values of p are set forth in Table XVII. 

The magnitude of the electrical effect in the 1,3-dipolar cycloaddition reaction is roughly comparable to that of substituted dienophiles in the Diels-Alder reaction. 

No useful comparisons can be made with other systems in so far as the magnitude of the electrical effect is concerned, because unfortunately, no data are available for a set of /rans-substituted ends. The values of a for the frans-heterovinylene sets can be correlated graphically with eq. (21) with b i2 = 1 12 = 2.5. The values of /3 do not seem to be a function of eq. (22). 

The magnitude of the localized electrical effect in the case of the substituted ethylenes is the same as that of the substituted acetylenes. The latter are significantly more susceptible to resonance effects than are the former, however. 

Data are extant in the literature for two other sets of ethynylene compounds. The sets studied are reported in Table XXXIII. Results of correlation are set forth in Table XXXIV and values of Pr in Table XXXV. The ionization constants of 3-substituted propiolic acids, which were first correlated with the Hammett equation by Charton (18), gave an excellent correlation with eq. (2) (set 22-27-1). The results are very much improved by the elimination of the value for X = H (set 33-27-2). The composition of the electrical effect corresponds approximately to that of the Op constants. The magnitude of the electrical effect is significantly less than that observed for the rrans-3-substituted acrylic acids. 

The Eo values for 2-substituted 1,4-benzoquinones (sets 45-4 through 45-7, 45-10) show an average value of pr of 59. Thus the resonance effect predominates. For most of these sets, the Op constants are not the best parameters for correlation. By contrast, the electron reduction potentials (set 45-8) show a Pr value of 39, which indicates predominance of the localized effect. The 2,5-disubstituted 1,4-benzoquinones differ distinctly in their behavior from the 2-substituted 1,4-benzoquinones in that they show an average Pr value of 53. The one-electron reduction potentials of these compounds show about the same composition of the electrical effect, with a value of Pr of 50. The only set of Eq values available for the 2,6-disubstituted 1,4-benzoquinones pve a Pr value of 51, comparable to the values observed for the 2,5-disubsti-tuted 1,4-benzoquinones. The 2,3,5,6-tetrasubstituted 1,4-benzoquinones have... 

The earliest successful parameterization of electrical effects is that of Hammett6-8. Burkhardt reported the existence of QSRR two years before Hammett but did not develop a general relationship9. Hammett defined the am and ap constants using the ionization constants (Kx) of 3- and 4-substituted benzoic acids in water at 25 °C as the reference set and hydrogen as the reference substituent (i.e. K ) to which all others are compared. For hydrogen the values of the am and ap constants were defined as zero. 

These interactions (dd, di, ii) are a function of dipole moment and polarizability. It has been shown that the dipole moment cannot be replaced entirely by the use of electrical effect substituent constants as parameters52. This is because the dipole moment has no sign. Either an overall electron donor group or an overall electron acceptor group may have the same value of /x. It has also been shown that the bond moment rather than the molecular dipole moment is the parameter of choice. The dipole moments of MeX and PhX were taken as measures of the bond moments of substituents bonded to sp3- and sp2-hybridized carbon atoms, respectively, of a skeletal group. Application to substituents bonded to sp-hybridized carbon atoms should require a set of dipole moments for substituted ethynes. 

In the second edition of his book33 Stewart proposed a parallel between the rate of esterification of 2-substituted benzoic acids and the molecular weights of the substituents, the nitro group strongly deviating from this relationship. The first actual attempt to define a set of steric parameters is due to Kindler39. It was unsuccessful these parameters were later shown to be a function of electrical effects. The first successful parametrization of the steric effect is due to Taft40, who defined the steric parameter Es for aliphatic systems by the expression ... 

Arnett and Moe have reported enthalpies of deprotonation, hHdpnn, for 4-substituted phenols and for the alcohols XCH2OH (sets CRl and CR2, Table 9) with LiN(SiMe3)2 in THF at 25 °C. For the phenols only electrical effects were considered. Best results were obtained with the LD equation (equation 4). The regression equation is equation 25 ... 

The magnitude of p is in accord with a reaction at a ring C atom. Winkler and Winkler have also reported equilibrium and rate constants (sets CR7 and CR8, Table 9). We described above a modified Yukawa-Tsuno (MYT) equation (equation 6) with a taking the value for 3-substituted benzene derivatives and <750 for 4-substimted benzene derivatives, while for substituents in the meta position is 0. For this application the appropriate value of was unknown and all three pure electrical effect parameters were used for the para substituents thus the MYT equation used was equation 37 ... 

Quantum chemical calculations on substituted alkanes are in agreement with a through-space (field) transmission of substituent electrical effects . In view of the ever-increasing evidence for through-space transmission of electrical effects, it is time to make a conclusive statement. There is no inductive-effect component in electrical-effect transmission ... 

---