THERMAL DECOMPOSITION OF o-IODODIPHENYL DICHLORIDE

A. N. NOVIKOV, A. N. MOSKALCHUK, E. S. NOVIKOVA

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

By the action of molecular chlorine on diphenyl and its monosubstituted derivatives, it is practically impossible to obtain monochloro derivatives. Di- and polychlorobiphenyls are formed.

In order to introduce one atom, it is necessary that chlorine enter the reaction mixture in an insufficient amount. For this purpose, dichlorides of aryl iodides can be used; the rate of decomposition of these compounds can be regulated over a wide range by changing the reaction temperature.

To determine the possibility of using dichlorides for the synthesis of monochloro-substituted diphenyls, we prepared o-iododiphenyl dichloride and then thermally decomposed it in ethyl alcohol in the presence of ethylene. Ethylene and ethyl alcohol should trap excess amounts of chlorine and thus promote monochlorination of diphenyl.

From the products of the thermal decomposition reaction of the dichloride, a substance with m.p. 110–111°C was isolated. The qualitative reaction for iodine and chlorine was positive. Molecular weight: found cryoscopically, 320; calculated for iodoclorodiphenyl, 314.5—$\mathrm{C_{12}H_8ClI}$. Found, %: C—46.1; H—2.4; I + Cl—50.7. Calculated: C—45.8; H—2.6; I + Cl—51.6.

In its IR spectrum, the characteristic frequencies for the benzene ring in diphenyl (695–710 cm$^{-1}$) are absent; there is a frequency for the C—Cl bond (685 cm$^{-1}$), the band of in-plane deformation vibrations for 1,2-substituted benzene, the frequencies of out-of-plane deformation vibrations for two adjacent hydrogen atoms (830 cm$^{-1}$), and for the bonds

\[ >C=C< \ (1471\ \mathrm{cm}^{-1}) \]

and $=C-H$ (1795 cm$^{-1}$) of the benzene ring.

On the basis of the foregoing, we believe that the thermal decomposition of o-iododiphenyl dichloride in ethanol in the presence of ethylene forms 2-iodo-4-chlorodiphenyl, not described in the literature.

Experimental Part

Iodination of diphenyl [1]. To a mixture of 38.5 g of diphenyl, glacial acetic acid, 31.8 g of iodine, and 27.5 ml of sulfuric acid \((d = 1.83)\), at a temperature of \(36^\circ\)C with stirring, 4.8 ml of nitric acid \((d = 1.4)\) was added over the course of 1 hour 15 minutes. After completion of the reaction, the mass was cooled, diluted with water, the precipitate was filtered off and washed, and the iodine that had not entered into the reaction was removed by steam distillation.

From the mixture of monoiododiphenyl isomers the \(n\)-isomer was isolated by crystallization from ethanol. After distillation of part of the diphenyl, an oily liquid separated, which then changed into an oily, low-melting substance with a melting point of \(53\text{–}55^\circ\)C. Yield 12%.

Since the isomeric iododiphenyls differ only slightly from one another, it was not possible to purify \(o\)-iododiphenyl by vacuum distillation on the basis of boiling points. Only by repeated recrystallization from petroleum ether was \(o\)-iododiphenyl obtained with a melting point of \(55^\circ\)C. Its IR spectrum contains frequencies characteristic of monosubstituted \((701\ \text{cm}^{-1})\) and 1,2-disubstituted \((762\ \text{cm}^{-1})\) benzene rings, as well as bands for the bonds

\[ \mathrm{>C{=}C<} \]

and

\[ \mathrm{=C{-}H}. \]

Preparation of the Dichloride

\(O\)-iododiphenyl [2]. 13.3 g of \(o\)-iododiphenyl was dissolved with heating in 50 ml of glacial acetic acid, and 13.1 g of sulfuryl chloride was added with stirring. After standing for one hour, the dichloride precipitate was filtered off and washed with acetic acid until a negative reaction for free chlorine was obtained. The yield of dichloride was almost quantitative.

Thermal Decomposition of \(o\)-Iododiphenyl Dichloride

A flask with a reflux condenser, containing 15 g of dichloride and 100 ml of ethanol, was placed in a water bath, and at a temperature of \(40^\circ\)C, 3.5 liters of ethylene was passed through the mixture for 2 hours. Then part of the ethyl alcohol was distilled off, and immediately after cooling a solid product was isolated, with a melting point of \(104\text{–}105^\circ\)C, in an amount of 7 g. (Yield 55%.) After crystallization from ethanol, the melting point increased to \(110\text{–}111^\circ\)C. Upon further crystallization the m.p. did not change.

Conclusions

1. 2-Iododiphenyl dichloride was obtained, which in alcoholic solution decomposes to 4-chloro-2¹-iododiphenyl.

2. The structure of 4-chloro-2¹-iododiphenyl was confirmed by elemental analysis, determination of molecular weight, and IR spectra.

References

1. A. N. Novikov. ZhOKh, 29, 58, 1959.

2. B. Karele, O. Neiland. ZhOKh, 98, 18, 1968.