Apparent resonance effects

1 Apparent resonance effects (see also Section VIIB) 

The overall effect of substituents may be separated into inductive (oi) and resonance (o r) contributions, where o = oi + o r. Some of the data derived on this basis are given in Table 4.4 they indicate an apparent resonance contribution by various polyfluoroalkyl groups. 

The problem of describing such a resonance contribution then arises, and it is immediately tempting to draw an analogy with hydrocarbon systems and invoke fluorine 

In molecular orbital terms, the unshared /r-electrons at the carbanion site are donated into the a orbital of the adjacent C-F bond when the orbitals are in an anfi-periplanar configuration which ensures maximum orbital overlap [19, 20]. 

Another probe technique that has been used is to compare the effects of trifluoromethyl, at the meta and para positions, in both phenol and benzoic acid [29]. Only in the case where the substituent is in the para position in phenol is it directly conjugated with the ionising centre and therefore allowing a resonance effect to be important. Values of pK for the phenols led to the following substituent parameters o-(p-CF3) = +0.54 and o-(ot-CF3) = +0.43, the ratio o-(p-CF3)/o-(w-CF3) being 1.25, and this is essentially 

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It is apparent from the size of the conjugated system here that numerous resonance possibilities exist in this species in both the radical and the molecular form. Styrene also has resonance structures in both forms. On the principle that these effects are larger for radicals than monomers, we conclude that the difference ep. - ej > 0 for both hemin and styrene. On the principle that greater resonance effects result from greater delocalization, we expect the difference to be larger for hemin than for styrene. According to Eq. (7.23), r j oc > 1. According to Eq. (7.24), i2 < 1. 

A further difficulty in the case of fluoro-, chloro- and bromobenzenes is that with them apparently no choice of the 8 values seems to be reconcilable with the observed order of ease of substitution at the various positions unsubstituted benzene > para > ortho > meta. Both the inductive and the resonance effects are seen to leave the charge on the w-position practically unchanged, and approximately equal to 1.00c, while the observed order demands a considerably smaller value. As in the case of naphthalene, however, we shall find later that this discrepancy can apparently be explained by taking into account the polarization of the molecule by the attacking group. 

The electron paramagnetic resonance effect was discovered in 1944 by E. K. Zavoisky in Kazan, in the Tartar republic of the then-USSR, as an outcome of what we would nowadays call a purely curiosity-driven research program apparently not directly related to WW-II associated technological developments (Kochelaev and Yablokov 1995). However, a surplus of radar components following the end of the war did boost the development of EPR spectroscopy, in particular, after the X-band ( X meaning to be kept a secret from the enemy) was entered in Oxford, U.K., in 1947 (Bagguley and Griffith 1947). 

As we move to A-methylaniline, we see only a modest change in pK ,. This is undoubtedly due to the electron-donating effect of the methyl group, and this would be expected to stabilized the conjugate acid, increasing observed basicity. There is a modest increase in basicity, but it is apparent that the resonance effect, as in aniline, is also paramount here, and this compound is also a weak base. However, diphenylamine (A-phenylaniline) is an extremely weak base this can be ascribed to the resonance effect allowing electron delocalization into two rings. 

Two of three nitrofluorobenzene isomers react with methoxide, but the third is unreactive. Obtain energies of methoxide anion (at left), ortho, meta and para-nitrofluorobenzene, and the corresponding ortho, meta and para-methoxide anion adducts (so-called Meisenheimer complexes). Calculate the energy of methoxide addition to each of the three substrates. Which substrate is probably unreactive What is the apparent directing effect of a nitro group Does a nitro group have the same effect on nucleophilic aromatic substitution that it has on electrophilic aromatic substitution (see Chapter 13, Problem 4) Examine the structures and electrostatic potential maps of the Meisenheimer complexes. Use resonance arguments to rationalize what you observe. 

X = Y = Z)] give a completely linear Y-T correlation against the a scale with an r value of 0.76 for the substituent range from p-dimethylamino to p-nitro (Yukawa et al., 1966). On the contrary, less satisfactory Y-T correlations are obtained for the pXr+ values for monosubstituted triphenylmethanols [3(X,H,H)j or for the log(k/ko) values for the solvolysis of the corresponding chlorides, in which the s-tt-ED p-methoxy substituent exhibits a higher r value than that for the other w-ED groups. In the monosubstituted [3C (X,H,H)], apparently only the s-7r-ED-substituted aryl tends to enter into coplanarity with the cationic orbital, to exert its maximum resonance effect. 

The reaction apparently proceeds by the electrophilic attack of an acylium ion or protonated mixed anhydride [ArCO(H)OS02CF3], upon the para-position of an aromatic ether (Fig. 37). Loss of a proton results in the formation of 256. The nonsubstituted aryl group of the diphenyl ether was found to be much less reactive toward electrophilic substitution. This group is deactivated by protonation of the keto group in the strongly acidic environment. Therefore, monomers must be designed so that this type of resonance effect does not inhibit substitution at the second site of substitution [Eq. (53)] [162]. 

We don t usually look to aromatic systems for examples of inductive effects, because the pi system of electrons is ripe for resonance effects. However, in analyzing the resonance forms of phenoxide on the next page, it becomes apparent that the negative charge is never distributed on the meta carbons. Meta substituents cannot exert any resonance stabilization or destabilization at the meta position, substituents can exert only an inductive effect. The series of phenols demonstrates this phenomenon, consistent with aliphatic carboxylic acids. 

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