Iodination, of benzene

Chlorination is cariied out in a manner similar- to bromination and provides a ready route to chlorobenzene and related ar-yl chlorides. Fluorination and iodination of benzene and other arenes are rarely perfor-med. Fluorine is so reactive that its reaction with benzene is difficult to control. Iodination is very slow and has an unfavorable equilibrium constant. Syntheses of aryl fluorides and aryl iodides are nor-mally cariied out by way of functional group transformations of arylffluines these reactions will be described in Chapter 22. 

Iodine acetate also has been proposed331 as the electrophile in the peroxyacetic catalysed-iodination of benzene in acetic acid at 50 °C, which obeys the kinetic equation... 

Kinetic smdies of the iodination of benzene and acetanilide by iodine, diiodine pentoxide, and sulfuric acid in acetic acid indicate that benzene is involved in an equilibrium reaction prior to the rate-limiting <7-complex formation." It is proposed that this equilibrium involves the formation of a itt-complex between iodine adsorbed on diiodine pentoxide and the benzene as it is adsorbed. In the case of acetanilide the a-complex is formed directly with activated iodine adsorbed on the diiodine pentoxide. 

Step 2 of the iodination of benzene shows water acting as a base and removing a proton from the sigma complex. We did not consider the possibility of water acting as a nucleophile and attacking the carbocation, as in an electrophilic addition to an alkene. Draw the reaction that would occur if water reacted as a nucleophile and added to the carbocation. Explain why this type of addition is rarely observed. 

The preparation of iodobenzene by iodination of benzene, with iodine and nitric add, and a survey of preparative methods have been given in an earlier volume. The present procedure, based upon the method of Gattermann, gives a purer product. 

Once the electrophile is formed, iodination of benzene occurs by the same mechanism as bromination and chlorination. 

The halogenating species in the conversion of benzene to iodobenzene with iodine and peroxyacetic acid, shown in Section 12.5, is thought to be acetyl hypoiodite. Write a mechanism for the electrophilic iodination of benzene with this species. 

Direct iodination of benzene is possible only in the presence of strong oxidizing agents like HNO3 and HIO3, which destroy HI and prevent the backwanl reaction. 

How would you obtain (a) chlorobenzene (b) bromobenzene starting from benzene. Why direct iodination of benzene is not possible ... 

The most widely used reactions are those of electrophilic substitution, and under controlled conditions a maximum of three substituting groups, e.g. -NO2 (in the 1,3,5 positions) can be introduced by a nitric acid/sul-phuric acid mixture. Hot cone, sulphuric acid gives sulphonalion whilst halogens and a Lewis acid catalyst allow, e.g., chlorination or brom-ination. Other methods are required for introducing fluorine and iodine atoms. Benzene undergoes the Friedel-Crafts reaction. ... 

To determine which halogen is present, take 1-2 ml. of the filtrate from the sodium fusion, and add dilute sulphuric acid until just acid to litmus. Add about 1 ml. of benzene and then about 1 ml. of chlorine water and shake. A yellowish-brown colour in the benzene indicates bromine, and a violet colour iodine. If neither colour appears, the halogen is chlorine. The result may be confirmed by testing the solubility of the silver halide (free from cyanide) in dilute ammonia solution silver chloride is readily soluble, whereas the bromide dissolves with difficulty, and the iodide not at all. 

The heats of formation of Tt-complexes are small thus, — A//2soc for complexes of benzene and mesitylene with iodine in carbon tetrachloride are 5-5 and i2-o kj mol , respectively. Although substituent effects which increase the rates of electrophilic substitutions also increase the stabilities of the 7r-complexes, these effects are very much weaker in the latter circumstances than in the former the heats of formation just quoted should be compared with the relative rates of chlorination and bromination of benzene and mesitylene (i 3 o6 x 10 and i a-Sq x 10 , respectively, in acetic acid at 25 °C). 

The charge-tranter concept of Mulliken was introduced to account for a type of molecular complex formation in which a new electronic absorption band, attributable to neither of the isolated interactants, is observed. The iodine (solute)— benzene (solvent) system studied by Benesi and Hildebrand shows such behavior. Let D represent an interactant capable of functioning as an electron donor and A an interactant that can serve as an electron acceptor. The ground state of the 1 1 complex of D and A is described by the wave function i [Pg.394]

When distilled with phosphorus pentoxide, camphor yields cymene, and with iodine, carvacrol. Both of these bodies are para-derivatives of benzene. On oxidation with nitric acid camphor yields many acids, of which the chief are camphoric acid, CjgHjgO, camphanic acid, CjoHj O, and camphoronic acid, CgHj Og. The constitution of these acids has an important bearing on that of camphor. Many formulae have been suggested for camphor during the past few years, but that of Bredt is now universally accepted, and has received complete confirmation by Komppa s synthesis of camphoric acid. This synthesis confirms the formula for camphoric acid as—... 

The reaction mixture is cooled and the crude amines which separate are collected on a suction funnel and washed twice with 400-cc. portions of water. The filtrate and washings should be saved for the recovery of iodine (Note 8). The precipitate on the funnel is transferred to a 2-1. beaker, dissolved in about 11. of benzene, filtered, and the benzene-insoluble part washed three times with 75-cc. portions of benzene. The benzene solution and washings are combined and separated mechanically from as much water as is possible. The water is then completely removed by distilling until the distillate comes over clear. If necessary, dry benzene is added to the solution in order to have a final volume of about 1200 cc. 

The heat of solution of iodine in benzene is +4.2 kcal/mole (heat is absorbed). Assuming the increase in randomness is the same when iodine dissolves in liquid benzene as it is in ethyl alcohol and in CC14, justify the prediction that the solubility of I2 in benzene is higher than in CC14 but lower than in alcohol. 

In common with a number of heterocyclic iodinations, kinetic effects are found in the iodination of indole and 2-methylindole [68AC(R)1435], When the substituent effects for the reaction are examined it is clear that any resonance effects from the fused benzene ring are only poorly relayed to the reactive 3-position, and the rates appear to be controlled by inductive effects. A 5-methyl group was more activating than 5-methoxy [69AC(R)799]. 

Iodine acetate would seem to be unambiguously present in the iodination of pentamethylbenzene in acetic acid by iodine and mercuric acetate, since the latter components form an equilibrium mixture of iodine acetate and acetoxy-mercuric iodide and mercuric acetate speeds up the iodination332. Second-order rate coefficients of 0.078 (25 °C) and 0.299 (45 °C) were obtained, and these values are intermediate between those obtained for the reaction of bromine acetate with benzene (2.5 xlO-3) and toluene (1.2) at 25 °C, indicating that bromine acetate is the stronger electrophile. 

Iodination of moderately reactive aromatics can be effected by mixtures of iodine and silver or mercuric salts.31 Hypoiodites are presumably the active iodinating species. Bis-(pyridi nc)iodonium salts can iodinate benzene and activated derivatives in the presence of strong acids such as HBF4 or CF3S03H.32... 

The first report of an adduct of the type to be discussed here was that of Guthrie in 1863 [1], who described the compound HsN- T2. The spectroscopy of the interaction of benzene with molecular iodine in the UV/visible... 

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