Ortho-distributed benzenes

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

In a similar study, arc generated atoms were allowed to react with tert-butyl-benzene in order to use the tert-butyl group as an intramolecular trap for the carbene. This reaction gave drmethylindane-3- C (56) and 3,3-dimethyl-3-phen-ylpropene-l- C (57). The label distribution in 56 indicates that it arises from an initial insertion into an ortho C H bond to give o-tert-butylphenylcarbene (58), whUe 57 results from insertion into a methyl group C—H (Eq. 35). The fact that... 

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. 

Dibenzo[6,e][l,4]dioxin, phenoxathiin and thianthrene all react with butyllithium with proton abstraction from a benzene ring. Dibenzo[f ,e][l,4]dioxin and thianthrene are metallated at the 1-position (128), while lithiation of phenoxathiin occurs ortho to the C—O bond rather than the C—S bond, i.e. at C-4, (129). The lithiated products provide excellent intermediates for functionalizing the rings at these positions, usefully complementing the product distribution pattern in electrophilic substitution reactions. 

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. 

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. 

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). 

C6o solubility in pyridine is identical to that in benzene. Pyridine has a pronounced "aromatic" nature. zi-electron distribution in a pyridine molecule is identical to that in benzene. Pyridine has six mobile 7i-bonds, one of them is formed by an unshared pair of -electrons of a nitrogen atom. Pyridine can be nitrated. A nitro group enters the P-position. Because carbon with the highest electron density is a center for electrophilic substitution, one can make a logical assumption that the reaction center for charge-transfer interaction between pyridine molecules and C6o is also in the P-position or, what is equivalent, in the ortho-position relative to a nitrogen atom (Table 6). 

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. 

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%). 

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