METHOD FOR PRODUCING DIPHENIC ACID

V. P. Shabrov, A. G. Pechenkin, V. D. Volkova

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

Among the methods for producing diphenic acid by oxidation of phenanthrene \[1, 2\], a special place is occupied by methods based on the ozonization of phenanthrene in solvents. Carrying out the ozonolysis process in a medium of aliphatic acids, ketones, or alcohols leads to the production, in high yield, of peroxide derivatives of diphenic aldehyde \[3–5\].

The peroxides formed are valuable starting materials for the synthesis of diphenic aldehyde, diphenic aldehydic acid, and their derivatives. The synthesis of these compounds can be carried out by catalytic cleavage of the peroxides obtained, regulated by selection of the appropriate catalysts and reaction conditions.

To obtain diphenic acid from the decomposition products of the peroxides, it is necessary to oxidize the aldehyde groups to carboxyl groups; for this purpose, alongside ozonized oxygen, permanganates, nitric acid, and hydrogen peroxide are used \[6–9\]. It should be noted that the use of ozone and permanganates for this purpose is irrational because of their relatively high cost, while the use of nitric acid encounters a number of technological difficulties in the utilization of exhaust gases. When hydrogen peroxide is used, no wastes are formed except water; however, the low yields of diphenic acid, as well as the high consumption of oxidant in the known procedures, have until now prevented preference from being given to this reagent. We assumed, however, that by oxidizing the aldehyde group to a carboxyl group with hydrogen peroxide through oxyhydroperoxide, better yields of diphenic acid could be achieved with a simultaneous reduction in oxidant consumption.

Indeed, our experiments showed that the interaction of hydrogen peroxide with aldehyde groups under conditions of acid catalysis leads to the formation of the corresponding hydroperoxides in 90–95% yield.

To obtain diphenic acid, it is more advantageous to take diphenic aldehydic acid or its methyl ester, which are most readily accessible by cleavage of dimethoxy peroxide (3,8-dimethoxy-4,5,6,7-dibenzo-1,2-dioxacyclooctane), and which, when converted into the acid, require a minimal consumption of oxidant. Catalytic cleavage of dimethoxy peroxide in methanol by known methods \[9, 10\] leads to the methyl ester of diphenic aldehydic acid, which can be oxidized without isolation by hydrogen peroxide into the corresponding methoxyhydroperoxide. This allowed us to develop a one-stage method for producing diphenic acid from dimethoxy peroxide according to the scheme:

![Reaction scheme: conversion of a substituted biphenyl dimethoxy peroxide through aldehyde ester and methoxy hydroperoxide intermediates, followed by alkaline hydrolysis and acidification to diphenic acid.]

Methods for carrying out the first stage of the process are known [10], whereas the synthesis of methoxy hydroperoxide III and its alkaline hydrolysis were investigated by us for the first time. It was found that, when a methanolic solution of aldehyde ether II is treated with hydrogen peroxide, the methoxy hydroperoxide III that forms precipitates, shifting the reaction equilibrium to the right. However, to obtain a 95–98% yield of methoxy hydroperoxide, it is necessary to use an excess of hydrogen peroxide of 0.5 to 0.8 mol. Methoxy hydroperoxide III is a white crystalline substance, mp 106–107°C, stable to mechanical action, and is readily displaced by iodine from its salts.

The yield of methoxy hydroperoxide is almost independent of the amount of methanol used at ratios from 1.5 to 4 parts of alcohol per 1 part of aldehyde ether, and also of the process temperature in the range from 0°C to 30°C. In alkaline hydrolysis of methoxy hydroperoxide we obtained diphenic acid in up to 99% yield. In the proposed method, however, the methoxy hydroperoxide is not isolated in pure form, but is saponified directly in the whole reaction mixture to diphenic acid. The dependence of the yield of diphenic acid on the amount of hydrogen peroxide used is shown in Fig. 1. When hydrogen peroxide of different concentrations (from 30% to 65%) was used, we observed a decrease in its specific consumption with increasing concentration.

![Graph plotting yield, % versus consumption of H₂O₂, mol, showing increasing diphenic acid yield with increasing hydrogen peroxide consumption.]

Fig. 1. Dependence of the yield of diphenic acid on the amount of hydrogen peroxide used.

Experimental Part

50 g of dimethoxy peroxide is placed in a flask fitted with a reflux condenser, and 75 ml of methanol containing 0.1 g of NaOH is added. On heating the resulting mixture to 60°C, hydrolysis of the dimethoxy peroxide begins and is completed when the solution boils, within 10–12 min.

The resulting solution, after cooling, is acidified with 1 ml of HCl ($d = 1.18$), mixed with 35 ml of 30% hydrogen peroxide, and kept for 2.5 h at 20–25°C. Then 48 ml of a 30% aqueous solution of KOH or NaOH is added to the reaction mixture, the mixture is heated to 50°C, and held at this temperature for 30 min. After this holding period, another 22 ml of alkali solution is introduced, and then the methanol is distilled off using a column. The alkaline solution remaining in the flask is neutralized with concentrated hydrochloric acid to pH 7 and, if impurities are present, filtered. More hydrochloric acid is added to the filtrate to pH 2, whereupon diphenic acid precipitates completely. M.p. 226–228°C. Yield—43.8 g (98.5%). Recrystallization from 50% acetic acid gives diphenic acid in 90–95% yield, m.p. 231–233°C.

Conclusions

1. A one-stage method has been developed for obtaining diphenic acid from dimethoxy peroxide (using hydrogen peroxide as the oxidizing agent), which provides a 98–99% yield of the acid and regeneration of the methanol participating in the process.

2. A reduction in the specific consumption of hydrogen peroxide by a factor of 4–6 has been achieved by the developed method in comparison with previously known methods.

References

1. A. A. Dolgalev, I. M. Kamakin, R. I. Polataiko, Khim. prom. 9, 17 (1966).

2. W. F. O’Connor, E. J. Moriconi, Ind. Eng. Chem. 45, 277 (1953).

3. P. S. Bailey, S. B. Mainthia, J. Org. Chem. 23, 1089 (1958).

4. W. F. O’Connor, E. J. Moriconi, W. J. Schmitt, Ind. Eng. Chem. 49, 1701 (1957).

5. R. H. Gallighan, J. O. Hawthorne, U.S. Patent 3118934, C.A. 60, 10606.

6. P. S. Bailey, J. Am. Chem. Soc. 78, 3811 (1956).

7. A. K. Lebedev, V. P. Shabrov, USSR Author’s Certificate 189829 (1965), Bull. No. 24 (1966).

8. R. M. Pentz, U.S. Patent 2872480, RZhKhim 85802p (1960).

9. P. G. Copeland, British Patent 1007012, C.A. 64, 2036 (1966).

10. A. K. Lebedev, A. N. Novikov, Izv. TPI 151, 312 (1966).