Henry J. Shime Texas Tech University, Lubbock, TX 79409
The year 1987 marked the centenary of the death of Alexandr Porfir'evich Borodin. Borodin is known to the world for his music, and, on a smaller scale, for his contributions to education. includine chemistw. In this connection. and in commemoration of the centenary event, five articles (1-4) appeared in American and English chemistry periodicals in 1987. These articles describe the commitments Borodin had to both music and education. and thev teach us that Borodin, in fact, would rather do chemistr; than write music. In three of the articles (I-3), it is said of Borodin that his "investigations of benzidine derivatives (1859-1860) led to an understanding of the mechanisms of the henzidine, biphenyline [sic], and semidine rearrangements!' This attribution was very surprising to me when I first read it (I), hecause in abnost 40 vears of research and readine in benzidine rearrangements I had never come across a reference to contributions bv Borodin. This curious anomalv led to a year-long correspondence and exchange of information with George B. Kauffman and Ian D. Rae, and to a search into the available literature of the period. 1 t is evident now that Borodin's work really had no impact at all on the mechanisms of the henzidine rearrangements. The claim made in his behalf is an exaggeration derived from the Russian biography of Borodin (5). That the claim is an exaggeration at best is likely to he evident onlv to the small (and. alas. decreasine) number of chemists who were actively soking'out the mechanisms of benzidine rearrangements during the last 20-40 years. Now, I do not want to make a mountain out of a historical molehill, and I do not mean to disparage Borodin's work with benzidine. Yet, the history is not correct, and in regard to the role which unraveling the reactions and structure of benzidine had in the mechanisms of benzidine rearrangements, it is proper, in the popular chemical literature, to give credit where it is properly due. The term benzidine rearrangement refers, in its bestknown sense, to the acid-catalyzed rearrangements of hydrazoarenes. These rearrangements go back, indeed, over 100 vears. Thev have been reviewed. alone with lesser known thermal and photochemical benzidine rearrangements, a number of times in the last 20 vears (6).It is sufficientto sav. now, that the rearrangemenis' name is derived from the prototype rearrangement of hydrazobenzene (1)into benzidine (2) in acid solutions. Another product, diphenyline (3), an isomer of 1 and 2, is also formed when 1 is treated with aqueous acids (eq 1).
If hvdrazohenzene has para substituents, other rearranneme&, called ortho- and para-semidine, can occur, too.-1n some cases, for example with 2.2'-hvdrazonaphthalene, an obenzidine rearrangement can alsobccur. 1'these ways the term henzidine rearrangement now covers all of these types. Now. none of this was known in the ~ e r i o d1859-1860. ~ e n z i d k was e known and had been prodiced from azobenzene by Zinin, in 1845 (7). Zinin reduced azobenzene with
hydrogen sulfide in ethanol solution saturated with ammonia. It is likelv that Zinin first obtained hvdrazobenzene in this reaction, because he descrihes the product as becoming vellow on standine (i.e.. it became oxidized on its surface to Lobenzene, a prokern kith hydrazobenzene that bothers us even today). In order to purify the yellow substance, Zinin added dilute sulfuric acid to its solution in boiling alcohol, an act in which he unwittingly converted the hydrazobenzene (if, indeed, be had it) into benzidine. Thus, Zinin unknowingly reduced azobenzene to hydrazobenzene and unknowingly carried out the benzidine rearrangement, perhaps for the first time. In 1853 Zinin r e ~ o r t e dthe reactions of azobenzene and azoxybenzenewith h f u r o u s acid in alcohol solution. In each case benzidine was formed (8).Once aeain Zinin unknowinely reduced not only the azobekzene hut, ironically, also azok vbenzene to hvdrazohenzene. and caused the rearrangement bf hydrazohenzene to benzidine. Zinin showed quiteelearly with its acid salts that benzidine was dibasic, and he reported its elemental composition, ClzH~zNz,correctly (7). The structure of benzidine was unknown, however, and it is on that account, presumably, that Zinin's proteg6, Borodin, carried out its reactions with ethyl iodide (9), carbon disulfide. and other reactants (10). It is on account of these reactions that Borodin was &edited for his contribution to the mechanism of the benzidine.. di~henvline. & - .and semidine rearrangements (5). However, there is nothing in Borodin's reDorts on the reactions of benzidine that bears on the benzidine rearrangement. The most pertinent of these studies would have heen that on the reaction of benzidine with ethvl iodide because that study must have been designed partic". larlyto bear on the structure of benzidine. Nothing of lasting value was achieved, however, insofar as the sthcture of benzidine was concerned. Borodin obtained a product whose elemental analysis he compared with those that would be expected for tri- and tetraethylbenzidine. He appears to have coniectured that his nroduct was a mixture of these two derivatives. It seems fromthis that he must have had a very rood idea of the functional rrouos that he was seekine. but the results were inconclusiv~.1nihis regard, then, he & on the track of characterizing benzidine as having two primary m i n e groups but was unsuccessful in reaching the goal. In this regard also he was overshadowed by the elegant contemporaneous work of P. W. Hofmann (11). In successive