Nitrobenzene meta substitution

In nitration with nitronium salts in sulpholan, nitrobenzene was substituted in the following proportions 8% ortho, 90% meta and 2% paraf under the same conditions benzylidyne trifluoride yielded 8%, 88% and 4% of 0-, m- and p-nitro compound respectively Both of these aromatic compounds were stated to be io -10 times less reactive than benzene. "... 

In DMSO as solvent and in the presence of nitrobenzene, aryl-de-diazoniation of the unsubstituted benzenediazonium ion leads mainly via meta substitution to 3-nitrobiphenyl, whereas in the case of the 4-nitrobenzenediazonium ion the formation of o- and -substituted products (2,4 -and 4,4 -dinitrobiphenyl) prevails (Gloor et al., 1972). 

Chatteijee, A., Iwasaki, T., Ebina, T., and Mizukami, F. 2003. Intermolecular reactivity study to scale adsorption property of para- and meta-substituted nitrobenzene over 2 1 dioctahedral smectite. J. Chem. Phys. 118 10212-10220. 

Probtom 11.12 (a) Draw enthalpy-reaction diagrams for the first step of electrophilic attack on benzene, toluene (meia and para) and nitrobenzene (meta and para). Assume all ground states have the same energy, (b) Where would the para and meta substitution curves for C HjCI lie on this diagram ... 

A somewhat related report in which equally impressive selectivity and yield (60—90%) were obtained Cacou and Wolf have studied the low temperature direct fluorination of aromatic compounds in CC13 F (100). They have established that under appropriate conditions molecular fluorination proceeds on substituted benzenes and tolulenes with similar selectivity and orientation as has been commonly observed for other halogens. For example reaction with nitrobenzene gives 80% meta substitution and reaction with tolulene gives 60% ortho substitution. The conversions were reported in this study and found to average 0.01%. 

The decomposition is first order in nitrobenzene and tetrahydronaphthalene and the Arrhenius parameters obtained with various solvents for phenyl azide and some derivatives are collected in Table 16. As seen from the data, meta substitution has no efiect on the rate, and it was concluded that the rate-determining step is N2 evolution and not ring closure to form azepines (in aniline solution). In indene, Smith observed that para substituents increased the rate of Nj evolution by about eight times however, with both meta and para substituents the amine yields varied widely and in some cases the Nj yields were not quantitative with respect to the amount of azide decomposed. Benzenes and substituted benzenes have been observed in small amounts and it appears that a minor decomposition pathway would involve azide radical loss . On the other hand, Waters reports that the thermolysis of phenyl azides proceeds by different mechanisms, depending on the medium ... 

In nitrobenzene, ortho,para substitution is thus slower than meta substitution because electron withdrawal by —NO2 is more effective during attack at the positions ortho and para to it. 

Shinde et al. (2008) have prepared silica gel-supported sodium hydrogen sulfate and demonstrated its application in the synthesis of coumarins. The reactions were conducted in acetonitrile and the reaction time in most of the cases was 1 h, which included the weakly reactive naphthols. Application of nanocrystalline sulfated zirconia in the reaction of meta-substituted phenols has also been investigated (Tyagi et al. 2007) under different conditions. In view of the slow reaction rates observed in nitrobenzene and toluene, the solvent-free conditions were adopted under MW irradiation to obtain high yields (95%-99%), of 7-amino- and 7-hydroxy-4-methylcoumarins in the reaction of m-amino and m-hydroxy phenols with ethyl acetoacetate. A low percentage of catalyst is required and its reusability has been claimed as a novel feature of this synthesis. 

This pattern is also followed for other electrophilic aromatic substitutions— chlorination, bromination, sulfonation, and so on. Toluene undergoes mainly ortho,para substitution, whereas nitrobenzene undergoes meta substitution. 

So there really is a positive charge adjacent to the ring in nitrohenzene. Therefore the same factors that lead to meta substitution and a slow rate in the trimethylanilinium ion will operate for nitrobenzene as well (Fig. 14.75). 

There are some anomalous cases in which a specific atom shows both high electrophilicity and nucleophilicity due to the limitation of various basis set-dependent charge calculation procedures, and hence it is more appropriate to rationahze this concept of relative electrophilicity/nucleophUicity. Relative nucleophilicity is the nucleophilicity of a site relative to its own electrophilicity, and vice versa for relative electrophilicity. The idea of relative nucleophilicity/electrophilic-ity was first proposed by Roy et al. [81] to predict intramolecular reactivity sequences of carbonyl compounds. We have used a similar ratio for the first time to find the best di-octahedral smectite for nitrogen heterocyclics adsorption in terms of intermolecular interaction [82] and as well for the adsorption property of para and meta substituted nitrobenzene [83]. 

A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

Electrophilic aromatic substitution of nitrobenzene occurs selectively at the meta position. 

---