RALPHKLEISAND MILTOND. SCHEER
1874
and 10. (The ordinate unit as drawn in the figure is 0.1 log unit; the curves are displaced vertically by arbitrary amounts in order to simplify the diagram.) It will be heen that the plots are all nicely linear. The intercepts at c = 0, which give the corresponding values of log K-4, are given in Table 111. TABLE I11 DERIVED CONST4hTS KO.
1 2
3 4 5
.lo(T.’,
1
(70 1) 57 7 52 7 48.0 45 0
log K A
2 99 4 48
5 46 7 07 9 05
NO,
6 7 8 9 10
MW)
(20 24 27 29 30
22) 4
3 2 3
l o p KA
2.46 3 72 5 06 6.33 7 65
Figure 3 shows the logarithms of the association constants as a function of reciprocal dielectric constant. The plots are linear, showing first that the errors due to the various approximations made are negligible compared to the wide span covered by the experimental ~ a l u e sof K A (5-6 decades). The linearity shows furthermore that the model of sphere-in-continuum will describe either system over this wide range of variables, but the difference in dopes simultaneously shows that the same sphere will not describe tetrabutylammonium picrate in bo2h solvent mixtures. Calculation of contact distance? from the slopes of the lines of Fig. 3, by the equation (valid at 2 5 O )
Vol. 67
i z ~= 243.4/(A log KalAD-l)
(12)
gives i z ~= 7.22 for the MeCS-dioxane mixtures and 7.84 for the PSA-dioxane mixtures. These values bracket the value &K = 7.51 found for BudNPi in nitrobenzene-carbon tetrachloride mixtures18over the range 4.96 6 D 6 16.20, but is distinctly larger than the value18 &A = 5.91 calculated from limiting conductances. l7 It should be mentioned that data over a narrower range of dielectric constant, especially a t higher Dvalues, for these two systems could probably be approximated quite closely by two parallel lines (Le., equal U K values). This observation is a convincing argument for studying a given system over as wide a range of variables as possible. The facts that the Walden product is smaller in the PNA system and that the value of aK is larger both argue that different models must be used to represent the same ions in the two different solvent systems here considered. The hypothesis of specific solvation of the picrate ions by molecules of p-nitroaniline is consistent with the present experimental results. Finally, we point out that this is not an ad hoc hypothesis: it was surmised that such solvation should occur, and a n experimental test13 verified the expectation. (18) E. Hirsoh and R. M. Fuoss, J . Am. Chem. Soc., 82, 1018 (1960).
HYDROGEN ATOM ADDITION TO SOLID FOUR-CARBON OLEFINS BY RALPHKLEIKAND MILTOND. SCHEER 11-ational Bureau of Standards, Washington, D.C . Received April 8, 1963 An investigation of the hydrogen atom addition to condensed, solid four-carbon olefins in the 77’K. region has shown that the general reaction is terminal hydrogen atom addition to terminal double bonds followed by disproportionation and recombination of the resulting radicals. A11 of the butene-2 formed from butene-1 and from butadiene-1,3 is trans, but that formed from butadiene-1,2 is both cis and tmns. Butadiene-1,3 produces nine eight-carbon dimers, butadiene-1,2, three, and isobutene, none. One of the major products of the H atom addition reaction with butadiene-1,2 is butyne-2.
Several elementary reaction processes can be studied a t low temperatures. Low-temperature investigations (below 150°K.) differ from those at higher temperatures in the absence of secondary reactions. The mercuryphotosensitized hydrogenation of butene-2 a t room temperature gives products which may be separated into ten distinct fractions by gas-phase chromatography.l On the other hand, the hydrogen atom addition to trans-butene3 at 77 OK., reported here, yields reaction products with only three fractions. A suitable experimental technique for observing hydrogen atom addition t o solids a t low temperat u r e ~has ~,~ been developed previously. It was shown in the case of condensed olefins that the reaction occurs on the surface and the surface concentration of the clrfin is maintained by diffusion processes in the condensed layera4 The low temperature chemistry of the propylene-H atom system has been established.6 (I) P. J . Boddy and J. C. Robb, Proc. Roy. SOC.(London), A249, 532 (1959). (2) M. D. Scheei and R . Klein, J . P h y s . Ckem., 6 6 , 375 (1961). (3) M. D. Scheer and R. Klein, to be published. (4) R. Klein, and M . D. Scheer, J . Phys. Ckem., 66, 2677 (1962). (5) R. Klein, &f. D. Scheer, and J. 0.Waller, %bed., 64, 1247 (1960).
