Nitro-aromatic compounds substituents

Figure 9.19 shows the Hammett s equation for degradation rates of substituted nitrobenzenes from experimental data. Photocatalytic degradation of the nitro aromatic compounds by UV/Ti02 from the experimental data reported by Dillert et al. (1995) are used to construct the model. The degradation of nitrobenzenes can be described according to the number of nitro substituents nitrobenzene > dinitrobenzene > 1,3,5-trinitrobenzene (Dillert et al., 1995). 

The most common types of aryl halides m nucleophilic aromatic substitutions are those that bear o ox p nitro substituents Among other classes of reactive aryl halides a few merit special consideration One class includes highly fluormated aromatic compounds such as hexafluorobenzene which undergoes substitution of one of its fluorines on reac tion with nucleophiles such as sodium methoxide... 

PART 5 AROMATIC COMPOUNDS WITH NITRO SUBSTITUENTS... 

A wide range of mechanisms are involved in the degradation and transformation of aromatic compounds with nitro substituents. These include reduction of the nitro group, and dioxygenation, monooxygenation, and reduction of the aromatic ring. A review devoted to 2,4,6-trinitrotolune is available (Esteve-Nunez et al. 2001). 

A wide variety of aromatic compounds can be brominated. Highly reactive ones, such as anilines and phenols, may undergo bromination at all activated positions. More selective reagents such as pyridinium bromide perbromide or tetraalkylammonium tribromides can be used in such cases.18 Moderately reactive compounds such as anilides, haloaromatics, and hydrocarbons can be readily brominated and the usual directing effects control the regiochemistry. Use of Lewis acid catalysts permits bromination of rings with deactivating substituents, such as nitro and cyano. 

With the nitro group successfully introduced, the aromatic fluoride substituent in 11 was ready to undergo the nucleophilic aromatic substitution with the hydrox-ypyridine 9. The reaction proceeded smoothly in DMF at 55 °C using an equimolar amount of cesium carbonate as the base and provided a 90% isolated yield of 23 after crystallization. With compound 23 in hand, only the reduction of the nitro... 

Nucleophilic substitution is the widely accepted reaction route for the photosubstitution of aromatic nitro compounds. There are three possible mechanisms11,12, namely (i) direct displacement (S/v2Ar ) (equation 9), (ii) electron transfer from the nucleophile to the excited aromatic substrate (SR wlAr ) (equation 10) and (iii) electron transfer from the excited aromatic compound to an appropriate electron acceptor, followed by attack of the nucleophile on the resultant aromatic radical cation (SRi w 1 Ar ) (equation 11). Substituent effects are important criteria for probing the reaction mechanisms. While the SR wlAr mechanism, which requires no substituent activation, is insensitive to substituent effects, both the S/v2Ar and the Sr+n lAr mechanisms show strong and opposite substituent effects. 

The isolated double bonds in the dihydro product are much less easily reduced than the conjugated ring, so the reduction stops at the dihydro stage. Alkyl and alkoxy aromatics, phenols, and benzoate anions are the most useful reactants for Birch reduction. In aromatic ketones and nitro compounds, the substituents are reduced in preference to the aromatic ring. Substituents also govern the position of protonation, Alkyl and alkoxy aromatics... 

The value of 0 in a particular radical can be estimated by comparison of the experimental half-wave potential and HFSC with the results obtained from a series of HMO calculations using different assumed values of 0. A large number of phenyl-substituted aromatic compounds [62] and ethylenes [63] have been treated in this fashion. Similar evidence for the twisting of the nitro group in nitroaromatic anion radicals is summarized in Reference 1. Restricted rotation of alkyl substituents is also discussed in Reference 1, but this torsion does not significantly affect the electrochemical behavior. 

Frank, Horman and Scheibe [34] also found that asymmetric N02 vibrations could be shifted towards lower frequencies under the influence of conjugation in aromatic compounds. They examined their substances in potassium bromide. A number of aromatic nitro compounds in dilute solutions were recently examined by T. Urbanski and Dabrowska [35], They found that when a nitro group was placed in the para position with respect to another group, its symmetric stretching modes were of a lower frequency than in the meta position. This was most likely due to the strong conjugation of substituents with the ring ... 

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