Inductive and resonance substituent constant

It has been found that the chemical shift of the benzylidene carbon, Sct, in ring-substituted benzaldoximes (Scheme 2) correlates well with Hammett a constants. On the other hand, it is known that the chemical shift of that atom in the NMR spectra of benzaldoximes depends mainly on the substituents inductive effect and its resonance effect is of reduced importance. Multiparameter correlations of Scr with the inductive and resonance substituents constants are of better quality. It is known that the chemical shifts of para-substituted benzaldoximes are linearly dependent on Hammett a constants. Correlations between the NMR chemical shifts of the oximino oxygen and a, a and aj substituent constants for substituted benzaldoximes are poor. These results show that coplanarity of Ar—CH=NOH creates difficulties in transmission of the substituents effect, especially the resonance effect, to the CH=NOH group. ... 

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. ... 

For meta- and para-substituted phenols, log values spread over 2 log units from 3-dimethylaminophenol to 4-nitro-3-trifluoromethylphenol. Their order is well explained by classical electronic effects. A dual-substituent parameter analysis gives equations 16 and 17, where crp and ctr are the Taft field-inductive and resonance substituent constants, respectively. 

We have recently explored the possibility of using the NH3 molecule as a basis for measuring the electron-attracting tendencies of substituents We computed V, for the amine nitrogens in a series of NH2-X molecules at the STO-5G level. In order to focus on electronic factors, the HNX angles were constrained to their STO-3G values in NH3. Table 2 lists the resulting electrostatic potential minima of 31 NH2-X molecules, along with the inductive and resonance substituent constants, o, and for the substituents X. 

Table 2 Calculated STO-5G Electrostatic Potential Minima of NH2-X Molecules and Inductive and Resonance Substituent Constants ...

LINEAR FREE ENERGY RELATIONSHIPS Hammett substituent constants Taft substituent constants Inductive and resonance substituent constants Steric substituent constants Homolytic substituent constants Field and resonance components Aqueous solubility The parachor... 

The results of two-parameter correlations of the 13C and 15 N NMR chemical shifts of 2-substituted 5(6)-nitrobenzimidazoles with the induction and resonance constants (a, ak) are given in Table 3.26 [688, 689], The electronic influence of substituents on the chemical shifts of carbons (and protons) in positions 4 and 7 are mainly transmitted by a resonance mechanism, C-4 being more sensitive than C-7 to the substituent effect. For positions 5 and 6 a slightly smaller contribution from the resonance component to the total transmission of substituent effects is observed. However, analysis of correlation between 5 15 N and substituent parameters indicates an approximately equal influence of the induction and resonance substituent effects on the nitro group shielding (see Table 3.26) [688, 689], When other sets of substituent constants (F and R, c, and ck, etc.) are used in the correlation the percentage ratios of resonance and inductive contributions remain unchanged. 

P is the magnitude of the property Y for substituent X, expressed relative to the property for X=H Oj and Or are inductive or polar and resonance substituent constants, respectively, there being various scales for oR pi and Pr are the corresponding regression coefficients. 

The quatemization reaction of the thiazole nitrogen has been used to evaluate the steric effect of substituents in heterocyclic compounds since thiazole and its alkyl derivatives are good models for such study. In fact, substituents in the 2- and 4-positions of the ring only interact through their steric effects (inductive and resonance effects were constant in the studied series). The thiazole ring is planar, and the geometries of the ground and transition states are identical. Finally, the 2- and 4-positions have been shown to be different (259. 260). 

One underlying physical basis for the failure of Hammett reaction series is that substituent interactions are some mixture of resonance, field, and inductive effects. When direct resonance interaction is possible, the extent of the resonance increases, and the substituent constants appropriate to the normal mix of resonance and field effects then fail. There have been many attempts to develop sets of a values that take into account extra resonance interactions. 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Many other definitions of an ortho substituent constant have been made Shorter has reviewed these. Charton analyzed Oo in terms of Oi and CTr, i.e., = a(Ti -I- fpoR, finding that the distribution of inductive and resonance effects (the ratio a/b) varies widely with the substituent and, therefore, that no general Oo scale is possible. Charton also subjected to analysis according to Eq. (7-47),... 

B. The Separation of Inductive and Resonance Effects Substituent Constants from Spectroscopic Studies. 

Exner devoted some attention to SOPh and S02Ph in his extensive compilation of substituent constants of 197879. For the former, however, values based directly on chemical reactivity were found to be few, as is already apparent in the present section, and so Exner considered it useful to give values based on appropriate summation of inductive and resonance constants (see Section III.B). These were considered to be normal values, i.e. 7m° and a°, of 0.51 and 0.50 respectively. Corresponding values for S02Ph were 0.59 and 0.66 respectively, both slightly smaller (for no obvious reason) than the chemically based values of am and ap (and a value supposed to be of a°) quoted earlier in this section. 

