Benzene iodination

Insoluble in water but soluble in nonpolar solvents such as CCl4 or benzene. Iodine is typical of most molecular substances it is only slightly soluble in water (0.0013 mol/L at 25°C), much more soluble in benzene (0.48 mol/L). A few molecular substances, including ethyl alcohol, are very soluble in water. As you will see later in this section, such substances have intermolecular forces similar to those in water. 

Iodine pentafluoride Benzene Iodine heptafluoride Organic solvents... 

The iT-values show clearly that the mesitylene-iodine complex is more stable than the benzene-iodine complex. The ratio of the T-values of the two complexes corresponds roughly to the ratio of the. K -values obtained from an evaluation of vapour pressure measurements (Table 9). It must, however, be borne in mind that these measurements were carried out at very different temperatures, so that this comparison is only a qualitative one. 

The last condition, the orientation principle, is illustrated for the benzene-iodine and ethylene-platinum complexes. It is seen that the orientation depicted in Fig. 2a leads to a positive overlap integral and... 

Fig. 2. Orientation principle in the benzene-iodine charge-transfer complex (a) S and P Oi (b) jS and = 0.

I, I-B1s(trifluoroacetoxy)iodo]benzene Iodine, phenylbis(trifluoroacetato-O-)- (9) (2712-78-9)... 

A. H. Zewail The observation of coherent motion in the benzene-iodine system should be related to the l2 motion and hopefully with better time resolution we should be able to resolve it. As for the base-pair experiment, the key motion is that of the N N stretch and N—H asymmetric motions, and our time scale of observation was appropriate for the dynamics to be observed. 

Since the Braggs first determination, thousands of structures, most of them far more complicated than that of sodium chloride, have been determined by x-ray diffraction. For covalently bonded low molecular weight species (such as benzene, iodine, or stannic chloride), it is often of interest to see just how the discrete molecules are packed together in the crystalline state, but the crystal structures affect the chemistry of such substances only to a minor degree. However, for most predominantly ionic compounds, for metals, and for a large number of substances in which atoms are covalently bound into chains, sheets, or three dimensional networks, their chemistry is very largely determined by the structure of the solid. 

Direct excitation of electron-transfer states may yield surprising results. This is the case with the bimolecular benzene iodine charge-transfer complex. In solutions this system is the prototypical case of charge transfer as reported by Mulliken [262]. The characteristic 280 nm absorption band of the benzene-iodine system is distinct from any absorption features of neat iodine or benzene. It has been identified as being due to a promotion of the HOMO benzene n electron to a a LUMO orbital on iodine resulting in benzene iodine electron transfer. 

Hence, in a benzene-iodine cluster, excitation at 266 nm leads to the charge-transfer potential energy surface. Using femtosecond excitation one can detect the decay of the initially populated state, the appearance of the products and their kinetic energy distribution and alignment (with respect to the pump laser) ]55, 56,... 

Scheme 5. (Diacetoxyiodo)benzene-iodine-mediated formation of pyran and furan derivatives...

The yield of labeled benzene can be enhanced by the presence of the benzene-iodine complex during irradiation. Pozdeev et al. (1962a) ascribe this effect to stabilization of a highly excited intermediate which results in increased probability of hydrogen loss from the intermediate. Phase-effect studies on cyclohexane, cyclohexene, cyclohexadiene and benzene by Pozdeev et al. (1962b) lend further support to the excited-... 

A mild and effective procedure for the iodination of electron-deficient heterocyclic systems using the [bis(trifiuoroacetoxy)iodo]benzene-iodine system has been reported [108]. The usefiilness of this procedure can be illustrated by the preparation of 3-iodoindole derivatives 92 (Scheme 3.36), which are difficult to obtain by other methods due to their chemical instability. Sensitive protecting groups such as acetyl, Boc and terf-butyldimethylsilyl are stable imder these iodination reaction conditions [108]. 

Several useful synthetic methodologies are based on the generation of the oxygen-centered radicals from carboxylic acids and the (diacetoxyiodo)benzene-iodine system [613-617]. In particular, a direct conversion of 2-substituted benzoic acids 566 into lactones 567 via oxidative cyclization induced by [bis(acyloxy)iodo]arene/iodine has been reported (Scheme 3.224) [613,614]. 

The application of these techrriques is corrsiderably erthanced by the irttroduction of sarrrple-seeded supersonic jets. Gas-phase spectra are obtained at terrtperatures close to the absolrrte zero arrd the problem of Boltzmaim congestion is effectively overcome. Besides making the artalysis of previously hopelessly congested spectra tractable it has revealed a new farrrily of weakly-botmd van der Waals dimers or clusters. Some of the analyses are limited to general conclusions, as e.g. the distinction between end-on and sideways-on orientation of diatomic iodine in a benzene-iodine complex. Such data are not included in the present compilation. Other analyses, however, yield acciuate intemuclear distances as in the benzene-rate gas complexes. 

The effect of the former can be seen well, for instance, from a comparison of the reactions between iodine and tetraalkyl lead in benzene and in carbon tetrachloride [Pi 68]. In spite of the facts that both solvents are apolar and have small relative permittivities, and neither of them solvates the tetraalkyl lead molecule, the reaction takes place at a rate 15-20 times higher in benzene than in carbon tetrachloride solution. The explanation of the phenomenon is that with benzene iodine forms a charge-transfer complex of relatively high stability this causes a greater polarization of the iodine molecule than does the very weak interaction between iodine and carbon tetrachloride. 

Type of reaction C-I and C-Nu bond fomaation Reaction conditions Acetonitrile, room temperature Synthetic strategy Co-iodination of alkenes Catalysts (Diacetoxyiodo)benzene/iodine... 

Figure 7.14 Formation and dissociation of the excited benzene-iodine charge-transfer complex. Top Transient of free iodine atoms following the excitation of the Bz-l2 complex to its charge-transfer state. (The full line is a single exponential convoluted with the response time of the detector system.) Bottom The four panels (a), (b), (c) and (d) illustrates a possible series of structural changes see text...

Soon after thiophene was discovered by Victor Meyer, reports from his group gave the first examples of halogenated thiophenes - dibromothiophene (now known to be 2,5-dibromothiophene) and monoiodothiophene [77]. Monoiodothiophene (now known to be 2-iodothiophene) was synthesised from a mixture with benzene isolated from coal tar containing 50-60% of thiophene ( raw thiophene ). Here the higher reactivity of thiophene in comparison to benzene in halogenations was observed and used. Thus for benzene, iodination requires elevated temperatures, but thiophene was iodinated at room temperature. It was found that 2-iodothiophene can be synthesised either by the treatment with a mixture of iodine and iodic acid or... 

Weak donor-acceptor complexes, such as benzene/iodine (Bz/I ), are... 

Benzaldehyde, finely cut Zn-foil, and abs. ether added to a dry mixture of a-bromo-a,a-diethylacetamide and benzene, iodine crystals added at 20°, and refluxed 2 hrs. a, -diethyl- -hydroxy-j6-phenylpropionamide. Y 77%. F. e. s. A. V. Sivertseva, Tr. Leningr. Khim.-Farmatsevt. Inst. 1962(14), 7 G. A. 60, 15769h. 

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