EXCHANGE OF SUBSTITUENTS ON NITROGEN IN MOLTEN SALTSAND AMINES
fluorescence will be 5%. This agrees with the findings of Franck." ( 5 ) The model provides the electronic conductor that is needed to prevent back-reactions between the reducing and the oxidizing power. (6) The model provides a mechanism for the production of delayed light.
791
Two problems in solid state physics must be solved before the electron-hole picture of photosynthesis has to be taken seriously. (1) Can an exciton break up to give a bound electron and a free hole? (2) How do free electrons and holes move in an aggregate of a few hundred pigment molecules?
Exchange of Substituents on Nitrogen in Molten Ammonium Salts and Amines
by Heinz K. Hofmeister and John R. Van Wazer Monsanto Company, Central Research Department, St. Louis, Missouri
(Received February 19, 1964)
At 200-300° in sealed tubes, methyl groups and hydrogen atoms exchange places on the nitrogen atoms of ammonium ions in melts of tetramethylammonium chloride with ammonium chloride. Likewise, the three pure mixed methylammonium cations-CH3?;H3+, (CH&SH2+, and (CHS)&H+-undergo such rearrangements. I n all cases, there is an equilibrium between the various ammonium ions which is not much different from that expected for random sorting of the hydrogens and methyl groups. The kinetics of the thermal equilibration of methyl-, dimethyl-, and trimethylammonium ions a t 300" are presented. Mixtures of aniline and dimethylaniline in the same temperature range p , ! ~ , exchange hydrogens and methyl groups to give a near-random equilibrium in the presence of appreciable amounts of HCl. Shifting of phenyl groups between nitrogen atoms is immeasurably slow under the conditions studied.
As pointed out by Skinner,l the enthalpy of redistribution reactions gives a direct measure of the departure of bond-energy-term values from constancy. This information may alSo be obtained indirectly2 by measuring the deviations from statistically random sorting in the scrambling of substituents in such reactions. As part of a broad quantitative study of new families of inorganic compounds, ranging from small molecules .to macromolecular species, it has been deemed desirable to establish certain generalities concerning deviations from the statistically random sorting of substituents in scrambling reactions. The information presented below represents a specific experimental example studied for this purpose, since we could find no quantitative chemical data or any thermochemical information concerning the scrambling of substituents on quadruply connected nitrogen. Simultaneously,
data were also obtained for another general treatment we are attempting which deals with the kinetics of scrambling reactions.
Experimental Details The N,N-dimethylaniline, aniline, methylamine hydrogen chloride, and ammonium chloride were reagent ~~
-
(1) H. A. Skinner, Rec. trav. chim., 73, 991 (1954). (2) T h e values of AH may be estimated, rather closely in many cases, by calculating (in terms of free energy) the deviations from randomness of the measured equilibrium constants of the appropriate scrambling reactions. This assumes cancellation of the partition functions for translation, rotation, and vibration. See A. G. Evans and E. Wnrhurut, Trans. Faraday SOC.,44, 1S9 (1948); J. R. Van Wazer and L. Maier, J. Am. Chem. SOC.,86,811 (1964); K. Moedritzer and J. R. Van Wazer, Inorg. Chem., 3 , 139 (1964).
(3) Manuscripts concerned with (a) regularities in t h e deviations of scrambling reactions from randomness and (b) a theory of the variations t o be found in t h e kinetics of such reactions are now in preparation.
