Benzene derivatives resonance effects

Let us illustrate this with the example of the bromination of monosubstituted benzene derivatives. Observations on the product distributions and relative reaction rates compared with unsubstituted benzene led chemists to conceive the notion of inductive and resonance effects that made it possible to explain" the experimental observations. On an even more quantitative basis, linear free energy relationships of the form of the Hammett equation allowed the estimation of relative rates. It has to be emphasized that inductive and resonance effects were conceived, not from theoretical calculations, but as constructs to order observations. The explanation" is built on analogy, not on any theoretical method. 

The reduction of the C— Br and C—1 group moments from 1.10 and 0.90 in bromo- and iodo-benzene to about 0.80 and 0.50 in 2-bromo- and 2-iodo-thiophene has been ascribed to the larger weight of resonance forms such as (8) and (9) in the thiophene series. The chlorine, nuclear, quadrupole, resonance frequencies of chloro-substituted thiophenes are much higher than those of the corresponding benzene derivatives. This has been ascribed to a relayed inductive effect originating in the polarity of the C—S o-bond in thiophenes. The refractive indices, densities, and surface tension of thiophene, alkyl- and halo-thiophenes, and of some other derivatives have been... 

In aromatic compounds the effect of a functional group on retention may be enhanced or diminished by resonance. As illustrate in Fig. 4 the curves for monofunctional benzene derivatives exhibit a mo e or less parallel slope on the plot of log k against log eluent composition whereas the multifunctional derivatives, e.g., nitroanilines, cholestenotie, show distinctly different slopes. This demonstrates how difficult the prediction of retention behavior in adsorption chromatography is. The greater the deviation of the structure from the simple model compounds used for establishing the rules, the more difficult the prediction becomes. 

In particular, called resonance polar effect [Taft, 1956] is defined for any benzene derivative where there is no direct conjugation between substituent and reactive it can be considered constant for a particular solvent, therefore expressing resonance interactions between substituent and skeletal group. 6r is usually referred to as the effective resonance constant and of hold for electrophilic and nucleophilic reaction series, respectively. 

The ortho coupling in benzene derivatives varies over the relatively small range of 6.7-8.5 Hz, depending on the resonance and inductive effects of the substituents. The presence of heteroatoms in the ring expands the range at the lower end down to 2 Hz, because of the effects of electronegativity (pyridines) and of smaller rings (furans and pyrroles). 

Yukawa-Tsuno equation A multiparameter extension of the Hammett equation to quantify the role of enhanced RESONANCE effects on the reactivity of meta- and para-substituted benzene derivatives, for example,... 

Resonance and induction have a marked effect on orientation in aromatic substitution. One would therefore expect some correlation between orientation in substitution and the electronic absorption spectra of benzene derivatives. In Table 5.4 the position of the 200 mp benzene band10 and the spectroscopic moments14 have been compared with the percentage of the meta isomer produced in nitration. A fair parallelism is apparent in the three columns. 

Both inductive and resonance effects are involved. The favored reaction proceeds through the most stabilized (or least destabilized) intermediate carbocation. Study carefully the re.sonance forms pictured for the possible cations derived from electrophilic attack on meihylbenzene and (trilluoromethyl)benzene (Section I6-I), and on benzenamine (aniline), benzoic acid, and a haiobenzene (Section 16-2). Notice the types of groups that fall into the different categories in Table 16-1. In particular, notice the following two general trends ... 

Considerable effort has been devoted to the determination of rate and equilibrium constants for benzene derivatives with meta- and pnrn-trifluoro-methyl substituents in a search for an exalted substituent effect which would provide evidence for special electron delocalization. Resonance interactions are generally detectable by such procedures. To illustrate, two sigma constants Op-NOi p-N(h with values of 0.78 and 1.24, respectively are necessary to describe the behavior of the nitro group quantitatively by the Hammett equation... 

The stability and reactivity of benzene derivatives are dominated by their aromaticity and by the fact that the electronic effect of substituents can be transmitted around the ring by resonance ... 

The bathochromic effect of phenyl substituents depends on the angle between the benzene and the chelate rings, suggesting that resonance effects are involved. This is supported, on the whole, by the spectral behaviour of the NH, NH and COOR substituted derivatives. However, the effect of p-NH substituents does not show the expected dependence on the aforesaid twisting angle. The hypsochromic shift on passing from NH to NCH, is also observed in some cyanine dyes. 

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