Nitration of Bromobenzene

In a fractional crystallization, it is commonly possible to induce crystallization of the more soluble component, in this case the ortho isomer 19, once the less soluble material has been mostly removed. However, the low melting point of 19 means that this isomer is difficult to crystallize in the presence of impurities. Consequently, isolating this isomer involves column chromatography (Sec. 6.3). 

Purpose To demonstrate nitration by electrophilic aromatic substitution and to test the directing effects of a bromo substituent. 

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Nitration of bromobenzene with mixed acid yields largely p-bromo nitrobenzene accompanied by a little of the o-isomeride ... 

However, if halogen atoms are deactivating the ring due to inductive effects, they should not direct substitution to the meta position like other electron-withdrawing groups. Consider the nitration of bromobenzene. There are three resonance structures for each of the three intermediates leading to these products, but the crucial ones to consider are those which position a positive charge next to the substituent. 

Draw all the resonance forms of the sigma complex for nitration of bromobenzene at the ortho, meta, and para positions. Point out why the intermediate for meta substitution is less stable than the other two. 

If, however, we reverse the order of the steps and first form bromobenzene, we now have an ortho-para directing group on the ring. Nitration of bromobenzene then... 

Would nitration of bromobenzene or nitration of 4-bromonitrobenzene be expected to have the higher enthalpy of activation Explain your answer. 

The bromination of phenyl n-pentyl ether is more para-selective in anionic micelles than it is in water. This contrasts with the lower para-selectivity of nitration of bromobenzene in the cationic micelles formed by dissolving lauric acid in 95% H2S04. It is not clear whether these effects are due to substrate orientation or to micelle-induced changes in the selectivity parameter for electrophilic aromatic substitution. The rates of solvolysis of alkyl p-trimethyl-ammoniumbenzenesulphonate trifluoromethanesulphonates (42) are strongly inhibited by anionic micelles of sodium lauryl sulphate or sodium dodecanoate. In water, homomicelles of (42) or sodium dodecanoate micelles, undergo inversion of stereochemistry, but in sodium lauryl sulphate 22% retention of... 

Would nitration of bromobenzene be a useful alternative way to begin a synthesis of 3-bromobenzenamine ... 

Solutions of dinitrogen pentoxide have been used in preparative nitrations.Benzene, bromobenzene, and toluene were nitrated rapidly in solutions of the pentoxide in carbon tetrachloride nitrobenzene could not be nitrated under similar conditions, but reacted violently with solid dinitrogen pentoxide. 

An interesting case is provided by the nitration of aryl halides where the effect of the halogen is to deactivate the aromatic nucleus (by the -I effect) but to direct the incoming nitronium ion to the ortho and para positions as a result of the mesomeric interaction of the halogen lone electron pair with the charge developed in the corresponding intermediates [e.g. (5), in the formation of p-bromonitrobenzene from bromobenzene, Expt 6.20],... 

Unfortunately, the several investigators who have examined the nitration of the halobenzenes did not adopt similar conditions. Ingold and his associates (1931) nitrated benzene and toluene by the addition of nitric acid to acetic anhydride and nitromethane. In acetic anhydride, nitric acid is slowly converted to acetyl nitrate (Bordwell and Garbisch, 1960). The relative rates, kT/kB, observed were 23 and 21 for reaction in acetic anhydride and nitromethane respectively. Bird and Ingold (1938) adopted different conditions. In a study of the halobenzenes, these workers added pre-formed acetyl nitrate to the halobenzene in acetic anhydride, acetonitrile, and nitromethane. The results for fluoro-, chloro-, and bromobenzene are summarized in Table 17 (p. 78). 

Bromine is an ortho, para-directing group. If it is introduced first, nitration of the resulting bromobenzene yields a mixture of o-bromonitrobenzene and p-bromonitrobenzene. 

Trifluoroacetyl nitrate is more reactive than acetyl nitrate, and it has been successfully used for the nitration of deactivated aromatics such as nitrobenzene and bromobenzene[31] using fuming nitric acid and trifluoroacetic anhydride in equimolar proportion at 45-55°C. The reaction between trifluoroacetic anhydride and nitric acid gives the nitrating agent trifluoroacetyl nitrate according to Scheme 5.3. 

In practical terms, it is usually possible to get high yields of para products from these reactions. Both nitration and sulfonation of bromobenzene give enough material to make the synthesis worthwhile. Though mixtures of products are always bad in a synthesis, electrophilic aromatic substitution is usually simple to carry out on a large enough scale to make separation of the major product a workable method. 

Very important from both theoretical and practical points of view were experiments on the influence of substituents on the rate and the yield of nitration. For that purpose the method of competitive nitration was used. It was originally developed by Wibaut [17] to study the rate of nitration with nitric acid-acetic anhydride of toluene, chlorobenzene and bromobenzene in relation to benzene. The experiments established a higher rate of nitration of toluene and a lower rate of chloro- and bromobenzene ... 

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