Ortho, meta, para isomers positional

The classical argument concerting the equivalence of the positions on the benzene ring is based on the existence of three (ortho, meta. para) isomers of xylene (dimethylbenzene). How many isomers ore there of dimelhylhorazine ... 

To elevate p-selectivity in nitration of toluene is another important task. Commercial production of p-nitrotoluene up to now leads with twofold amount to the unwanted o-isomer. This stems from the statistical percentage of o m p nitration (63 3 34). Delaude et al. (1993) enumerate such a relative distribution of the unpaired electron densities in the toluene cation-radical—ipso 1/3, ortho 1/12, meta 1/12, and para 1/3. As seen, the para position is the one favored for nitration by the attack of NO (or NO2 ) radical. A procednre was described (Delande et al. 1993) that used montmorillonite clay supported copper (cupric) nitrate (claycop) in the presence of acetic anhydride (to remove excess humidity) and with carbon tetrachloride as a medinm, at room temperature. Nitrotoluene was isolated almost quantitatively with 23 1 76 ratio of ortho/meta/para mononitrotoluene. 

Arylnitrenium ions are likewise capable of adding to n nucleophiles. With substituted aromatics (e.g., toluene) there exists the possibility of three reactive sites on the nitrenium ion (the nitrogen, ortho- and para-ring positions), along with up to three possible sites on the arene (ortho, para, and meta in the case of a monosub-stituted trap). Thus in a typical case there is the possibility of nine distinct regio-isomers. Obviously, any synthetic utility of such chemistry relies on the ability of the reagents to react in a selective manner. 

At the equilibrium geometries of the neutrals, the symmetry assignments of these cationic doublet states, ordered by ascending vertical ionization potentials, are as follows. The obvious notations Bz+, E-Bz+, 1,2, 1,3, 1,4 and 1,2,3 correspond to the benzene, mono-fluorobenzene, ortho-, meta-, para-difluorobenzene isomers and 1,2,3-trifluorobenzene, respectively. (The numbers refer to the position of the... 

Iliev devoted the second part38 of his generalization of the Lunn-Senior mathematical model of isomerism to identification of the isomers of a given molecule with their structural formulas on the basis of their substitution reactions. A classical prototype of this identification is the Komer scheme of substitution reactions40 between the three di-substituted and the three tri-substituted homogeneous products of benzene, which allows all these products to be identified as ortho, meta, para, vicinal, asymmetric, symmetric, respectively, in agreement with the combinatorics of the substituent positions on the six-membered ring. 

After 15 min reaction time the starting material is almost completely consumed but, disappointingly, the ratio of the positional isomers has changed again the ortho isomer now is formed in excess over the meta/para isomers. 

The reaction of toluene with 1 could conceivably lead to four different trans-isomeric products by attack at the ortho, meta, para, or alpha positions. The carbanion chemistry of toluene would predict attack at the methyl group but, as in other transition metal arene activation studies (8,9,10), this is not found. The NMR spectrum of the toluene adducts displays two singlets attributable to trans-tolyl adducts in an intensity ratio of 1 1.6 in addition to resonances of cis-tolyl adducts. Selective mono-deuteration experiments show that the weaker downfield resonance is attributable to the para isomer while the stronger one is attributable to the meta isomer no ortho isomer is observed. If one corrects for statistical effects, the preference for para, meta, and ortho isomers is 1 0.8 0. We attribute this distribution of isomers primarily to... 

When nitrobenzene and nitrate ions are mixed in the presence of sulfuric acid, it reacts by adding nitrate to the aromatic ring (Figure 3.25.1). The main product of this electrofilic substitution is m-dinitrobenzene, but small amounts of the ortho- and para-isomers, (Figures 3.25.2 and 3.25.3) are also produced. M-dinitrobenzene is favored since the NO2 group is an electron-withdrawing substitute that deactivates the benzene ring but deactivates the meta-position the least. A small quantity of 1,3,5-trinitrobenzene is also produced. The reaction is allowed 5 minutes to complete. 

Three isomers are possible when two groups are substituted for hydrogen atoms on the benzene ring. The relative positions of the substituents are indicated by either the prefixes ortho-, meta-, para- (abbreviated 0-, m-, p-), or... 

The following scheme describes the products resulting from nitration followed by chlorination (reaction 27.12) and chlorination followed by nitration (reaction 27.13). It shows that the substitution is not random. The —NO2 group directs Cl to a meta position. Almost no ortho or para isomer is formed in reac-fion (27.12). The Cl group, on the other hand, is an ortho, para director. Essentially no meta isomer is produced in reaction (27.13). 

A typical example for shape selectivity is the alkylation of alkyl aromatics to dialkylated compounds with meta, ortho, and para isomers [105]. In general, alkyl cations attack the 2-, 4-, and 6-positions of alkyl aromatics because alkyl groups are ortho- and para-directing. Para substitution is more favorable over ortho substitution if the alkyl groups are bulky. However, the isomerization of para isomers to ortho isomers always occurs on the acidic site. Thus, the selectivity to para isomers is low. This is not the case for the alkylation reactions taking place in... 

Because the product composition is kinetically controlled, the isomer ratio will be governed by the relative magnitudes of AG, AGI, and AG, the energies of activation for the ortho, meta, and para transition states, respectively. In Fig. 4.7 a qualitative comparison of these AG values is made. At the transition state, a positive charge is present on the benzene ring, primarily at positions 2, 4, and 6 in relation to the entering bromine. 

In the case of nitrobenzene, the electron-withdrawing nitro group is not able to stabilize the positive charge in the complex intermediate. In fact, it strongly destabilizes die intermediate. This destabilization is greatest in the o- and />-intermediates, which place positive charge on the nitrosubstituted caibon. The meta transition state is also destabilized relative to that for benzene, but not as much as the ortho and para transition states. As a result, nitrobenzene is less reactive than benzene, and the product is mainly the meta isomer. 

In many cases, however, the ortho isomer is the predominant product, and it is the meta para ratio which is close to the statistical value, in reactions both on benzenoid compounds and on pyri-dine. " There has been no satisfactory explanation of this feature of the reaction. One theory, which lacks verification, is that the radical first forms a complex with the aromatic compound at the position of greatest electron density that this is invariably cither the substituent or the position ortho to the substituent, depending on whether the substituent is electron-attracting or -releasing and that when the preliminary complex collapses to the tr-complex, the new bond is most likely to be formed at the ortho position.For heterocyclic compounds such as pyridine it is possible that the phenyl radical complexes with the nitrogen atom and that a simple electronic reorganization forms the tj-complex at the 2-position. 

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