Volume 69,Number 9 March 1965
HEIXZK. HOFMEISTER AND JOHX R. VAS WAZER
792
grade chemicals froni the Fisher Scientific Co. Diand trimethylamine hydrogen chlorides, as well as aniline hydrogen chloride and tetramethylammonium chloride, were Eastnian White Label Chemicals. The S-iiiethylaniline was redistilled Eastnian practical grade (Yellow Label). The ammonium chloride and tetramethylaniiiioriium chloride were recrystallized from water and alcohol, respectively, and then dried in a desiccator over phosphorus pentoxide at room temperature. It was found that mixtures of tetramethylammonium chloride and ammonium chloride in sealed Pyrex glass tubes melted over a period of several hours when heated to about 300'. Even after heating a t this temperature for 4 days, no residual gas pressure was noted after cooling to room temperature and opening those tubes which contained more than 50 niole % amnioniurn chloride. Even when there were large amounts of tetraniethylaninioniuni chloride, deconiposition was inappreciable during the period needed to achieve equilibrium. Four or five replicate determinations were made at mole fractions of tetraniethylammonium chloride equal to 0.25, 0.50, and 0.75, and a few scattered determinations were carried out for other compositions. Proof that equilibrium was achieved in these studies was based on the fact that the same final coinpositions were obtained when the monoinethylaminonium, dimethylanimonium, and trimethylaninionium chlorides were subjected to the same thermal treatment. The majority of the equilibrium measurements were carried out at 300', but some runs were also made on nielts a t 200'. I n order to avoid shifting of the equilibrium with lowering teniperature and upon crystallization, quick quenching was employed, This consisted of dropping the sealed Pyrex tubes into a well-stirred mixture of ice and water. After the samples were quenched, the tubes were opened and the contents were dissolved in 20% hydrochloric acid for the nuclear magnetic resonance (n.m.r.) measurements. Equilibrium between aniline and K,N-dimethylaniline was also achieved in sealed Pyrex tubes in the temperature range of 200-300'. I n this case, the desired ratios of the two end-member compounds were combined with various proportions of hydrochloric acid, in the range of 0 to 0.5 mole of HCl/g.-atom of S. Achievement of equilibrium was checked by heating N-methylaniline under the same conditions. For this system a t 200', preliminary experiments indicated that equilibrium was reached within 2 days with 0.5 mole HCl/g.-atom of S as accelerator. Again rapid quenching was employed, following which the contents of the tubes were shaken with an aqueous The Journal of Phyeicd Chemistry
alkaline solution in a separatory funnel so that, after drying, the pure liquid amines could be used for the n.1n.r. measurements. A Varian A-60 analytical spectrometer running at a frequency of 60 ilfc./sec. was employed for the H1 n.ni.r. determinations, generally using the smallest available sweep width (50 c.p.s. for the ent're scale). A typical spectrum of the reaction products obtained from a molten mixture of tetramethylammonium chloride and aninionium chloride is shown in Figure 1, from which it can be seen that there were four resonance
- 2.3
-2.5
-2.7
-2.9
-3.1
Magnetic field i n p . p . m . (with respect t o water).
Figure 1. Typical H1 n.m.r. spectrum at 60 Mc. for an equilibrated mixture of 0.75 mole of tetramethylammonium chloride with 1.25 moles of ammonium chloride. Water is generally taken to have a shift of -5.3 p.p.m. with respect to tetramethylsilane.
peaks used for determination of the methylammonium ion as compared to one peak for the tetramethylammonium ion. The unsubstituted aninionium ion mas measured by subtracting the proper equivalence of hydrogen for the substituted ammonium ions from the single resonance peak corresponding to all of the hydrogens directly attached to the nitrogen atom. The N-hydrogens corresponding to the various ammonium ions were not resolvable but appeared as a single resonance. The full spectrum of Figure 1 demonstrates the type of resolution generally obtained in this study. However, on the few occasions when optimum conditions were achieved, the multiplet corresponding t o the spin-spin coupling between the hydrogen and nitrogen of the tetramethylammonium ion gave a wellseparated triplet, as shown in the insert of Figure 1. The coupling constant for this H-C-If configuration, as obtained from these measurements, is found to be J H N = 0.55 C.P.S. As expected, the unsymmetrical methylamines and methylammoniuni ions do not show splitting due to this coupling. The H-S-C-H coupling constant between the hydrogens on the nitrogen and on the amnioniuni ion was found to be 5.20 f 0.06
EXCHANGE OF SUBSTITUENTS ON NITROGEN IN MOLTENSALTS AND AMINES
/
100
80
gen equivalent of the substituted anilines from the total phenyl group resonance. A statistical study4of the data showed that the overall standard error in the n.m.r. determination of the various ammonium ions with respect to the weighed amounts of reactants in the system S(CH3)4+-NH4+ was 1.54% of the total nitrogen. The error was greatest (3.0%) for XH4+ion and least (0.5%) for the N(CH3)4+ ion, as calculated from measurements on known niix( C H ~ ) the ~ tures. For the C ~ & ~ H ~ - C ~ H E , ~ ' system, over-all standard error was rather high, being 10.0% of the total nitrogen. Since the standard deviation of the equilibrium constant for the aniline system (see eq. 2 below) was found to be only 3% of its value, this 10% standard error in the material balance is probably due in considerable part to inconsequential side reactions.
5: 1
2
793
-s
'2 60
2
4 ..
I 3 40 C
8
k 20
0 0
1
R
=
CHa,"
2 mole ratio.
