Para-distributed benzenes

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 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 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). 

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. 

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)]. 

Palmisano et al. [41] in a study on the selectivity of hydroxyl radical in the partial oxidation of different benzene derivatives have investigated how the substituent group affect the distribution of the hydroxylated compounds. The reported results show that the primary photocatalytic oxidation of compounds containing an electron donor group (phenol, phenylamine, etc.) leads to a selective substitution in ortho and para positions of aromatic molecules while in the presence of an electron-withdrawing group (nitrobenzene, benzoic acid, cyanobenzene, etc.) the attack of the OH radicals is nonselective, and a mixture of all the three possible isomers is obtained. 

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). 

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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