Direct Iodination of Xylenes
Abstract Generated abstract
The study examines the direct iodination of the three xylene isomers using a sulfuric and nitric acid mixture as an inexpensive oxidizing medium. Building on earlier work with aromatic iodination, the authors describe a controlled procedure in glacial acetic acid with iodine, sulfuric acid, and gradual addition of nitric acid, followed by isolation and distillation of the products. The method afforded 2-iodo-p-xylene, 4-iodo-m-xylene, and 4-iodo-o-xylene in 75%, 64%, and 70% yields, respectively, with negative qualitative tests for nitrogen, indicating formation of iodoxylenes without nitrated by-products under the stated conditions.
Full Text
DIRECT IODINATION OF XYLENES
A. N. NOVIKOV, A. M. SEDOV
(Presented at the scientific seminar of the departments and laboratories of organic synthesis)
In previously published works [1, 2] it was shown that many iodine-substituted compounds of the aromatic series can be obtained in good yields by direct iodination in the presence of nitric \((d\ 1.4)\) and sulfuric \((d\ 1.84)\) acids. The advantages of this method over known methods of direct iodination in the presence of other oxidizing agents are the availability of the nitrating mixture and the absence of reaction by-products, if the reaction is carried out under strictly controlled conditions.
In order to determine the possibility of broader use of the method in organic synthesis, we iodinated isomeric xylenes in the presence of a sulfuric–nitric acid mixture. In this way iodoxylenes were isolated, which have been described in the literature. They had been obtained by diazotization of the corresponding xylidines followed by decomposition of the diazonium salts with potassium iodide, by the action on xylenes of iodic sulfur and nitric acid [3], and by direct iodination in the presence of concentrated or fuming nitric and sulfuric acids and persulfates [4, 5, 6]. The yield of iodoxylenes was 50–70%.
The nitrating mixture used by us is a cheaper reagent than the oxidizing agents used previously. The yield of iodoxylenes reached 75%.
Experimental Part
To a mixture of 5.3 g (0.05 g-mol) of the corresponding xylene, 6.35 g (0.05 g-at) of iodine, 5.5 ml of sulfuric acid \((d\ 1.83—1.84)\), and 20 ml of glacial acetic acid, heated to 50°C (bath temperature), 1.5 ml of nitric acid \((d\ 1.4)\) was added gradually over 30 minutes with vigorous stirring. Heating was then continued for another 15 minutes.
After completion of the reaction, the mixture was diluted with water; the separated oily, heavy liquid was separated from the aqueous-acid layer, washed free of acids, treated with sodium sulfite solution to remove iodine that had not entered into the reaction, and washed with water. After drying over calcium chloride, the iodoxylenes were distilled under ordinary conditions.
Obtained: 2-iodo-p-xylene, yield 75%, b.p. 230—231°C; 4-iodo-m-xylene, yield 64%, b.p. 230—232°C; 4-iodo-o-xylene, yield 70%, b.p. 235°C.
The qualitative test for nitrogen was negative for all iodoxylenes.
Conclusions
By direct iodination in the presence of sulfuric acid ($d$ 1.83–1.84) and nitric acid ($d$ 1.4), 2-iodo-p-xylene, 4-iodo-m-xylene, and 4-iodo-o-xylene were obtained from the corresponding xylenes.
References
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