Meta production

Nitration of aromatic amines with urea nitrate in sulfuric acid is reported to yield the -nitro derivative exclusively (44). When the para position is blocked, the meta product is obtained in excellent yield. 

It is understood that the ortho and meta products form part of the waste produced. In determining AE, the balanced chemical equation is written with a generalized structure for the product indicating all possible isomers and since the molecular weights of all isomers are identical equation (4.2) is used without change. In the above example, the atom economy for the production of para, meta or ortho products is the same. 

A similar trend was postulated for final Tg s of the cured materials. This result was observed in some cases, as with the hydroquinone systems where the more flexible meta product gave a significantly lower final Tg. The highest cured Tg s were seen in the bisphenol A, dihydroxybiphenyl and resorcinol systems. The lowest final Tg was observed with the thiodiphenol product, at 174°C, less than 350°F. 

All levels of calculation (including semi-empirical calculations) provide a qualitatively correct account of the experimental regio and stereochemical preferences. The only (apparent) exceptions are that both B3LYP/6-31G and MP2/6-31G models show modest preferences for meta products in cycloaddition of 2-methylcyclopentadiene and acrylonitrile. Note, in particular, the success of the calculations in properly assigning the more crowded syn product for the cycloaddition of 5-methoxycyclopentadiene and acrylonitrile. Also note the large magnitude for the preference. Clearly factors other than sterics are at work. 

Problem 11.8 (a) Give the delocalized structure (Problem 11.1) for the 3 benzenonium ions resulting from the common ground state for electrophilic substitution, C H,G + E. (6) Give resonance structures for the par -benzenonium ion when G is OH. (c) Which ions have G attached to a positively charged C (d) If the products from this reaction are usually determined by rate control (Section 8.5), how can the Hamnrand principle be used to predict the relative yields of op (i.e., the mixture of ortho and para) as against m meta) products (e) In terms of electronic effects, what kind of G is a (i) op-director, (ii) m-director (/) Classify G in terms of its structure and its electronic effect. M... 

A mixture of 21 g 2,3-methylenedioxytoluene and 1 g Hg(OAc)2 in 59 mL acetic acid was stirred and heated to 80 °C. To this there was added, dropwise, 12.8 g of concentrated nitric acid. Heating and stirring was continued for 2 h. The reaction mixture was quenched by pouring it into ice-H20, and extracted with Et20. The extracts were pooled, dried with anhydrous MgS04, and the volatiles removed under vacuum. The orange solid residue was recrystallized from to provide 20 g (58%) of a mixture of the ortho- and meta-products, 2,3-methylenedioxy-5-nitrotoluene and 2,3-methylenedioxy-6-nitrotoluene, with a mp 65-67 °C. This unresolved mixture was employed in the next step without further purification. 

The Z-substituted benzene (benzaldehyde, Figure 11.2) is not activated toward electrophilic attack since the HOMO of benzene is scarcely affected. No preferred site for attack of the electrophile can be deduced from inspection of the HOMOs. The interaction diagram for a Z-substituted pentadienyl cation, substituted in the 1-, 2-, and 3-positions, as models of the transition states for the ortho, meta, and para channels are too complex to draw simple conclusions. The HOMO and LUMO of the three pentadienyl cations with a formyl substituent are shown in Figure 11.4. The stabilities of the transition states should be in the order of the Hiickel n energies. These are 6a — 9.204 / , 6a — 9.2031/ , and 6a -9.1291/ , respectively. Thus, by SHMO, the ortho and meta channels are favored over the para channel, with no distinction between the ortho and meta pathways. Experimentally, meta substitution products are usually the major ones, contrary to the SHMO predictions. Either the SHMO method fails in this case or the predominance of meta products may be attributed to steric effects. 

Oxidation of enolizable nitro, carbonyl and dicarbonyl compounds with Fem MnnI and Celv reagents in the presence of electron rich aromatic (or heteroaromatic) rings often provides modest to good yields of substituted products. Typical examples are shown in Scheme 81.233 234 The oxidant functions both to generate the initial radical (Scheme 71) and to trap the adduct radical. Products of ortho substitution usually predominate but significant amounts of para and meta products are often formed, and in some cases, reversibility in the addition step may influence the product distribution. A recent paper by Citterio and Santi provides a nice introduction to these types of reactions.219... 

