Benzene rings para-distributed

The relative rates of acetylation in competition experiments in the [m.n]paracyclophane series 38> may be interpreted in terms of trans-annular electronic and steric effects. If the rate of acetylation of [6.6]para-cyclophane [(7), m =n =6] is is taken as one, the relative acetylation rates of the [4.4]-, [4.3]-, and [2.2]paracyclophanes are 1.6, 11, and >48, respectively. As the aromatic rings come closer together, the rate of entry of the first acetyl group into the nucleus increases, while that of the second acetyl group decreases. Both effects clearly indicate that the positive. partial charge can be distributed over both benzene rings in the monoacetylation transition state (64). 

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 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. 

Some chemical structures exhibit typical distances that occur independently of secondary features, which mainly affect the intensity distribution. In particular, aromatic systems contain at least a distance pattern of ortho-, meta-, and para-carbon atoms in the aromatic ring. A monocyclic aromatic system shows additionally a typical frequency distribution. Consequently, Cartesian RDF descriptors for benzene, toluene, and xylene isomers show a typical pattern for the three C-C distances of ortho-, meta-, and para-position (1.4, 2.4, and 2.8 A, respectively) within a benzene ring. This pattern is unique and indicates a benzene ring. Additional patterns occur for the substituted derivatives (3.8 and 4.3 A) that are also typical for phenyl systems. The increasing distance of the methyl groups in meta- and para-Xylene is indicated by a peak shift at 5.1 and 5.8 A, respectively. These types of patterns are primarily used in rule bases for the modeling of structures explained in detail in the application for structure prediction with infrared spectra. 

The substitution of a single atom or group, X, for an H atom in benzene can occur at any one of the six positions on the ring. We say that the six positions are equivalent. If a group Y is substituted for an H atom in C HsX, this question arises To which of the remaining five positions does the Y group go If all the sites on the benzene ring were equally preferred, the distribution of the products would be a purely statistical one. That is, Y can be substituted in five possible positions, and we should get 20% of each one. Since two possibilities lead to an ortho isomer and two lead to a meta isomer, however, we should expect the distribution of products to be 40% ortho, 40% meta, and 20% para ... 

Whether a group is an ortho, para, or meta director depends on how the presence of one substituent alters the electron distribution in the benzene ring. As a result, attack by a second group is more likely at one type of position than another. Examination of many reactions leads to the following order ... 

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 =... 

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. 

The 1H NMR shifts of phenol give us an indication of the electron distribution in the n system. The more electron density that surrounds a nucleus, the more shielded it is and so the smaller the shift (see Chapter 11). All the shifts for the ring protons in phenol are less than those for benzene (7.26 p.p.m.), which means that overall there is greater electron density in the ring. There is little difference between the ortho and the para positions both are electron-rich. 

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