reactions with ethyliodide P. W. Hofmannmade, and characterized with meltine ooint and ~ l a t i n i cchloride salts. diethvland tetraethylhe%dine. ~ u i t h e r on , finding that ktraethylbenzidine reacted only slowly with eithyl iodide, he converted the tetraethyl derivative into its dimethylammonium salt with methyl iodide. Thus, P. W. Hofmann knew that benzidine was a diamine, but he did not know its structure, about which he wrote: "Es unterlied kein Zweifel. dass diese Base ein primtires Diamin ist, in welchem die ~ & m g r u ~ ~ e C12H8,was immer ihre Natur sein mag, als zweiatomiger Radical fungirt." (There is no doubt that this base is a primary diamine in which the group ClzHa whatever its nature may be, is difunctional.) P. W. Hofmann carried out this work in London in the laboratory of A. W. Hofmann, a t that time probably the Volume 66 Number 10 October 1989
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foremost laboratory for research in arylamines in the world. It was knowledge that work was in progress in London, evidently, which made Borodin publish his seemingly incomdete studv. The stow has been recorded bv Dianin (12). he-unfortunate but understandable consequence appears to he that later workera have ignored the article. Zinin's process for making henzidine (by the unrecognized rearrangement of hydrazobenzene) was then the method of choice. Recognition that Zinin's conversion of azobenzene into benzidine (7) was a two-step rather than a one-step Drocess came also in A. W. Hofmann's laboratow. In usine Zinin's procedure, A. W. Hofmann discovered the formation of hydrazobenzene, which he isolated and named, and its conversion into benzidine by acids (13). Hofmann pointed out that "some of the chemists who have been engaged in the examination of benzidine must have occasionally worked with hydrazobenzol," and he referred to an example in his own laboratory some years earlier when Noble, having what he thought was benzidiie in his hands, described its conversion in& azobenzene by the action of nitrous acid (14). At the time Hofmann published the discovery and rearrangement of hydrazobenzene, he must have still been unaware of the structure of benzidme (15). The structures of azo- and hydrazobenzene were also unknown, and in that respect Hofmann wrote: "whatever view may be taken regarding the nature of azobenzol. the constitution of which it must be admitted is utterly 'unknown, the intermediate substance has to be viewed as its hvdrorren . . com~ound,and it is this consideration which induced me to propose the name hydrazobenzol" (13). A solution to the structure of benzidine had been brought a little closer, however, by Fittig about a year earlier, in his studies of biphenyl. Fittig prepared dinitrobiphenyl by nitration of biphenyl, and he reduced the dinitro to the diaminobiphenyl with ammonium sulfide and hydrogen sulfide. The diamine was shown to be identical with benzidine obtained by Zinin's method (16). In spite of his discovery, however, Fittig could not put together the pieces of the structural ~uzzle.because he still acce~tedNoble's misleading claim (14) that benzidine could h'e converted into azobenzene bv the action of nitrous acid. It wan not until 1874 of the two amino groups in henzidine were that the deduced, by Schultz (17). With Griess's diazotization method Schultz converted benzidine into 4,4'-dibromobipbenyl, the correct structure of which was already known. Later, Schultz found that the rearrangement of hydrazobenzene gave not only benzidine but also diphenyline (I@,the structure of which was proved in 1881 (19). The history related briefly above shows that the principals in working out the rearrangement of hydrazobenzene were the Hofmanns, Fittig, and Schultz. Borodin truly had no hand in it. Following Schultz's work, efforts by numbers of chemists went into carrving out benzidine rearrangements. In 1922 Jacobson sum&&ed all that was then known about the rearrangements (20). Jacobson credited Hofmann with the real discovery of the rearrangement, although certainly
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Zinin was the fimt to carry it out. Perhaps that is the proper order of credits. For nearly 100 years after its discovery the rearrangement's mechanism remained the center of controversy (6). Only in the last five years or so has it been shown, with measurements of heavy-atom kinetic iaotope effects, that the rearrangements are sigmatropic ones, classifiable by the rules of pericyclic reactions (21). Acknowledgment Research in benzidine rearrangements a t Texas Tech University have been supported by grants from the Robert A. Welch Foundation (Grant D-028) and the National Science Foundation. I thank George B. Kauffman and Ian D. Rae for discussions and for the translation of pertinent parts of the book on Borodin (5). Clive Hunt called my attention to Dianin's book (12) and the story of Borodin's presumed hasty publication, for which I thank him. Martin Hoffmann, Universititt Hannover, kindly supplied copies of the papers by Borodin and by Zinin in Erlenmeyer's Annalen, and W. Walter, Universititt Hamburg, kindly helped me with understanding the nuances of the earlier German writing. Preparation for this article was begun during the tenure of an Alexander von Humboldt Foundation Senior US. Scientist OrganAward. I thank the Foundation, and the Institut ~III ische Chemie, Universitilt Hamburg, for hospitality. Literature cited 1. Kauffuff.G.B.Indust. Chem. 1987,8(1).4143. 2. KauNman. G. 6.:Rae. I. D.: Solov'ev, Yu. I.; Steinberg, C. Chsm. En#.News 1987, f i. i,, ',.,,,. 1-.. . . .
