Orientation of nitration

Biphenyl is a compound which raises problems as regards the orientation of nitration produced by different reagents. This compound is discussed later ( 10.1). 

Phenylboronic acid. The orientation of nitration in phenylboronic acid is very susceptible to changes in the medium (table 5.8). The high proportion of o-substitution in acetic anhydride is not attributable to a specific o-reaction, for the nt -ratios of the last tabulated pair of results are not constant. The marked change in the ratio was considered to be due to the formation in acetic anhydride of a complex, as illustrated below, which is 0 -orienting and activated as a result of the -t-1 effect. This species need only be formed in a small concentration to overwhelm... 

Similar difficulties arise in the nitrations of 2-chloro-4-nitroaniline and /)-nitroaniline. Consideration of the rate profiles and orientation of nitration ( 8.2.5) these compounds suggests that nitration involves the free bases. However, the concentrations of the latter are so small as to imply that if they are involved reaction between the amines and the nitronium ion must occur upon encounter that being so, the observed activation energies appear to be too high. The activation energy for the simple nitration of the free base in the case of/>-nitroaniline was calculated from the following equation ... 

There have been many studies of the orientation of nitration in di- and poly-substituted derivatives of benzene, but in very few cases have... 

M.o. theory has had limited success in dealing with electrophilic substitution in the azoles. The performances of 7r-electron densities as indices of reactivity depends very markedly on the assumptions made in calculating them. - Localisation energies have been calculated for pyrazole and pyrazolium, and also an attempt has been made to take into account the electrostatic energy involved in bringing the electrophile up to the point of attack the model predicts correctly the orientation of nitration in pyrazolium. ... 

As with cases mentioned earlier, Hiickel m.o. theory performs satisfactorily in predicting the orientation of nitration in these oxides, but again fails to reproduce their strong deactivation. ... 

Nitration was carried out in the usual way. the orientation of nitration is controlled by the more powerfully activating methoxy groups rather than by the weakly activating fert-butyl. 

Quinoline-l-oxide undergoes nitration at the 2-, 5-, and 8-positions. While it is known that isomer ratios are temperature dependent, it has been recently shown that the orientation of nitration is also dependent on the acidity of the reaction medium <1997CPB279>. 2-Nitroquinoline 1-oxide, arising from nitration of unprotonated substrate, predominates under weakly acidic conditions. Nitration in stronger acidic media occurs on the hydroxyquinolinium ion and yields 5- and 8-nitroquinoline 1-oxide as the major products with increasing amounts of the 5-isomer formed in very highly acidic media. 

In this way it has been found that every group can be put into one of two classes ortho,para directors or meta directors. Table 11.1 summarizes the orientation of nitration in a number of substituted benzenes. Of the five positions open to attack, three (60%) are ortho and para to the substituent group, and two (40%) are meta to the group if there were no selectivity in the substitution reaction, we... 

In electrophilic aromatic substitution of a monosubstituted benzene, three products are possible the new group may become oriented ortho, meta, or para to the existing group. Table 22.1 shows the orientation of nitration of a series of monosubstituted benzenes. 

The second-order rate constant for the nitration of pyridine 1-oxide in 87 9 per cent sulphuric acid and the Arrhenius parameters for the reaction have been determined200. The failure of the 1-methoxypyridinium ion to undergo nitration under these conditions, and the fact that the 1-hydroxy-pyridinium ion should be even less reactive than the pyridinium ion, prove that the nitration of pyridine 1-oxide occurs through the free base. The observed orientation of nitration agrees with theory, but the theory fails in predicting that the oxide should be more reactive than benzene (p. 275). 

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