Donor Hammett studies

The ability of compounds with double bonds to act both as electron donors and as electron acceptors in charge transfer complex formation is well known (81,82). Hammond (83) has studied the correlations of association constants and of the energy of the charge transfer absorption of 2-substituted-l,4-benzoquinones complexed with hexamethylbenzene with the Hammett equation. Charton (84) has studied the correlation with eq. (2) of association constants of 1-substituted propenes with Ag. ... 

Several formylfuranboronic acids and similar compounds have been prepared by regular methods and studied with respect to their 13C and H NMR spectra. As in the benzene series, the protons ortho to the boron are deshielded and the meta protons are shielded. Good conjugation exists between the furan ring (donor) and both the boron atom and the carbonyl group (acceptors). A Hammett-Jaffe analysis yielded ap+ + 0.256 for the B(OH)2 group.234... 

Most of the data in Table 12 come from the work of Shvo et al. (78). Careful band-shape analysis and solvent-effect studies permitted evaluation of the rate constants and AG values at 298 K, which renders the discussion of substituent effects more meaningful than usual. The authors obtained reasonably linear Hammett plots when correlating log km with Or (79) for X and Y, holding one of these substituents constant. They also found that the dihydropyridine system may act as an unusually efficient donor, giving a AG of 17.6 kcal/mol with X, Y = H, CN, the only barrier below 25 kcal/mol reported for any donor-substituted cyanoethylene. However, with other acceptor combinations the dihydropyridine moiety is not so outstanding, and this illustrates the difficulty of measuring donor and/or acceptor effects by rotational barriers alone (vide infra). 

A vast generalization beyond the Bronsted—Lowry acids and bases concepts is the concept of a Lewis29 base (an electron pair donor) and a Lewis acid (an electron pair acceptor). This concept has been used extensively in all branches of chemistry. In physical organic chemistry, quantities of the type pA = —logio[A] have used extensively to study reactivities—for example, in the Hammett equation. 

In dilute aqueous solution, the acidity is measured using pH values. For concentrated acid solutions and non-aqueous acid solutions, pH values are no longer available. Hence, the Hammett acidity function Ho is used as a measure of the acidity of such media [130]. The proton donor ability of an acid in such media is measured by studying the equilibria of a series of indicator bases B (mostly nitroanilines), the UV/Vis absorption spectra of which are markedly different from those of their conjugate acids, so that the indicator concentration ratio I = [B]/[BH+] can be measured spectrophotometri-cally. The acidity function Ho is then given by Hq = p.Kbh+ + Ig f with the subscript zero indicating that the Hq function applies only to neutral bases B [130, 170], For dilute solutions, Ho corresponds exactly to pH in concentrated solutions, the two functions differ appreciably. 

With electron-donor substituents on the aryl group, the stability of N-arylpentazoles increases (except for orffzo-substituents). With increasing Hammett a values for the para-substituent in the N-aryl group, crystalline Af-arylpentazoles decompose at increasing temperatures between —10 °C for phenyl and 4-55 °C for dimethylamino (64AHC373, 85AGE513). The latter pentazole could even be studied by X-ray diffraction (83CC910). 

Several oxovanadium(IV) complexes (141) with ON-donor ligands were prepared from [YOCl2(thf)2] (thf = tetrahydrofuran) and /// /////-substituted phenols to study the electronic effects of para substituents in order to develop better [VCl2(OR)2]-type olefin polymerization catalysts.676 The hyperfine coupling constants, the HOMO-LUMO transitions, and the oxidation potentials were all found to be linearly related to the Hammett a constant of the substituent on the monoanionic aryloxy ring.676... 

Essentially electron-donor substituents retarded polymerization, whereas electron-acceptor substituents accelerated the process. In fact, these workers obtained a good correlation between the Hammett a values of the substituents and the polymerization rate. Remaining with benzophenone, the photopolymerization of 1,3,5-trithiane has been found to be inhibited by amines and also the absence of oxygen,whereas in the hydrogen peroxide-initiated photopolymerization of methyl methacrylate-benzophenone has been found to be a powerful accelerator. In the latter study different solvents were found to have different effects on initiation. In solvents giving low conversions, degradative initiator transfer was found to be a dominant process. 

Several theoretical studies have been directed at phosphonic acids among the earliest being that of Jaffe, Freedman, and Doak [296], who measured the pK s of 25 meta- and / ara-substituted benzenephosphonic acids and utilized Hammett s equation to rationalize the values observed, They subsequently measured the pK s of 16 orfljo-substituted compounds and concluded that hydrogen bonds were formed between the phosphono group (-PO3H2) and many of the ortho substituents. The phosphono group could act as either a donor or acceptor. 

The nonlinearity of the correlation between log k/ko) and Hammett (Tp constant at the cyclization of compounds 3.971 suggests that there is a change in reaction mechanism of C -C cyclization of the enyne carbodiimide, possibly from carbene to polar intermediates. Electron donor substituents X and Y increase probability of formation of the high polar zwitterionic carbene intermediate (Scheme 3.147, structures B and C), whereas electron-withdrawing substituents lead to a less polar carbenes (Scheme 3.147, structure A). The mechanism of polar intermediate is supported by a noticeable increase of the reaction rate with increasing the donor properties of the solvent that was observed for carbodiimide 3.971a. However, more detailed study of conditions of the... 

---