SULFUR IN AMINESOLVENTS
June 5, 1962 [CONTRIBUTION FROX
THE
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DEPARTMENT OF CHEMISTRY, PURDUE UNIVERSITY, LAFAYETTE, ISD.]
Sulfur in Amine Solvents1 BY ROBERT EARLDAVISAND HISHAMF. NAKSHBENDI RECEIVED NOVEMBER 15, 1961 When yellow octatomic sulfur is dissolved in amine solvents, unusual colors are observed raiigixig from deep yellow, to orange and even green. Solutions in aliphatic primary and secondary amines conduct current quite well. It is suggested that ionic ammonium polythioamine salts are produced. Solutions in tertiary amines and pyridine bases contain very few ions, but sulfur is much more yellow than expected. This has been explained in terms of a contact charge transfer process occurring during excitation. A cubic relationship between the extinction coefficient and the refractive index of the solvent has been obtained from quantum theory. This relationship holds quite well for sulfur in inert solvents. However, sulfur The significance of the data is discussed solutions in amines d o not fit this curve, suggesting strong chemical interaction. in terms of two important areas of sulfur chemistry: the Willgerodt reaction and the interaction between an amino nitrogen and a thiol sulfur in the aminothiols which are radio-protective compounds.
Introduction The Willgerodt reaction2 originally referred to the conversion of an aryl alkyl ketone to a terminal carboxamide with reduction of the ketone carbonyl using ammonium polysulfide. Kindler3 introduced the use of dialkylamines and sulfur in which case thioamides are formed. Morpholine is a very convenient solvent4 and the thiornorpholide is usually obtained. Typical is the reaction of acetophenone 0
H
Such modifications usually are classified together as the Willgerodt-Kindler reaction. A total cons p e c t ~ s ~of- ~ this reaction is available in the writings of several authors. Pryor’ has considered this reaction as a typical example of one type of sulfur oxidation. Emphasis upon understanding this reaction always has been placed upon the organic substrate. Davis8 placed strong emphasis on the nature of the interaction between the sulfur and the amine and reviewed this rather neglected area of research. Before judgment can be made upon the mechanisms of the Willgerodt reaction, the nature of the interaction of sulfur and the amine must be more clearly understood. I t is the purpose of this investigation to provide some insight into this area. Several physical techniques have been used to obtain information. (1) P a r t 111, Studies on the Willgerodt Reaction. (2) (a) C . Willgerodt, Chem. Ber., 20, 2467 (1887): (b) 21, 534 (1888); (c) J . prakt. Chem., 8 0 , 183 (1909); (d) C. Willgerodt and F. H. Merk, ibid., 81, 74 (1910); (e) C . Willgerodt and T. Schlotz ibid., 81, 382 (1911); (f) C . Willgerodt and B. Albert, abid., 8 3 , 383 (1911). (3) (a) K. Kindler, A n n . , 4S1, 187 (1923); (b) K . Kindler and T. Li, (,‘hem.Ber., 7 4 8 , 321 (1941); (c) K. Kindler, Arch. Phavm., 266, 388 (1927); (d) K. Kindler and W. Peschke, ibid., 270, 340 (1932). (4) E. Scbwenk and E. Bloch, J . A m . Chem. Soc., 6 4 , 3051 (1942). ( 5 ) M. Carmack and M. A. Spielman, “Organic Reactions,” Vol. 111, John Wile? and Sons, Inc., New York, h-,Y . , 1946, pp. 83-107. (6) R. Wegler, E. Kuhle and W. Schafer, Arrgew. Chem., 7 0 , 351 (1958). (7) W. A. Pryor, monograph in preparation, McGraw-Hill Book Co., Inc., New York. S . Y. (8) R . E. Davis, “Organic Sulfur Compounds. Vol. 11. A Critique of Some Reactions of Elemental Sulfurs,’’ Chapter 1, IC’. Kharasch, ed., Pergamon Press, New York, N. Y . , 1962, paper I in this series.
