Nitration bromobenzene

The rates of the reactions fall into two pairs and follow a U-shaped sequence fluorobenzene nitrates most quickly, followed closely by iodobenzene chloro-, and bromobenzene nitrate at around half these rates. Chlorine and bromine suffer because both are quite electronegative and neither has good lone pair overlap in fluorine, overlap is good in iodine, electronegativity is much less. 

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

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. 

Nitration using this reagent was first investigated, by Francis. He showed that benzene and some of its homologues bromobenzene, benzonitrile, benzoyl chloride, benzaldehyde and some related compounds, and phenol were mono-nitrated in solutions of benzoyl nitrate in carbon tetrachloride anilines would not react cleanly and a series of naphthols yielded dinitro compounds. Further work on the orientation of substitution associated this reagent with higher proportions of o-substitution than that brought about by nitric acid this point is discussed below ( 5.3.4). 

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. 

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. 

Answer Nitration in the ortho position can be accomplished by using acetyl nitrate. Thus reacting bromobenzene via procedure V-7 will yield C. 

Recently Melander [88] attempted to clarify the influence of the proton loss on the nitration reaction rate by investigating this process using a number of aromatic compounds benzene, toluene, bromobenzene and naphthalene, in which one hydrogen atom had been replaced by tritium. 

Since toluene, when nitrated, yields the ortho- and met a- isomers in the quantities shown in diagram IV, and bromobenzene in those shown in diagram V, for p- bromotoluene the quantities can be calculated. They are shown in diagram VI. 

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. 

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. 

Even though the reaction conditions may lead to formation of the metal enolate in high yield, further reduction may occur during the quenching step of the reaction. Alcohols such as methanol and ethanol convert metal enolates to saturated ketones much faster than they react with metals in ammonia, and quenching of reduction mixtures with these alcohols will usually lead to partial or complete conversion to alcoholic product rather than to the saturated ketone. Rapid addition of excess solid ammonium chloride is the commonly employed quench procedure if ketonic products are desired,but other reagents that destroy solvated electrons before neutralization may be employed, such as sodium ben-zoate, iron(III) nitrate, - sodium nitrite, bromobenzene, sodium bromate, 1,2-dibromoethane and acetone. 

Tin triethyl phenyl is obtained as an oil, B.pt. 254 C., from the reaction mixture derived from heating together tin triethyl iodide (1 moL), bromobenzene (1 mol.), and metallic sodium. It forms a colourless liquid, which has to be distilled in hydrogen, since it is partially oxidised in the air, has a fairly pleasant odour, burns with a sooty flame, depositing metallic tin. It is highly refractive, and has a density 1-2639 at 0 C., is easily soluble in alcohol and ether, but insoluble in water. With alcoholic silver nitrate, iodine, fuming hydrochloric acid, and stannic chloride, the following reactions take place —... 

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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