Four-carbon olefins are particularly interesting because they possess structural isomerism. There are the terminal double bonds in butene-1 and isobutene, the internal double bonds in cis- and trans-butene-2, and the two isomeric dienes (butadiene-1,2 and butadiene-1,3).
Experimental The four-carbon olefins used in these experiments, butene-1, cis- and trans-butene-2, isobutene, butadiene-1 ,Z, and butadiene1,3, showed less than 0.?5y0 impurities in all cases. In a typical experiment, the required olefin, either with or without a diluent such as propane, was condensed on a 7 5 - ~ r n flat . ~ bottom of a reaction vessel. During the reaction, the vessel was immersed in liquid nitrogen. The arrangement for hydrogen dissociation has been given earlier.6 Usually a quantity of about 100 pmoles of the olefin was taken, and the reaction with atomic hydrogen was allowed to proceed to the extent of about 59’0, based on the olefin. This gave sufficient product for reliable analysis using gas chromatographic methods. Products were identified by comparison of retention times with standard compounds. Except for some of the dimers formed in the diene systems, all peaks were positively identified. The products formed when H atoms are added to condensed four-carbon olefins arc given in Table I. ( 6 ) R. Klein and Ill. D. Scheer, J . Am. Chem. Soc., 60, 1007 (19%).
Sept., 1963
HYDlZOGES ATOAT ADDITIOK TO
SOLID
FOUR-CARBON OLEEISs
1875
TABLE I PRODUCTS FORMED IX THE H ATOM ADDITION TO CONDENSED FOUR-CARBON OLEFINS The C,/Cg product ratios are > 1.5 in all cases and the C4’s are listed in order of product abundance Olefin
Four-carbon products
Radlcal recombination products
Butene-1
n-B u t ane trans-Butene-2
3,PDimethylhexane
trans-Butene-2
n-Butane Butene-1
3,4-Dimethylhexane
cis-Butene-2
n-Butane trans-Butene-2 Butene-1
3,4-Dimethylhexane
Isobutene
Isobutane
Sone
Isobutane
2,3-Dimethylbutane 2,2,3-’l’rimethylbutane
Butadiene-1,3
n-Butane trans-Butene-2 Butene-1
3,4dDimethylhexane 3-hlethylheptane and 7 other olefinic eight-carbon dimers
Butadiene-1,2
n-Butane trans-Butene-2 cis-Butene-2 Butene-1 Butyne-2
3,CDimethylhexane and 2 other olefinic eight-carbon dimers
Isobutene
+ propylene
Results and Discussion Butene-1.--Butene-1, on hydrogen atom addition a t 77”K., gives only 3,4-dimethylhexane as the eight-carbon product.’ Any normal butyl radicals resulting from a nonterininal addition would be revealed by the formation of a t least the cross dimer between the secbutyl and n-butyl to give 3-methylheptane. This was never detected within the limit of the analytical technique. It is estimated that at least 95% H atom addition is terminal. The reactions system are CH&HZCH=CHz
+ H --+ CH3CHzCHCH3
2CH3CH2CHCH3 +3,4-dimethylhexane -+ n-butane
+
trans-butene-2 --+n-butane
CH3CH&HCH3
+ butene-1 (4)
+ H +Hz + trans-butene-2 --+ Hz + butene-1
(5) (6)
n-butane (7) No other products are found. The butene-2 formed in ( 3 ) and (5) a t 77°K. is found to be the trans isomer.* The thermodynamic equilibrium between cis and trans a t 77°K. requires the cisltrans ratio to be about 0.01. This suggests that the butene-:! from disproportionation occurs by a mechanism giving the equilibrium ratio. Considering the free rotation about the carboiicarbon bonds in the sec-butyl radical. equilibration is not unexpected. Reactions 4 and 6 result in butene-1. Evidence for ( 5 ) and (6) is furnished by the formation of HD when deuterium is used for the reaction with butene-1.8 The ratio of butane to 3,4-dimethylhexane a t 77°K. is found to be approximately 12. Gas phase experim e n t ~a t~ 300°K. gave a ratio of 2.3. There are at --+-
(7) R. Klein and M. D. Soheer, J. Phys. Chem., 62, 1011 (1958) (8) M. D. Scheer and R . Klein, abzd., 68, 1517 (1959). (9) J. W. Krauss and J. G. Calvert, J. Ant. C h e n . Soc., 79, 5921 (1957).
