On the Ortho/Para Isomer Ratio in the Electrophilic Iodination of Diphenyl

A. N. Moskalychuk, A. N. Novikov

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

In recent years several new methods have been proposed for the direct iodination of aromatic compounds [1—6]. However, there is no comparative characterization of them, which hinders the correct choice of the method and conditions for iodination.

In all methods, from iodine and additives introduced into the reaction or electrochemically, an electrophilic iodinating agent is formed, which attacks the substrate, as a result of which organic iodides are produced. A serious task in electrophilic substitution is to determine the influence of conditions on the ratio of the isomers obtained. In this respect, the iodination reaction has also not been studied.

From the concepts of electrophilic aromatic substitution it follows that the selectivity of reagents, their activity, can be comparatively evaluated by quantitative determination of the isomers. The weaker the electrophile, the more selective it is toward particular positions of the substrate molecule. With strong electrophiles, the substituting agent should give an isomer ratio close to the statistical one.

In the present work the relative activity and selectivity of iodinating electrophiles arising in acetic-acid solutions of iodine containing sulfuric–nitric (nitrating) mixture, silver sulfate, peracetic acid, peracetic and sulfuric acids, diacetates of iodobenzene and iodomesitylene toward diphenyl have been determined, and the effect of diphenyl iodination conditions in the presence of a nitrating mixture on the ratio \((\alpha)\) of the ortho/para isomers formed has been elucidated.

The results of the work show that a weak iodinating agent is found in the iodine–iodic acid mixture \((\alpha = 0.08)\), Table 3, while a stronger one is found in the system with sulfuric–nitric mixture and silver sulfate \((\alpha = 0.13\) and \(0.13)\). In selectivity, the electrophiles arising from iodine in the presence of peracetic acid and diacetates of iodine compounds occupy an intermediate position \((\alpha = 0.10)\).

The activities obtained correspond to the experimental results of studies of the iodination reaction in the presence of iodic and peracetic acids [3, 4]. Mixtures of iodine, acetic and iodic acids and of iodine, acetic and peracetic acids do not iodinate benzoic acid or nitrobenzene, whereas the latter are iodinated in the presence of a nitrating mixture and silver salts. Consequently, systems with iodic and peracetic acids do indeed contain less active iodinating agents and therefore exhibit higher selectivity \((\alpha = 0.08\) and \(0.10)\). It may be assumed that in the presence of diacetates

iodo compounds, benzoic acid and nitrobenzene will not be iodinated ($\alpha = 0.10$).

The reasons for the slight change in ($\alpha$) in Table 1, depending on the iodination conditions in the presence of a nitrating mixture, are the rearrangement of one isomer into another by the Jacobsen reaction [7], and not the action of different iodinating agents.

Table 1

Isomer content in the products of iodination of diphenyl
in the presence of a nitrating mixture

Diphenyl is not iodinated when acetic acid is replaced by nitromethane, dimethyl sulfoxide, benzene, carbon tetrachloride, nitrobenzene, or acetic anhydride. If it is assumed that the active electrophile is the product of addition of the iodine cation to acetic acid (CH$_3$COOHI$^+$) [4], it is possible to understand why the iodinating agents formed in acetic-acid solutions of iodine, with various additives, have similar selectivity. In all cases the same electrophile acts. Some difference in activity is the result of the influence of anions of different nature on the electrophile. Thus, any of the iodination methods considered will give a similar isomeric distribution of the products. Since sulfuric and nitric acids are the most readily available additives, the method of iodination in the presence of a nitrating mixture is a convenient and simple method for obtaining aromatic iodides.

Experimental part

Iodination of diphenyl in the presence of a sulfuric–nitric acid mixture was carried out according to the procedure [8]. The quantitative composition of the products was determined from the IR spectra. The absorption bands used were: for diphenyl—737 cm$^{-1}$; for $p$-iododiphenyl—830 cm$^{-1}$; for $o$-iododiphenyl—754 cm$^{-1}$; the $m$-isomer was not determined, since its content was less than 1%. The spectra were recorded in benzene solution. The accuracy of isomer determination was 1.5%. The $o$-iododiphenyl necessary for construction of the calibration curve was prepared from $o$-iodoaniline by analogy with [9].

In all experiments, 7.7 g of diphenyl, 6.15 g of iodine, 15 ml of glacial acetic acid, 2.85 ml of sulfuric acid $d = 1.83$—1.84, and 0.6 ml of nitric acid $d = 1.4$ were taken. The reaction time was 1 h. The results of iodination are presented in Table 1.

Since within the range of the indicated temperatures and reaction times (Table 1) the selectivity of the electrophile changes only slightly, iodination of diphenyl in the presence of other additives was carried out according to (1,3–5). The conditions and results are presented in Tables 2 and 3.

Table 2

Conditions for the iodination of diphenyl in the presence of various substances

Note: a) 1.5 ml of sulfuric acid, 7.5 ml of water, and 4 ml of carbon tetrachloride were added; b) 12 ml of a 12% solution in acetic acid; c) 30.5 ml of a 12% solution in acetic acid, 1.3 ml of sulfuric acid, 5.0 g of iodine.

Table 3

Isomer content in the products of diphenyl iodination
in the presence of various substances

Conclusions

1. A comparative evaluation has been carried out of methods for the iodination of aromatic compounds. From the ortho/para ratio of the isomers \((\alpha)\) formed from diphenyl, the activity of the iodinating agents was determined.

2. Relatively strong agents are found in acetic-acid solutions of iodine containing a nitrating mixture or silver sulfate \((\alpha = 0{,}13)\); a weak one is found in solution with iodic acid \((\alpha = 0{,}08)\).

3. It has been shown that iodination in the presence of a nitrating mixture is the simplest method for the synthesis of many aryl iodides.

LITERATURE

1. D. H. Derbyshire, W. A. Waters. J. Chem. Soc., 3694, (1950).

2. B. V. Tronov, A. N. Novikov. Izv. vuzov, “Khimiya i khim. tekhnologiya,” 3, 872, (1960); A. N. Novikov, V. T. Slyusarchuk. Zhurnal organicheskoi khimii, 3, 1323, (1967); P. I. Sinyanko, A. N. Novikov. Zhurnal Vsesoyuznogo khimicheskogo obshchestva im. D. I. Mendeleeva, 14, 587, (1969).

3. H. O. Wirth, O. Königstein, W. Kern. Leib. Ann., 634, (1960).

4. G. Ogata, K. Nakajima. Tetrahedron, 20, 43, (1964).

5. G. Ogata, K. Aoki. J. Amer. Chem. Soc., 90, 6187, (1968).

6. L. L. Miller. Tetrahedron Lett., 1831, (1968).

7. Organic Reactions. Collection 1, Moscow, IL, 1948.

8. A. N. Novikov. Zhurnal obshchei khimii, 29658, (1959).

9. Organic Reactions. Collection 2, Moscow, IL, 1950.