Resonance substituents

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. ... 

Substituent f7 inductive resonance Substituent [Pg.11]

Young, P. R. Jencks, W. P. Separation of polar and resonance substituent effects in the reactions of acetophenones with bisulfite and of... 

Tab. 5.3 Inductive and Resonance Substituent Effects of Various Functional Groups Electron donation...

The absolute rate constants for reaction of the carbon-substituted silenes 2b, 6, 7, 23 and 85a-c are plotted against the resonance substituent parameter o 86 in Figure 10. The data appear to correlate reasonably well, if one assumes that the curvature in the plot results from the fact that MeOH for the more reactive derivatives in the series approaches... 

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. 

Here (f is the normal substituent constant which does not involve any additional rr-electronic interaction between the substituent and the reaction centre Actr is the resonance substituent constant measuring the capability for TT-delocalization of rr-electron donor substituents and is defined by o- - a"... 

Equation (2) for a reaction giving a set of apparent substituent constants o can be rewritten in the form, (o- t/ ) = r a - cr ), where r is constant for the reaction regardless of substituents. As the increment of any o-from (f should be a reasonable measure of the resonance capability of the respective substituents, this proportionality represents a linear resonance energy relationship. The original form (Yukawa and Tsuno, 1959, 1965) using cr instead of (f in (2) has the same significance since the proportionality relation holds for the resonance increment (a - t/ ) or (a - cr). The definition of resonance substituent constants Ao by any set of (o— o") is arbitrary, and the definition of the r scale is also arbitrary. While the definition of the r = 0 scale by [Pg.269]

In (27), ct g) is a normal substituent constant and AaR(G) is a resonance substituent constant defined as (o- - ro is a resonance demand parameter, and po is a susceptibility parameter of the system in the gas phase. The gas-phase substituent parameter values are given in Table 15. 

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 the v(R)-parameters no correlation involving the group polarizability a(R) exists. However, it was found that v(R) depends on a, and the resonance substituent constants according to equation 8, both substituent effects having significant weights for v(R). 

Resonance substituent constants IC, Rr, and R9 are derived from the corresponding a values as ... 

Reynolds, W.F. and Topsom, R.D. (1984). Field and Resonance Substituent Constants for Aromatic Derivatives Limitations of Swain s Revised Fand R Constants for Predicting Aromatic Substituent Effects. J.Org.Chem., 49,1989-1992. 

Analysis of 19F chemical shifts show that the 1-aziridinyl group is not as conjugatively electron-donating as either theNH2 orNMe2 group51. The polar and resonance substituent constants based on 19F chemical shifts of the respective p-substituted fluorobenzenes are given in Table 551. 

TABLE 7. Resonance substituent constants for the amino groups in nonfluorinated anilines corresponding to the structures of p-fluoro derivatives 9-22 (Figure 9)... 

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. 

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. 

PR. Young and WP. Jencks, Separation of Polar and Resonance Substituent Effects in the Reaction of Acetophenones with Bisulfite and of Benzyl Halides with Nucleophiles, J. Am. Chem. Soc., 1979, 101, 3288. 

Several sets of resonance constants were defined according to the different types of Gp values. Resonance substituent constants, K, and K are derived from the corresponding g values... 

Reynolds, W.E. and Topsom, R.D. (1984) Field and resonance substituent constants for aromatic derivatives limitations of Swain s revised F and R constants for predicting aromatic substituent effects. /. Oig. Chem., 49, 1989—1992. 

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 ...

Qualitatively comparing these numbers to the traditional polar and resonance substituent effects, we see that for a donor substituent > s i. EWG substituents make s 2 > However, the trend that dominates the sum of the differences in... 

If a substituent has a lone pair on the atom that is directly attached to the benzene ring, the lone pair can be delocalized into the ring these substituents are said to donate electrons by resonance. Substituents such as NH2, OH, OR, and Cl donate electrons by resonance. These substituents also withdraw electrons inductively because the atom attached to the benzene ring is more electronegative than a hydrogen. 

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