Triflic acid effectively promotes the phenylamination of aromatics with phenylazide in a fast, convenient, high-yield process598 (Table 5.34). The high ortho/para selectivity with only a small amount of meta product and high substrate selectivity (kT/kB = 11) indicate the involvement of a substantially electron-deficient species, the phenylami-nodiazonium ion intermediate with the possible protosolvation by triflic acid. 

The nitration of 3-phenyl-1,2,4-oxadiazoles (45a-c) with mixed acid at -5 to 0°C gives a different orientation according to the substituent R. For the series 45a, meta products are obtained, for 45b both meta and para products are obtained, whereas for 45c only the 4-nitrophenyl product is obtained (63G1205). 

Methyl-4-phenylsydnone is nitrated using nitric acid acetic acid at RT to give the 4-nitrophenyl product (80%), with small amounts (<3%) of ortho and possibly the meta product. The use of mixed acid, or fuming nitric acid, at 0°C forms the 2,4-dinitrophenyl product (60-70%) [62CI(L) 1718]. 

Other workers confirmed the above results but did not isolate any of the 4-nitrophenyl product. The nitration of 5-bromo and 5-chloro-2-phenylpyrimidine was also shown to yield the ortho and meta products (in the ratio of about 3 7) with no para isomer being observed (80H1989). Further work has indicated that 5-nitro-2-phenylpyrimidine undergoes nitration in mixed acid at 0°C to give a similar mixture of ortho and meta products (1 7) with no para isomer, whereas 6-methyl-2-phenylpyrimidin-4-one (72) forms only a 3-nitro phenyl product (91UP1). 

The nitration of phenylazetidinones (91) with acetyl nitrate gives ortho and para nitrophenyl products with no meta products being observed. The ortho meta para ratio depends on the substituents. However, these results were not fully quantitative and may not be complete (85ACH191). 

The proportionality of S and p made possible the adoption of a slight modification of the approach involving the reversal of the conventional Hammett procedure. The Selectivity Factor, 8, rather than p was chosen as the reference parameter. This procedure is the application of the p approach with a single substituent, the methyl group, defining the reaction constant. This scheme is useful because 8f is an experimental quantity determined by the para/meta product ratio in toluene (10). 

Note that the order of each step in the sequence is important. The benzene must be nitrated first and then chlorinated to attain meta orientation. Chlorination of iodobenzene would not give meta product, so this indirect route must be used. 

As is usually the case, intramolecular reactions are easier than intermolecular and often do not obey the usual rules. Some do not need the carbonyl group, some show exo rather than endo selectivity, and the cyclisation of 56 gives the meta product 58. The mechanism 57 makes it clear that the expected para product (cf. 42) cannot be formed. This is a particularly impressive example as the product 58 is a bridgehead alkene with a strained geometry.10 The alkene is cis inside the six-membered ring but trans in the outer 10-membered ring. 

The reaction mixture was quenched by pouring it into ice-H20, and extracted with Et20. The extracts were pooled, dried with anhydrous MgSChi, and the volatiles removed under vacuum. The orange solid residue was recrystallized from EtOH to provide 20 g (58%) of a mixture of the ortho- and meta-products, 2,3-methylenedioxy-5-nitrotolucnc and... 

Explain why these compounds react more slowly than benzene in electrophilic aromatic substitution reactions and give predominantly meta products ... 

The ethyl group, like other alkyl groups, is weakly activating and directs to the ortho and para positions. The small amount of meta product that is formed is usually not shown. 

The first examples illustrate that the order of addition of the substituents is important in controlling their orientation. For example, suppose we needed to prepare m-chloronitrobenzene from benzene. Because the chlorine is an ortho/para director and the nitro group is a meta director, it is apparent that the nitro group must be added first if the meta product is desired ... 

Butadienes with alkyl substituents in the 2-position favor the formation of the so-called para products (Figure 12.24, X = H) in their reactions with acceptor-substituted dienophiles. The so-called meta product is formed in smaller amounts. This orientation selectivity increases if the dienophile carries two geminal acceptors (Figure 12.24, X = CN). 2-Phenyl-l,3-butadiene exhibits a higher para selectivity... 

Electron-withdrawing substituents give meta products... 

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