least two reasons for this discrepancy. First, the occurrence of reaction 7 would result in an increase in this ratio. The extent of this reaction depends on the concentration of radicals and H atoms on the surface. The radical conceintration is determined in pari by the diffusion rate of radicals into the matrix. This has been clearly demonstrated in the case of isopropyl radicals in “loose’’ and “rigid” matrices. The resulting propane-:!,3-dimethylbutaiie ratio is considerably higher in the former.1° This is doubtless general and is applicable to the addition of H atoms to the fourcarbon olefins. Second, the djsproportionahion-dimerizatioii ratio is temperature dependent. The dimerization reaction is associated with a higher activation energy so that lower temperatures favor disproportioiiation. We have demonstrated and measured this effect quantitatively for isopropyl Butene-2.--The H atom addition to either cis- or truns-butene-:! is similar to that of butene-1. The products for all three are identical. Undiluted cis- and trans-butene-:! show no hydrogen pick-up because the reaction is diffusion limited. With mixtures highly diluted with propane (lye mixture), the rate of addition to butene-2 is the same as that to butene-1.4 At this concentration the reaction is chemical rate and not diffmion limited. The rate of nontermiiial H atom addition to butene-2 is about the same as that for terminal addition to butene-1, but in an olefin when terminal or nonterminal addition is possible, as iii butene-l, terminal addition occurs preponderantly. 1sobutene.-Isobutene adds H atoms at 77°K. and the only product: is isobutane. The dimerization of the t-butyl radicals to give 2,2,3,3-tetramethylbutaiie occurs to a negligible extent. This is the only fourcarboii olefin system in which no radical dimerization a t 77°K. is detected. Either the concentration of tbutyl radicals at the surface is vanishingly small, or the recombination reaction has a considerably higher activation energy than the disproportionation. The first of these implications was examined by treating a (10) R. Klein and M. n. Soheer, t o be pubhshed. (11) M.D. Soheer and R. Klein, in preparation.
1876
RALPHKLEINAND MILTOND. SCHEER
condensed mixture of isobutene and propylene a t 77 OK. with atomic hydrogen. Six- and seven- but no eightcarbon alkanes were produced. Tertiary butyl radicals were present in significant concentration as shown by the formation of 2,2,3-trimethylbutane. This is possible only by the combination of propyl with a tbutyl radical. I n contrast, 2,2,3,4-tetramethylbutane was found a t room temperature and above in both the gas phase photolysis of di-t-butyl ketoneg and the H atom addition to isobutene.12 This would suggest an energy barrier for t-butyl, t-butyl recombination. An activation energy difference between recombination and disproportionation for alkyl radicals appears to be general. It is necessary to assume that there are two transition states, one for recombination and one for dimerization. The model proposed by Bradley13 assumes the same transition complex for both reactions and could not account for relative temperature effects. Butadiene-1,d.-The dienes are an excellent example of the value of the low temperature hydrogen atom addition studies for elucidating the reaction system. The only previous work on the reaction of H atoms with butadiene-1,3 was done in the gas phase by White and Winkler.14 The main product was ethane, with butane, butene, propane, and methane occurring in smaller concentrations. Nost of these are doubtless products of "atomic cracking'' reactions and do not reveal the detailed features of the reaction steps. In a preliminary communication on H atom addition to butadiene-1,3 at 77"K., it was reported that the reaction was slow and incomplete.6 It is now recognized that this