Iodobenzene, nitration

Under conditions in which benzene and its homologues were nitrated at the zeroth-order rate, the reactions of the halogenobenzenes ([aromatic] = c. o-1 mol 1 ) obeyed no simple kinetic law. The reactions of fluorobenzene and iodobenzene initially followed the same rates as that of benzene but, as the concentration of the aromatic was depleted by the progress of the reaction, the rate deviated to a dependence on the first power of the concentration of aromatic. The same situation was observed with chloro- andjbromo-benzene, but these compounds could not maintain a zeroth-order dependence as easily as the other halogenobenzenes, and the first-order character of the reaction was more marked. 

Partially or fully reduced thiadiazoles can be oxidized to yield 1,3,4-thiadiazoles. The 2,5-disubstituted 3-acyl-l,3,4-thiadiazole 157 can be deacylated by numerous methods <2004H(63)2243>. The oxidative deacylation of compound 157 to thiadiazole 158 can be achieved using oxidants such as KMn04, cerium(iv) ammonium nitrate (CAN), and (diacetoxy)iodobenzene (Equation 58). Better yields and cleaner products are obtained using CAN as oxidant. 

Roberts and his associates (1954) re-examined the nitration of the halobenzenes using fuming nitric acid as the reagent in both acetic anhydride and nitromethane. These workers detected a significant solvent effect on the relative rate. Iodobenzene was nitrated by nitric acid in acetic anhydride at a rate 0.13 times that of benzene, in nitromethane this reagent provided a rate ratio of 0.22. 

Note that the order of each step in the sequence is important. The benzene must be nitrated first and then chlorinated to attain meta orientation. Chlorination of iodobenzene would not give meta product, so this indirect route must be used. 

C o solubility increases in the similar sequence, from iodobenzene to chlorobenzene. The exception is fluorobenzene in which C6o solubility is lower than in iodobenzene. Apparently, in the case of interaction between fluorobenzene and C6o fullerene the factor of high fluorine electronegativity prevails. Moreover, as Table 3 indicates the fluorobenzene nitration gives rise to mainly para-isomer and very little ort/zo-isomers. Consequently, the entire negative charge is localized in the /jara-position in a fluorobenzene molecule. Therefore, as with Cgo solubility in alkyl derivatives of benzene (Table 2), one can anticipate that for the C6o molecule that is an electrophilic reagent, the ortho-position will be the more preferential location for electrophilic attack than the /w/ra-position. 

Nitration of halogenobenzenes yields 12% of the ortho product for fluorobenzene, and 41% for the iodobenzene. 

Hence, this synthesis can be performed when iodonitro-benzene is obtained from iodobenzene. This can be accomplished by a nitration reaction, where concentrated nitric and sulfuric acids are mixed to produce the nitronium ion C N02) which is the electrophilic reagent that attacks the ring. 

So far we have steered clear of the reactions of halogenated derivatives of benzene. Before we explain their reactions, have a look at the table, which shows the rates of nitration of fluoro, chloro, bromo, and iodobenzene relative to benzene itself, and also gives an indication of the products formed in each case. ----... 

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. 

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