3. Kauffman, 0 . 6 . ; Soiw'ev,Y. I.: Steinberg, C. Educ. Chem. 1981.24.138-140. 4. (a) Hunt.C.B.Chem.Bril. 1987,23,547-550. (b) White,A.D. J.Chom.Edue. 1987.64, ?.,L.,""
5. Pigurovskii,N.A:Solw'ev.Yu.I.AleksondrPoriir'ouichBadii;IIdateL'~'ttALademii Nauk SSSR M o m . 1950. Thia bmk has heen tramhtd into English by Chadene Steinbsq and George B. Kauffman as Aleksondr Parfir'mirh Boradin: A Chemist's Biography: Springer-Verlag: Berlin. 1988.1am indebted to Kauffman for sending me the translation ofthat part ofthe book mncemingth8 aork of Bordin with banzidina (pp 33,361. 6. (a) Banthorp. D. V.: Hughen, E. D.: Insold.C. K. J. Chem. Sor. 1%4,2%&1901. (b) Shine,H. J. Amm~ticReorrongemnl8:Elsevier:Amtordam, 1967: pp 126179. (e) Shine, H. J. MTP Jntl. Re". Sci., Oq. Chom. Ser. 11973, 3, 7% SBI.11 1976.3. . Top. Corbocyd.Chsm. 1959.1,1-62: ChemReu. 1970. 1W-110. (d)Banthorpe,O. V 70. 236322. (el Cox. R.A,; Buneel. E. The Chemistry 01the Hydrolo, h o , and A m q OIUUPS; Patai. S.,Ed.:Wiley:New York. 1975; pp 777-805. 7. Zinin,N. J.1Sokt.Chem. 1845, [1].36,93-107. 8 Zinin,N.Ann. Chem.Phom. 1853.85.325329. ~l Chem.Phorm. 1MO. 4 5 m . 9. Bordin, A. E l l ~ n m e yundLewinstein'*Z. 10. Barodin,A.Erlanmyer und LowimtainBZ. C h o m . P h . 1860.3.641443. 11. Hofmann, P. W. Justus Liabig'a Ann. 1860.115.362368. 12. Disnin. S. Boradin: translated by R. Lord; Oxford Universihc London. 1963, p 28; ' " B o d i nmade haste +apublish this article since he had fovod out through Zinin that Hofmann inLondon was working in thissame field." 13. H0fmann.A. W . h . R a y . Soc. 1863.12,576578. 14. Nob1e.A.J Chom. S w . L855.8,292-We Juatua Liebi#b Ann. 1856,98,2%256. 15. Hofmann. A. W. h . Roy. Sac. 1863.12.389-394. la thia paper Hofmann described the nature of banridme as worked out by P. W. Hoimaon but d w not d e p a ite
16. Fitfig, R.JusfurLieb@'*Ann. L862.124.276289. 17. Schultz. G. JvstvsLiebiZB Ann 1874.J74. 20-235. See& Schmidt.. H.:.khulte.0. JU& ~ i d i g . 8A"". k 8 1 . 207,328j47. la schmidt. H.:schultr. G.c h m . &,. 1878.11,1751-1755. HJJustus Liebihh Ann. 1881,207.34E350. 19. Scbuitz, G.;khmidt, H.; Str-r. 20. Jambson. P. JustusLiobigb Ann. 1922,428.76121. 21. KupnykSubatkoanka, L.; Shine, H. J.; Subotkouski, W.; zygmuot, J. Doll. Chim. J.Am.Chom.Sae. I%LIOB,lOXL Jtol. 1987,117,513516;Rhe,ES.:Shin8,H.J. 1W6: 1987. 109. 5052: J. Om. Chem. 1987.52. 5633-5636: and earlier references