4
3
Figure 2. Equilibrium between tetramethylammonium chloride and ammonium chloride at 300". The dotted lines correspond to an ideal random system and the solid lines to Ki = 0.68, Ki = 0.33, and K 3 = 0.115; 1, NHI'; 2, CHaNH3'; 3, (C€L)pNH?+;4, (CHa)aNH+; 5, (CHa)rN+.
100
\
80 0
0
20
40
60 Time, min.
80
100
120
Figure 3. Reorganization of the methylammonium ion at 300" in molten CH3NH3C1: 0 , CHaNH3+; 0, NHd+; X, (CH3)2XHz+; A, (CH3)3NH+; 0 , (CHa)nN+.
c.p.s. for the (CHI)INH+ ion, 5.78 f 0.07 for the (CH3)2NH+, and 6.20 f 0.05 for the CH3NHa+ion. Analysis of the aniline system was carried out by measuring the integral of the individual methyl group 1i.ni.r. peaks as well as that of the total phenyl. Thus, the X,N-dimethylaniline and the N-methylaniline were determined directly, and the amount of unsubstituted aniline was obtained by subtracting the proper hydro-
Results and Conclusions The Molten Ammonium Chloride System. The experimental data on the system ammonium chloride us. tetramethylanimoniuni chloride is compared with
the smooth curves for the observed averaged equilibrium constants in Figure 2. There are three such constants, corresponding to values of i = 1, 2, or 3 in eq. 1. Based on the 16 separate preparations, the
equilibrium constants are calculated4 to be 0.115 with a standard deviation, s, of 0.008 for i = 1; 0.33 with s = 0.02 for i = 2; and 0.68 with s = 0.05 fori = 3. Were the system ideally r a n d ~ n i ,the ~ constant for i = 1 and 3 would be 0.375, whereas the constant for i = 2 would be 0.444. The greatest deviation from randomness is thus seen to be found for the reaction having i = 1 in eq. 1 and the least for i = 2. However, in all cases, the deviations are small. The kinetics of reorganization of originally pure samples of (CH3)XH3+,(CH3),SH2+,and (CH3)NH3+ were studied a t 300'. The resulting data are presented in Figures 3, 4, and 5, from which it can be seen that, for all practical purposes, equilibrium is reached within 20 min. for the triiiiethylamiiionium ion, 70 min. for the diniethylniethylammonium ion, and 60 min. for the monomethylammonium ion. Although the kinetics are quite involved, they may be fitted to first-order rate equations. When this is done, the (4)
L. C. D. Groenweghe and J. R. Van Waaer, Anal. Chem., 36, 303
(1964). ( 5 ) G. Calingaert and H. A. Beatty, J . A m . Chem. SOC.,61, 2748
(1939).
Volume 69,Number 3 March 1966
HEINZK. HOFMEISTER AND JOHN R. VAN WAZER
794
100
80
accelerator. The equilibrium constant was found to be 0.30 with s = 0.09, for the reaction ~ C B H , N ( C H ~= ) HCeHbXHz
+ CsH,N(CHs)z
(2)
Since for ideal randomness, the equilibrium constant 1s 0.250, we see that-within experimental error-the exchange of methyl groups and hydrogens on aniline is indistinguishable from random behavior. It is interesting to note that methyl and hydrogen substituents on ammonia readily exchange places under conditions where the phenyl groups do not. I n one preliminary experiment, a mixture of aniline and dimethylaniline with 0.5 mole of HCl/g.-atom of ?J was heated for a week a t 200’ with no evidence of ex80 100 0 20 40 60 Time, min. change of the phenyl group with either hydrogen or Figure 4. Reorganization of the dimethylammonium ion at the methyl group. With no added HC1, decomposition 300” in molten (CH3)ZNHzCl: X, (CHa)zNHz+; A, ( ~ H d s N H + ; rather than exchange was observed. e, CH3NH3+;V, NH4+; 0, (CH&N+. Miscellaneous Studies. The poor exchangeability of phenyl groups on nitrogen was also evidenced by an 100 I experiment in which 1 mole of diphenylamine with 0.5 mole of HC1 was heated for two days a t 300’. No exchange was found, although as little as 1% of substituent interchange could have been detected. Likewise, a mixture of triphenylamine and tetramethylammonium chloride was heated a t 300’ for 1 week. Again no exchange was detectable although the n.m.r. spectrum showed some small peaks due to side reactions. These findings, using modern C.P. reagents, probably mean that the old commercial process6 for the synthesis of diphenylamine from aniline by redistribution of the phenyl groups was accelerated by impurities in the HC1 catalyst or the aniline or by the autoclave walls. Upon heating monobutylammonium chloride a t 300’ 0 for 2 days, considerable gas pressure built up in the 0 20 40 