Hammett substituent constant, effect

Hammett substituent constant, effect of cyclo-amyloses on, in hydrolysis of phenyl esters, 23 222... 

The fundamental assumption of SAR and QSAR (Structure-Activity Relationships and Quantitative Structure-Activity Relationships) is that the activity of a compound is related to its structural and/or physicochemical properties. In a classic article Corwin Hansch formulated Eq. (15) as a linear frcc-cncrgy related model for the biological activity (e.g.. toxicity) of a group of congeneric chemicals [37, in which the inverse of C, the concentration effect of the toxicant, is related to a hy-drophobidty term, FI, an electronic term, a (the Hammett substituent constant). Stcric terms can be added to this equation (typically Taft s steric parameter, E,). 

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 x-band in malachite green arises from an NBMO—>n transition, so that 3- and 4-substituents affect the energy of the excited state only and bring about spectral shifts of the first absorption band which vary linearly with the appropriate Hammett substituent constants. Thus, electron-withdrawing groups cause bathochromic shifts of the x-band whereas donor substituents cause hypsochromic shifts (Table 6.6) [64,67]. The 3-band arises from a n—>n transition [68] so that substituent effects are less predictable. As the donor strength of the 4-substituent increases, however, the 3-band moves bathochromically and eventually coalesces with the x-band - at 589 nm in the case of crystal violet (6.164), which possesses two NBMOs that are necessarily degenerate [69]. 

Attempts have been made to correlate the effects of acceptor groups on the C=C barriers by Hammett substituent constants. Since the important interaction is concerned with the delocalization of a negative charge, the cr scale should be more appropriate than the normal a scale (136). However, in the ctp scale, CN is more efficient than COMe, which is contrary to experience, and therefore Shvo et al. preferred the ctr = ctp - o, scale (78 see also Sect. II-C-5). Similarly, in a series of ketene mercaptals, a reasonable correlation was found... 

A monoparametric equation results when a composite electrical effect parameter is obtained by combining all three pure electrical effect parameters with fixed values of both Pd and t]. The Hammett substituent constants are of this type. The Hammett equation is given by equation 5 ... 

Effects of structural variation on organolithium compounds TABLE 3. Values of Hammett substituent constants... 

The addition of hydroxide ions to substituted benzaldehydes (ArCHO + OH <=> ArCH(0H)0 ) is used to establish J-acidity scales in water-ethanol and water-DMSO mixtures containing sodium hydroxide as a base. The pK-values in such mixtures are linearly correlated with Hammett substituent constants. The independence of reaction constant p of solvent composition confirms that substituted benzaldehydes are suitable J- indicators for hydroxide solutions in water-ethanol and water-DMSO mixtures. Dependence of J- values on sodium hydroxide concentration is only slightly affected by ethanol up to 90 % and at a constant sodium hydroxide concentration shows only small increase between 90 and 98 % ethanol. J- increases more with increasing DMSO concentration, but the effect is much smaller than that of DMSO on H- values based on proton abstraction from aniline. 

Elimination reactions of ( )- and (Z)-benzaldehyde Opivaloyloximes (19a) and (19b) with DBU in MeCN have been found to occur by a nitrile-forming E2 mechanism which is ca 2000-fold faster for the latter isomer in each case.15 The corresponding Hammett substituent constants, activation parameters, and primary deuterium isotope effects, suggest that the anti elimination from (19b) (for which p = 2.4 0.1, H/ D = 2.7 0.3, A/H = 12.5 0.2 kcal mol-1, and A= —31.0 0.6eu) proceeds to (20) via a more symmetrical transition state with a smaller degree of proton transfer, less charge development at the jS-carbon and greater extent of triple bond formation than for syn elimination from (19a) (for which p = 1.4 0.1, kn/kn = 7.8 0.3, AH = 8.8 0.1 kcal mol 1 and A= -23.6 0.4 eu). 

Because of the bulk of comparable material available, it has been possible to use half-wave potentials for some types of linear free energy relationships that have not been used in connection with rate and equilibrium constants. For example, it has been shown (7, 777) that the effects of substituents on quinone rings on their reactivity towards oxidation-reduction reactions, can be approximately expressed by Hammett substituent constants a. The susceptibility of the reactivity of a cyclic system to substitution in various positions can be expressed quantitatively (7). The numbers on formulae XIII—XV give the reaction constants Qn, r for the given position (values in brackets only very approximate) ... 

In Figure 4.2, log f1/2 is plotted against Hammett substituent constant straight line correlation, and a large negative value of p (—3). Such a result is consistent with that already observed for acylation of aromatic compounds on CeY zeolite[28] and for the conventional acid catalysed Friedel-Crafts acylation. These electronic effects are analogous to those reported in the case of classical electrophilic aromatic substitutions. 

Van Wauwe and coworkers182 studied the effect of various para-substituents on the binding of phenyl a-D-mannopyranosides to the lentil lectin. As with con A and the pea lectin, binding of p-substituted-phenyl a-D-mannopyranosides correlated fairly well with the Hammett substituent constant trH in which electron-releasing... 

The effea of substituents on ionization has already been discussed in Chapter 2, where the Hammett substituent constant was described, and in Chapter 4 in connection with the acidity ol phenols, These effects influence the relative acidity of benzoic acid derivatives in a similar way. 

This approach to separating the different types of interactions contributing to a net solvent effect has elicited much interest. Tests of the ir, a, and p scales on other solvatochromic or related processes have been made, an alternative ir scale based on chemically different solvatochromic dyes has been proposed, and the contribution of solvent polarizability to it has been studied. Opinion is not unanimous, however, that the Kamlet-Taft system constitutes the best or ultimate extrathermodynamic approach to the study of solvent effects. There are two objections One of these is to the averaging process by which many model phenomena are combined to yield a single best-fit value. We encountered this problem in Section 7.2 when we considered alternative definitions of the Hammett substituent constant, and similar comments apply here Reichardt has discussed this in the context of the Kamlet-Taft parameters. The second objection is to the claim of generality for the parameters and the correlation equation we will return to this controversy later. 

The original Hammett substituent constants [Hammett, 1937 Hammett, 1970] measuring the overall electronic effect of the meta- and paro-substituents of benzene derivatives having the functional group in the side chain. They were originally calculated from the variation of the acid dissociation constant of substituted benzoic acids (m-, P-XC6H4COOH) in water at 25°C, with respect to the unsubstituted compound (i.e. benzoic acid) ... 

A substituent effect on electrochemical and xerographic properties of triarylamines has recently been reported16. The oxidation potentials correlate with both the Hammett substituent constants and the calculated HOMO energies of the molecules under investigation, and this approach was used16 to rationalize xerographic properties, including transport phenomena of solid-state solutions in polycarbonate-Z. 

The analysis of the effect that the different substituents may have on the rate coefficients in this case is particularly hard to perform by using simple inductive effects, as described for instance by the Hammett substituent constants. Note that, despite the fact that most of the 28 carbenium ions exhibit para substitution, the general structure on top of Chart 6, shows a complex substitution pattern at the carbocation centre. In order to asses the effect of multiple substitution at this site, we first considered those compounds that have two fixed hydrogen atoms at the -position of the phenyl group, which according to Hammett classification have crp(H) = 0.0, and the third phenyl group substituted at -position with H (compound 96 in Chart 6) OCH3 (compound 97 in... 

There are limitations to the electronic constants which we have described so far. For example, Hammett Substituent Constants cannot be measured for ortho substituents since such substituents have an important steric, as well as electronic, effect. 

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