On the Direct Iodination of Monoiodobenzoic Acids

A. N. Novikov

(Presented by Professor, Doctor of Chemical Sciences B. V. Tronov).

The present work is a continuation of earlier studies [1] on the reactions of direct iodination and bromination of aromatic substances in the presence of a nitrating mixture. As objects of direct iodination, m- and p-iodobenzoic acids were taken. As a result of a series of experiments, 3,4-diiodobenzoic acid was prepared from p-iodobenzoic acid in 48% yield (m.p. 255°). According to the literature, it is recommended that it be prepared by diazotization of 3-iodo-4-aminobenzoic acid or 4,5-diiodo-2-aminobenzoic acid, followed by replacement of the diazonium group by iodine in the first case and by hydrogen in the second.

Under analogous conditions, m-iodobenzoic acid was practically not iodinated (it was isolated unchanged). Apparently, the reaction rate is influenced by the coinciding orientation in p-iodobenzoic acid and the noncoinciding orientation in m-iodobenzoic acid. Under more severe conditions, m-iodobenzoic acid was iodinated, but with complications. From the reaction products a substance with m.p. 230° was isolated, apparently 3,5-diiodobenzoic acid, in 3–4% yield, and a mixture of substances with m.p. 170–178° (35%). According to the halogen content, this mixture should contain diiodobenzoic acids with a small content of m-iodobenzoic acid.

Experimental Part

Iodination of p-iodobenzoic acid

The optimal conditions are as follows.

In a 100 ml round-bottom flask, p-iodobenzoic acid (4.1 g), iodine (2.5 g), sulfuric acid of specific gravity 1.84 (10 ml), glacial acetic acid (55 ml), and carbon tetrachloride (2.0 ml) were mixed. The flask was placed in a sulfuric-acid bath heated by an electric hot plate connected to the mains through a thermoregulator. A reflux condenser, a dropping funnel, and a stirrer for mechanical stirring were attached to the flask through an adapter. At a temperature of 130–134°, nitric acid of specific gravity 1.4, in an amount of 2.6 ml, was gradually added to the mixture over 25 min with vigorous mechanical stirring. Heating was then continued for another 35 min. The reaction mixture was cooled, the precipitated solid was filtered off, washed with water, and recrystallized from ethyl alcohol. This gave 2.9 g of substance with m.p. 255° (yield 46.8%). The qualitative test for nitrogen was negative. Halogen content by Stepanov: 67.1%; calculated amount for diiodobenzoic acid: 67.9%. A 2-hour synthesis did not increase the yield of the reaction product; a half-hour synthesis decreased it.

Iodination of m-iodobenzoic acid

Iodination of m-iodobenzoic acid under conditions analogous to the iodination of p-iodobenzoic acid did not lead to the expected results. Unchanged m-iodobenzoic acid was isolated from the reaction mixture. Increasing the amount of nitric acid by 2 ml and the synthesis time to three hours changed almost nothing.

Then the amount of solvent (acetic acid) was reduced

by a factor of two. Instead of 55 ml, 25 ml of acid was taken. Under these conditions, as has already been stated above, according to the data of the quantitative determination of halogen by Stepanov’s method, the acid was iodinated, but it was not possible to isolate pure individual reaction products. Apparently, owing to the noncoinciding orientation, a mixture of isomeric diiodobenzoic acids is formed, which is difficult to separate by ordinary crystallization.

Conclusions

1. By direct iodination in the presence of a nitrating mixture, 3,4-diiodobenzoic acid was obtained from p-iodobenzoic acid.

2. m-Iodobenzoic acid, under analogous conditions, is practically not iodinated.

References

1. Tronov B. V., Novikov A. N. ZhOKh, 23, 1022, 1953.
   Novikov A. N. ZhOKh, 24, 655, 1954.
   Novikov A. N., Izv. Tomskogo politekhn. in-ta, 77, 188, 1953.

Corrections and Noted Misprints

Izv. TPI, vol. 102.