Results Ultraviolet Spectrum of Octatomic Sulfur.The ultraviolet spectrum of octatomic sulfur in a hydrocarbon solvent as methylpentane, cyclohexane or n-hexane is characterized by a gentle maximum between 260 and 280 mp) a gentle minimum a t 251 mp and a maximum a t 225 mp a t room temperature. A long wing extends above 300 mp and this is responsible for the familiar canary-yellow color. At low temperatures (130’K.) in methylpentane the broad band between 260 and 280 mp is split into two bands a t 263 mp (38020 cm.-l) and 278 mp (35970 cm.-l). It is a general phenomenon that absorption bands sharpen with decreasing temperature. As a consequence of the Franck-Condon principle, a change in distribution of molecules over the various vibrational levels of the ground state leads to sharpening of the absorption bands with lowering the temperature. At low temperatures the vibrational quantum numbers of the absorbing molecules will be zero in accordance with the Boltzmann exponential distribution law. The most probable electronic transitions for these molecules then involve excitation from the lowest level. If sulfur is now dissolved in a different solvent, the extinction coefficient will change. If the sdvent does not interact with sulfur and only exerts a bulk solvent effect due to an internal electric field, one would expect that the extinction coefficient (E) should increase with the increase in the refractive index (n) of the medium.$ The relation-. ship E
=
C (n3
+ i n + 4/n)
C a constant
(2)
has been derived from the quantum theory and this derivation is described in the Appendix. The equation accurately correlates E and n for sulfur in inert solvent^'^ ; however, deviations occur for basic solvents (Fig. I ) . Sulfur in Amines.-The color of sulfur in amine solvents is much more intense than predicted (9) Friedman and Kerkerlo studied the ultraviolet absorption of sulfur in methanol, ethanol, chloroform, hexane and water and presented an empirical relationship which was nearly linear between the extinction coefficient and the refractive index. They also observed a small shift toward higher wave length in the position of maximum and minimum of the curves with increase in refractive index (KOndt’s rule) . l b 1 2 (10) H. L. Friedman and M. Kerker, J . Colloid Scz., 8 , 80 (1953). (11) A. Kundt, An% P h y s . Chem., 4 , 31 (1878). (12) A. L. Le Rosen and C. E. Reid, J . Chem. Phys., 2 0 , 233 (1952). (13) R. E . Davis, Proc. Indiana A c a . Sci., 71,paper I1 (1962).
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ROBERTEARLDAVISAND HISHAMF. NAKSHBENDI
Vol. 84
In3
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3000
3
5 0 I1 380 M Y
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Fig. 1.-Ultraviolet spectrum of sulfur. Extinction 4% 4/n for sulfur in: 1, methacoefficient, e, versus n3 nol; 2, water; 3, 95% ethanol; 4, ethanol; 5, n-hexane; 6, chloroform; 7 , 2,6-lutidine; 8, 4-picoline; 9, pyridine; 10, triethylamine; 11, di-n-butylamine; 12, tri-n-butylamine; 13, ethylenediamine; 14, %-butylamine; and 15, morpholine. Plot A, solvents 1-6 at 300 mp; plot B, all solvents at 380 mp. A solid line joins the points which satisfy the conditions of the cubic equation.
+ +
by either the empirical or theoretical relationships. The sulfur sample initially contains less than M sulfur dioxide and hydrogen sulfide. Thus there is no hydrogen sulfide or sulfur dioxide initially present due to contamination of the sulfur. Both iiiaterials will enhance the color of sulfur by forming open-chain ionic polysulfur compounds. l 4 - I 6 The extinction coefficients of sulfur have been measured in various solvents (Table I). There are three classes of behavior. The first class includes amines in which the suliur spectrum is initially only slightly affected. The amines in this class are tertiary amines and pyridine bases as 2,6lutidine, pyridine,8 4-picoline, tri-n-butylamine and triethylamines in order of their increasing effect. The series 2,6-lutidine < pyridine < 4picoline < triethylamine tri-n-butylamine indicates the basicity of the nitrogen is important but that sterically hindered amines as 2,6-lutidine are much less effective in enhancing the color. All of these solutions (except in pyridine) change color slowly with time. The rate of change is first order (14) P. n. Bdrtlett. E. F. Cox and R. E. Davis, J. A m . C h e m . Soc.. 83, 103 (1961) (15) P. D Bartlett, A. K. Colter, R. E. Davis and W. R. Roderick, ibtd., 83, 109 (1961) (16) R. E. Davis, Dissertation, Harvard Univ., 19.58.
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SULFURIN AMINESOLVENTS
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in initial concentration of sulfur (Table 11). It is TABLE I1 not known what detailed processes are responsible FIRST-ORDER RATE CONSTANTS FOR COLORCHANGESOF for the observed changes. SULFUR SOLUTIONS AT 25.00" kl. sec.-l The next class of amines is secondary amines Amine A, mir like di-n-butylamine and morpholine. The dilute Ethylenediamine 320 1 . 9 x 10-4 2 . 4 x 10-4 solutions in di-n-butylamine are deep orangen-Butylamine 310 Di-n-butylamine 340 1 . 4 x 10-5 yellow and tend to be orange-red a t high concentration. Dilute solutions in morpholine sometimes Tri-n-butylamine 340 1 . 0 x 10" Pyridine 340