Oj and aR are respectively the inductive and resonance constants of Taft s analysis of ordinary composite Hammett constants (values obtained by Charton were used) and v is the steric substituent constant developed by Charton161,162. The intercept term, h, notionally corresponds to log k for H as an orfho-substituent, but is not found in practice always to agree closely with the observed value of log k for the parent system. 

The most fruitful treatment of the electronic effects of ozt/zo-substituents involves the use of the same cr/ and correlation analysis for meta- and para-substituents by means of the dual substituent-parameter equation 91 or the extended Hammett equation 95 (Section II.B). Obviously it is a considerable assumption that these are valid for ort/zo-substituents and the implication is that in the correlation analysis any peculiarities may be adequately expressed through the coefficients of the inductive and resonance terms. Really satisfactory correlation analysis for any given reaction system requires a large amount of data and can only rarely be accomplished. 

The electronic effects of cycloalkyl annelation can be divided into inductive and resonance effects. Few measurements of the resonance contributions of cycloalkyl substituents exist. A single determination of the resonance parameter para to substitution in o-xylene, tetralin and indan suggests that hyperconjugative release is fairly constant within a series of benzocycloalkenes, and that strain induced electronic effects must act inductively. ... 

A simple case where the general a constants in Table 8.5 do not succeed in correlating acidity constants is when the acid or base function is in direct resonance with the substituent. This may occur in cases such as substituted phenols, anilines, and pyridines. For example, owing to resonance (see Fig. 8.4), a para nitro group decreases the pKa of phenol much more than would be predicted from the o para constant obtained from the dissociation of p-nitrobenzoic acid. In such resonance cases (another example would be the anilines), a special set of o values (denoted as oJpara) has been derived (Table 8.5) to try to account for both inductive and resonance... 

Calculated Inductive and Resonance Effect p Values for the Rates of Alkylation of 3- and 4-Substituted Pyridines Using [Pg.84]

Hammett s success in treating the electronic effect of substituents on the equilibria rates of organic reactions led Taft to apply the same principles to steric, inductive, and resonance effects. The Hammett o constants appear to be made up primarily of two electronic vectors field-inductive effect and resonance effect. For substituents on saturated systems, such as aliphatic compounds, the resonance effect is rarely a factor, so the o form the benzoic acid systems is not applicable. Taft extended Hammett s idea to aliphatics by introducing a steric parameter ( .). He assumed that for the hydrolysis of esters, steric and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base. Rate differences would be caused only by the field-inductive effects of R and R in esters of the general formula (XCOOR), where X is the substituent being evaluated and R is held constant. Field effects of substituents X could be determined by measuring the rates of acid and base catalysis of a series XCOOR. From these rate constants, a value a could be determined by Equation (5.9) ... 

Conjugation of the nitrogen lone pair with the adjacent phosphoryl or carbonyl function was tested by the 13C NMR parameters of the N-phosphorylated and N-acetylated aniline, as well as of their complexes with Lewis acids. (h ). The inductive and resonance constants for the neutral and charged amide groups were determined using the dual substituent parameter (dsp) approach. ( 5). Results are given in the Table. 

If steric effects of substituents do not affect the formation of the complex and A (e.g. TCNE) is constant, coefficients a and b in equation 19 depend on D only130. In approximation 18, hvcr and Ip depend on the inductive and resonance effects of the X substituents for constant D128,129. As the donor properties of X become stronger, the HOMO energy increases and the hvcr and Ip values decrease. 

Correlations of fluorine-19 chemical shifts with substituent constants enable the empirical susceptibility of the chemical shift to inductive and resonance effects to be separated. For example, examination of the fluorine-19 chemical shifts of a series of fluorobenzenes gave the following correlation2,20-22 ... 

Some differences in the change of 13C NMR chemical shifts of nitrobenzothiazoles compared with nitrobenzimidazoles and nitrobenzoxazoles have been discussed. In Table 3.30 the 13C NMR chemical shifts of nitrobenzothiazoles are presented. The nitro group introduction into position 2 leads to a 10 ppm down field shift of the ipso-carbon resonance, whereas a similar effect of the ipso-substitution in the phenylene fragment of benzothiazoles is 20 ppm [778-781], The results of regression analysis of the 13C NMR chemical shifts of benzothiazoles in terms of the inductive and resonance constants of substituents (F and R, c, and cR, c, and aR°) provide evidence for the fact that the substituent effect transmission from positions 2-6 is approximately 30% weaker than in the opposite direction [779], As stated previously, an analogous picture is observed for benzimidazoles. 

---