Meta-distributed benzenes

The photographs obtained for pyridine and py-razine are so closely similar to those of benzene as to leave no doubt that the three molecules have nearly identical structures. The radial distribution curve for pyridine (Fig. 1) calculated from the data given in Table II has well-defined peaks at 1.38, 2.39 (= V3 X 1.38), and 2.76 (= 2 X 1.38) A. The sharpness of the 2.39 peak indicates that the six meta distances in the ring are nearly equal. The calculated intensity curve for the model with C-C = 1.39 A., C-N = 1.33 A., C-H = 1.08 A., the angle C-N-C = 119° and the angles C-C-C = 121°, and having nearly equal meta and para distances, is shown in Fig. 3. The comparison of s0 and s values for this model (Table II) leads to the values C-H =... 

Distribution of ortho-, meta-, and para-isomers in the divinyl benzene copolymer... 

The 5 ring H s of monosubstituted benzenes, C H G, are not equally reactive. Introduction of E into Cf,HjG rarely gives the statistical distribution of 40% ortho, 40% meta, and 20% para disubstituted benzenes. The ring substituent(s) determine(s) (a) the orientation of E (meta or a mixture of ortho and para) and (b) the reactivity of the ring toward substitution. 

With suitably substituted benzene derivatives a preference can be seen in the product distributions for replacement of a group meta to the substituent which is not displaced (and which can be reckoned as... 

Study of isomer distribution in substitution of benzene rings already carrying one substituent presents some potential pitfalls. Inspection of product ratios for ortho, meta, and para substitution, as in investigation of electrophilic substitution (Section 7.4, p. 392), might be expected to give misleading results because of the side reactions that occur in radical substitution. The isomeric substituted cyclo-hexadienyl radicals first formed by radical attack partition between the simple substitution route and other pathways (Equation 9.102). In order for the... 

The high acidity of the Nafion-H catalyst is further demonstrated by its ability to promote both polyalkylation and isomerization. In reaction between benzene and ethylene at 190°C, 20% of the alkylated products are diethylbenzenes.187 The isomer distribution of the diethylbenzenes is 1 % of the ortho, 75% of the meta, and 24% of the para isomers. This composition is very close to the equilibrium composition of diethylbenzenes determined in solution chemistry with AICI3 catalyst and indicates that the reaction is thermodynamically controlled. 

Acyl chlorides were also tested in acylations promoted by B(OTf)3.231 Acylation of benzene and toluene in competitive reactions (molar ratio = 5 1) with acetyl chloride shows high para selectivity (92-95% with 2.5-7% of meta, kT/kB — 31-73), whereas the para isomer is formed only with 72-75% selectivity (8-10% of meta, k lkK = 78) in benzoylation with benzoyl chloride. Acetylation appears not to be affected by significant isomerization as indicated by isomer distributions and relative reactivity data. 

In benzene derivatives, electron-donating substituents direct into the ortho-and para-positions, while in the case of the electron-withdrawing substituents considerable meta-addition is observed (Table 3.1) otherwise a more equal distribution is established [reactions (6)-(9) and Table 3.1]. In agreement with the pronounced regioselectivity, ipso-addition at a bulky substituent such as the chlorine substituent in chlorobenzene is disfavored. Evidence for this is the low HC1 yield in the case of chlorobenzene, the low yield of para adduct in 4-methyl-phenol (Table 3.1), or the decarboxylation in the case of benzoic acid [reactions (6) and (10)]. 

This model predicts the existence of a barrier to the reaction whose height depends on the dipole moment of the chromophore. The efficiency of this process for distributed benzenes is clearly correlated with their dipole moments it is larger for ortho-disubstituted benzene than for meta- and para-distributed benzenes. This explains the cluster experiments (Brutschy et al. 1991) as well as the variation of reactivity in the gas phase (Tholman and Grutzmacher 1991). 

The examples of C6o dissolution in benzene derivatives considered in the present work evidence the clear dependence of C6o solubility on the electron density distribution in the benzene ring. We have identified a priori the electron density with the distribution of ortho-, meta-, para-isomers which form in the reactions of electrophilic substitution of the benzene derivative considered. This identification is evaluated but in some cases, such as in a series of homologs for alkyl derivatives of benzene, the total agreement between the C6o solubility and the amount of ort/io-isomers is observed (Table 2 and Fig. 7). 

Again, because of photoexcitation, the important frontier orbital of the electrophile (the LUMO) is able to interact productively with the LUMO of the benzene ring which was not productive in the ground state of any drop in energy. Certainly i/ 5 has an electron distribution ideal for explaining ortho/meta attack in anisole, and, as we saw in Chapter 4, the LUMOs of nitrobenzene do lead to reactivity at the para position. Now that photoexcitation has placed an electron in these orbitals, an electrophile can take advantage of this electron distribution, whereas, in the ground state, only a nucleophile could. 

The distribution rates for iodination of monosubstituted benzene derivatives have been reported. Under conditions of thermodynamic control (elevated temperature), meta substitution is observed. Under conditions of kinetic control (room temperature), a significant preference for para substitution is observed for compounds containing oriha- puru-directing substituent groups. Ortho substitution results when chelation of TTFA with the directing substituent permits intramolecular delivery of the electrophile. For example, methyl benzoate gives almost exclusively or/ho-lhallation (95%). 

In order to get some insight on how ELF works, we will analyse a number of parent molecules CeHsX (X = H, OH, F, Cl, Br and I). Their localization domains are displayed in Figure 14. Except for the substituent itself, all these molecules have 6 V(C, C), 5 V(C, H) and one V(C, X) basins. The differences are to be found in the hierarchy of the V(C, C) basins which is ruled by the nature of the substituent. In benzene, all the V(C, C) basins are equivalent and therefore the six critical points of index 1 between these basins have the same value, i.e. rj(rc) = 0.659. In the phenyl halides where the molecular symmetry is lowered from D h to C2v, the former critical points are then distributed in four sets according to the common carbon position ipso, ortho, meta and para. In phenol with a Cj symmetry, the two ortho and the two meta positions are not totally equivalent. In all studied molecules, the r) rc) values are enhanced in the ipso, ortho and para positions and decreased in the meta position. It has been remarked that the electrophilic substitution sites correspond to the carbon for which r) rc) is enhanced. Moreover, it is worthwhile to introduce electrophilic substitution positional indices defined by equation 26,... 

Equimolar mixtures of toluene and benzene were passed over beds of DHY at low temperatures (25-60°) in experiments where the two aromatics of different reactivity competed for the electrophilic deuterium (75). The distribution of deuterium between toluene and benzene (apparent ACeHsCHs/fcCeHe) and among the ring positions of the toluene-di samples was determined. A plot of log pt vs selectivity factor (St) for these data from the competitive experiments at 25-60° (Fig. 20, black circles) falls on the line obtained from a study of 47 electrophilic substitution reactions by H. C. Brown and associates (83). The partial rate factors pt and mt give the rate of substitution of the para position and one of the meta positions in toluene, relative to the rate of substitution of one of the six equivalent ring positions of benzene. Points a, b, c, d, and e fall quite close to the line, which represents a linear free energy relationship in both positional and substrate selectivity. 

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