[CONTRIBUTION FROM THE CHEMICAL LABORATORY, UNIVERSITY OF TORONTO 1
THE IMPROVED PREPARATION OF CHLOROFURFURAL W. J. CHUTE
AND
GEORGE F WRIGHT
Received July 30, 19.46
The original method for preparation of 5-chloro-2-furfural previously involved an optimum yield of 31% on the basis of furfural diacetate which was treated with two moles of sulfuryl chloride (1). Since this represents a 15% yield on the furfural basis (the yield of the furfural diacetate intermediate being 5070) some effort was made in the earlier work to chlorinate furfural in carbon disulfide with sulfuryl chloride. Such attempts a t simplification led to insignificant yields of 5%. A t the same time it was found that the action of chlorine on furfural diacetate in carbon disulfide led to yields of 34%. W i l e such yields were by no means impressive, the appearance of the reaction mixture seemed t o indicate first, that the more costly sulfuryl chloride was less advantageous than elemental chlorine, and second, that the reaction was incomplete under the conditions imposed on the system. A renewed need for chlorofurfural led us to re-investigate the preparation with these observations in view. We soon found that if one mole of chlorine was passed into a boiling solution of one mole of furfural in carbon disulfide over one hour with three hours subsequent reflux, a 12y0 yield of chlorofurfural could be obtained by distillation of the carbon disulfide, followed by three hours' heating at 100" to remove hydrogen chloride, and subsequent steam distillation. Furthermore this yield could be raised to 24% by increasing the chlorine input to 1.5-2 moles. This yield was that remaining after a by-product oil had been separated by crystallization of the chlorofurfural. The amount of this oil did not exceed one weight per cent when 800-600 cc. of carbon disulfide was used per mole of furfural. Since this oil was quite vesicant it was desirable to avoid its formation. The use of less carbon disulfide therefore seemed inadvisable, because experiment showed that this variation caused an increase in oil formation to 5-6 weight per cent. The yield of chlorofurfural was also decreased, and in the extreme case where no solvent was used, a large amount of furfural was recovered. In this instance the chlorofurfural yield was about 1% and about 2 weight per cent of vesicant oil was obtained. During the study of this revised procedure the solvent was purified by shaking with calcium carbonate, dried, and distilled for re-use. The chlorofurfural yield at once dropped to 1-2% when this recovered carbon disulfide was employed, while the quantity of vesicant oil increased to 6-7 weight per cent. This indicated that a catalyst, which was present in the stock solvent, was removed during the purification. This catalyst turned out to be sulfur. Addition of 0.020 atom equivalent of this substance to the recovered solvent restored the 24% yield. Some slight 541
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W. J. CHUTE AND GEORGE F WRIGHT
understanding of this catalyst behavior may be gained from the observation that the yield was decreased markedly when yet a larger amount of sulfur was used, and considerable increase in decomposition was apparent. This may have been owing to sulfur monochloride formation, since addition of this substance decreased the chlorofurfural yield to 5%. Carbon disulfide has always seemed to be a desirable solvent for furan halogenation (3). The discovery that sulfur is catalytic toward chlorination of furfural might imply that this solvent was effective because it ordinarily contained sulfur as an impurity. We found, however, that carbon tetrachloride, with or without added sulfur, could not replace the disulfide as a furfural chlorination solvent. During the search for the lost catalyst, aluminium, antimony, ferric and phosphorus chlorides as well as sulfuryl chloride, diphenylamine, pyridine, and acetic anhydride' were tried without effect, while iodine, as well as sunlight, inhibited the reaction. However the addition of 0.004 equivalent of benzoyl peroxide to the purified sulfur-free solvent increased the yield from 1-2% to 10%. This was not entirely unexpected in view of previous reports on peroxide catalysis of chlorination reactions (2). After the effect of sulfur had been discovered it seemed worth while to ascertain whether benzoyl peroxide was alternative or co-catalytic with respect to sulfur. It was found to be co-catalytic in the sense that it caused a yield increase to 33% under conditions where sulfur as the single catalyst would effect a 24% yield. By contrast to sulfur a larger amount of benzoyl peroxide, a t least up to four times the minimum of 0.004 peroxide equivalent, did not further affect the yield. A further advantage of the composite sulfur and peroxide catalyst system accrued from the fact that no vesicant oil contaminated the crude product. No further attempt was made toward greater yield increase since the cheapness of the reagents rendered the preparation quite adequate for laboratory use. However we do not believe that the 33% yield we obtained is the ultimate maximum. Destruction of the furan ring is unavoidable when aqueous mineral acid is present. The tar and coke formation characteristic of furan reactions, especially halogenation, is undoubtedly owing to the fact that an aqueous environment is created i n situ, by decomposition of the furan ring. The discovery of more potent catalysts which will accelerate reactions with furan compounds before this decomposition can set in probably will result in higher yields. EXPERIMENTAL
Preparation of chlorofurfural. A solution of 96 g. (1 mole) of technical furfural (dried by azeotropic distillation a t 30 mm. pressure) in 800 cc. of dry carbon disulfide containing 0.64 g. (0.020 atom) sulfur and 0.968 g. (0.004 mole) benzoyl peroxide, was heated under reflux while 142 g. (2 moles) of chlorine was added over one hour. After three hours reflux, with some hydrogen chloride evolution,.the solution was poured into a flask equipped for steam distillation. The solvent was removed over a water-bath and the remainder heated at 100" for three hours to complete the hydrogen chloride evolution. The residue was steam distilled to yield 42.9 g. of chlorofurfural or 33% of theoretical. This product melted at 1 Acetic anhydride has been effective in raising an otherwise low yield caused by use of undried solvent.
PBEPARATION OF C HLOROFURFURAL
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If insufficient catalyst was present, a vesicant oil also steam distilled. The oil was separated by suction filtration of the cold distillate. The recovered carbon disulfide was purified for re-use by agitation with 37 g. of precipitated calcium carbonfite per litre. It was then distilled t o separate the first fraction which contained water.
31.5-33' and was non-vesicant.
SUMMARY
The preparation of chlorofurfural by chlorination of furfural has been improved by inclusion of sulfur and benzoyl peroxide as catalysts. TORONTO, ONTARIO
REFERENCES (1) GILMANAND WRIGHT,Rec. trap. chim.,60, 833 (1931). (2) KHARASCH AND BROWN,J . Am. Chem. Soc., 61, 2142, 3432, (1939). (3) GILMANAND WRIGHT,J . Am. Cheni. Soc., 62, 1170 (1930).