60 Time, min. sealed tube. Although a n analysis was not carried Figure 5. Reorganization of the trimethylammonium cation out, this gas was probably butylene. Proton n.m.r. st 300” in molten (CHa)3NHCI: A, (CHa)aNH+; study of a solution of the solid resulting from quenching 0, (CHZ)~PYI+; X, ( C H ~ ) Z N H Z e, + ; CH3NHJ+. showed a number of small unidentified resonance peaks. A similar situation, with many small n.m.r. peaks being found in both the CH2-CHs region and in the phenyl pseudo-first-order rate constants for the diminution region of the proton n.m.r. spectrum, was observed of the original species and the growth of the species when benzylamnionium chloride was heated a t 300’ formed are found to be equal within experimental for 2 days. I n this case, however, gas pressure was error. At 30Oo, the half-life for reorganization of the not noted. trimethylamiiioriiuiii cation is 0.8 min., for the diReaction Mechanism. It is quite apparent that the methylainrnoniuin cation it is 7.2 min., and for the reorganization mechanism must involve the ammomonoinethylammonium cation it is 17.5 min. From nium ion, since the pure amines were always found to comparison with measurements made a t 20O0, an actiundergo decomposition before any substituent intervation energy of 38 kcal. was found for the reorganization of the diiiiethylaininonium cation. The rZniline System. The experimental data were (6) P. H. Groggins, “Aniline and Its Derivatives,” D . Van Nostrand and Co., New York, N. Y., 1924, p. 164. Alternatively, see K. obtained on the system aniline us. diniethylaniline with Winnacker, Ed., “Chemische Technologie, Organ. Tech. 11,” Carl 0.3 mole of HCl/g.-atom of N being used as a rate Hanser, Munchen, 1954, p. 56. The Journal of Physieal Chemistry
‘
EXCHANGE OF SUBSTITUENTS ON NITROGEN IN MOLTEN SALTSAND AMINES
change could be detected. Likewise, it seems reasonable to conclude that because permanent gases did not form over most of the range of interest in either the substituted ammonium chloride melts or the substituted aniline mixtures, the redistribution reactions probably follow ionic and not free-radical mechanisms. A reasonable mechanism’ for this process is given below, as exemplified by the exchange between two trimethylammonium ions (CH3)3KH+
+ C1-I-
(CH3)aN:
+ HC1
(3)
H ‘\
H
H (CH3)i“
(CHJzNH:
+ HC1
+ :NH(CHa)2
(CH&NHZ+
+ C1-
(4) (5)
This mechanism indicates that the reactions should go the fastest for mixtures of amine and ammonium salt, and several experiments have shown this to be the case. llilechanisms involving reaction between methyl chloride and the free amines or the ammonium ions are not ruled out by these experiments. However, it is difficult to picture a transition state for the reaction between methyl chloride and the ions.
Discussion From available thermodynamic data8 on amines, one can compare the methyl vs. hydrogen exchange on
795
triply connected nitrogen (in the amines) with the same exchange on quadruply connected nitrogen (in the ammonium ions). Thus, the reaction of eq. 6 may be compared with that of eq. 1
For i = 1, the equilibrium a t 300’ is calculated to be 8 X for the gaseous and 6 X lo-* for the liquid amines as compared to our measured value of 11.5 X for the ammonium chloride melts. Also, estiniation2 of AH from the equilibrium constants given in this paper consistently leads to somewhat smaller numerical values for these exchange reactions on the quadruply as compared to the triply connected nitrogen. One is tempted to ascribe these apparently soniewhat greater deviations from random scrambling on the amines to the ready ability of triply connected nitrogen to undergo changes in rehybridization by formation of an abortive bond involving its unshared pair of electrons. Acknowledgment. We wish to thank Dr. Harold Weingarten for helpful discussions concerning the reaction mechanism. (7) For related work see H. R. Snyder, R. E. Carnahan, and E. R. Lovejoy, J . Am. Chem. Soc., 76, 1301 (1954). (8) F. D. Rossini, D. D. Wagman, W. H. Evans, S. Levine, and I. Jaffe, “Selected Values of Chemical Thermodynamic Properties.” National Bureau of Standards Circular 500, U. S. Government Printing Office, Washington, D. C., 1952. (9) C. A. Coulson, “Valence,” Oxford University Press, Oxford, 1952, p. 210.
Volum8 69,Number 8
March 1966
