THE HISTORY OF A MELTING POINT R. MACY Chemical Corps Technical Command, Edgewood Arsenal, Maryland
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POWER of the printed word is well known. Once a statement or datum gets into print, it acquires a kind of sanctity which i t ordinarily would not attain. If the statement should be in error, to prove it wrong may sometimes be quite difficult. This principle is illustrated by the melting point of chloroacetophenone, the lacrimator known in military circles as CN. The history of the melting point of CN may be outlined as follows: Graehe [Ber., 4, 35 (1871)l was the first to prepare CN, by chlorination of acetophenone, and found its m. p. to be 41°C. It was probably very crude. Staedel [Ber., 10, 1831 (1877)l prepared a sample of CN which he stated melted a t 58-59T. This i s the m. p. reference that is generally quoted in chemical handbooks and in textbooks of .chemical warfare agents; it influenced most of the later workers on C N . Friedel and Crafts [Ann. chim., 6,1,507 (1884)l prepared CN from benzene and chloroacetyl chloride in the presence of aluminum chloride, and "obtained a solid substance very irritating to the eyes, melting a t 56', a t 53' after crystallization from alcohol, and boiliig a t 2.4-242O." Gautier [Ann. chim., 6, 14, 380-1 (1888)l prepared CN by the Graebe method and by the method of Friedel-Crafts. The products in each case melted a t 55%. and boiled without decomposition a t 244°C. Cdlet [Bull. soc. chim., 3, 17, 506 (1897)l made CN by the Friedel-Crafts reaction, and obtained a m. p. of 57-58°C. after two crystallizations from alcohol. Rust [Ber., 30, 2833 (1897)l prepared.cN by an indirect process and identified the CN by obtaining what he termed the right (LLrichtige") m.'p of 58-59°C. This indicates the intluence of Staedel's figure. Korten and Scholl [Ber., 34, 1902 (1901)l recrystallized CN from alcohol "to a melting point of 58-59°C." Tutin [J. Chem. Soc., 97,2500 (1910)l crystallized CN from light petroleum and gave the m. p. as 59°C. Clihbens and Nierenstein [ J . Chem. Soc., 107, 1492 (1915)l ohtained CN as one of the products of a reaction and gave the m. p. as 5S59"C. It was crystallized from light petroleum. Helfrich and Reid (unpublished report to the CWS in 1918) made CN by established procedures and (pointing out its military use) said, "These crystals melt a t 58-59' and boil undecomposed a t 246O.l' Stolteenberg ("Preparation of Poison Gases," Hamburg, 1930) lists the m. p. of CN as 56-58' on p. 9, and on p. 12 states that the pure compound melts a t 58' but the literature is not in agreement on this value. Hoogeveen [Rec. trau. chim., 50, 672 (1932)l refers to the preceding remark in Stoltzenberg. He found it impossible to attain a m. p. of 58' by repeated vacuum
distillation or recrystallization from light petroleuni. The highest m. p. he obtained was 56.5"C. This was the first work in which the m. p. of CN was obtained with the object of establishing the correct value. Baker [ J . Chem. Soc., 134,1151 (1932)l prepared pure CN for physicochemical measurements. The material was crystallized repeatedly from alcohol and finally from ligroin and had a m. p. of 56OC. Pusin and Hrustanovic [Ber., 71,798 (1938)l crystallized CN from acetic acid. They stated, "It melted a t 51' (instead of 58"). Our preparation therefore was not entirely pure." This was a remarkably timid statement, for this melting point was the best value in all the published literature on CN. It is obvious from this history that Staedel's work back in 1877 had the sanctity of t h e printed word. Some of the succedmg workers undoubtedly simply copied his value for the m. p. into their records, but most of them measured it and strove to obtain his result. The writer purified CN by the following procedures: (a) One crystallization from 95 per cent alcohol. (b) One crystallization as in (a) followed by crystallization from benzene. Crystallization three t i ~ ~ from @ 95 per cent alco(c) hol. . .. Several days of airdrying or vacuum-drying were required to remove the solvent. All these samples gave the same results in the melting point and freezing point measurements. The product from (c) was pure by chlorine analysis. The melting point was h t determined by capillary tube in a Thiele apparatus. The capillary tubes had a diameter of 1 to 1.5 mm., and the calibrated Anschutz thermometer was graduated to 0.2'C. The first melting points went over 55", but in careful measurements i t was found that the temperature a t which sufficient had melted to form a meniscfs was a little over 54.5"C. To fix the melting point more definitely, it was finally measured in a Beckmann apparatus. When seeded a t 53" with the bath a t 50°, the sample froze a t constant temperature a t 54.0°. When melted with the bath a t 58-60', the melting temperature was constant a t 54.0". The amount of CN used in this test gave a volume of about 20 cc. when melted, ample to cover the bulb of the thermometer. The purification of CN is such a simple matter and the correct m. p. is so readily determined, one is forced to the conclusion that the perpetuation of an incorrect value for them. p. was due as much to the psychological effect of the printed word as it was to the tendency of organic chemists for running melting points in capillary tubes, which often give high results.
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