is a matrix effect, and when the butadiene-1,3 is sufficiently diluted (with propane, for example) so that diffusion may occur readily, the reaction is comparable in rate to that of butene-1. In the region of high propane concentration, the rate of H atom addition to butadiene is concentration controlled. At high butadiene concentrations, the rate is diffusion limited. There is an intermediate dilution in which the over-all rate is a maximum. ,4t this dilution a build-up of reaction products occurs in the surface layer even for small over-all conversions. The products formed in the hydrogen atom addition to butadiene-1,3 under these conditions were n-butane, butene-1, transbutene-2, and at least nine eight-carbon dimers. Ethane, propane, and methane were absent. The appearance of the large number of dimers can be readily understood if three distinct four-carbon radicals are present. The butadiene-1,3 molecule has very little resonance energy. Dewar and Schmeissing estimate it to be less than 2 kcal./mole.l6 The possibility of more than one form of butadiene-1,3 participating in the H atom addition reaction may be dismissed. Further, it is clear that only terminal addition. occurs. Nonterminal addition would give the radical CH2CH2CH=CH2 with no resonance stabilizatiop. Terminal addition, on the other hand, gives CH,CHCH=CHz, resonance stabilized with the equivalent CH&H= CHcH2. Using D(CHsH) - D(R-H) as.&~riterion,'~ the resonance energy of the radical CH3CHCH=CH2 is'estimated to be about 23 kcal. This is 15 to 20 (12) P. J. Boddy and J. C. Robb, Proc. R o p Soc. (London), A249, 547 (1959). (13) J. N. Bradley, J . Chem. Phus., 36,748 (1961). (14) W. H. White and C. A . Winklei, Can. J . Res., 26B,3 (1948). (15) XI. J. S. Delrar and H. N. Schmeissiny, Tetrahedron, 11, 96 (1960).
Vol. 67
kcal. greater than for CH2CH2CH=CH2. On energetic grounds, therefore, the terminal H atom addition to butadiene-1,3 is expected to give the resonance stabilized niethylallyl radical. These radicals dimerize and disproportionate to give, among other products, butene-1. The latter gives the sec-butyl radical by H atom addition. Six eight-carbon compounds may be produced by association of these radicals. Considering further hydrogenation of double bonds, ten distinct eight-carbon molecules are possible. These are 3,4-dimethylhexane, 3-methylheptanej n-octane, 3,4dimethylhexene-1, octene-2, 5-methylheptene-2, 3methylheptene-1, 3,4-dimethylhexadiene-l,5, octadiene2,6, and 3-niethylheptadiene-lj5.Sine eight-carbon compounds have been found with gas chromatography. Butadieiie-1,3, on addition of H atoms, undergoes the reactions
+ H +CHz=CHCHCHa
CH2=CHCH=CH2
CH3CH=CHCH2 2CH*=CHCHCH, + CH2=CHCH=CH2
+
CH2=CHCH&H3 2CH3CH=CHC:E,
--+
CH2=CHCH=CH2
+
trans-CH3CH=CHCHs CH?=CHCH&H3
+ H +CH&HCH&H,
CH2=C=CHCH,
+ H +CH&H=CHCH,
(9) (10)
(11) plus mixed radical disproportionations, H atom radical disproportioiiatioiis, etc. It is not clear why butadiene-1,2 was not found among the products. Butadiene-l,Z.-Butadiene. 1,2 offers some interesting contrasts. Nonterminal H atom addition would give
(12)
as in butadiene-1,3. In spite of the large resonance energy associated with this radical, the nontermiiial addition does not occur. This is evident from the very marked differences in products obtained between butadiene-1,3 and butadiene-1,2. Butadiene-1,2 gives as products cis-butene-2, trans-butene-2, butene-1, butyne2, and three eight-carbon products. The series of reactions can be represented as
+ H +CH&=CHCH3 + CH3CH=CHCH3 +
(13)
CH3C-CCHs
(14)
CHZ==C=CHCH~ 2CH3C=CHCH3
2CH&=CHCHa -+ CH,CN=C (CH,)C (CH3)=CHCH,
(15)
Also
+
CH&===CHCH3 H --+Hz
+ CH3CeCCH8 (16)
and CH&=CHCH,
+H
CH3CH=CHCHa
(17) Several results are to be emphasized. The formation of butyne-2 as a major product is the result of disproportionations of type 14 or 16. It niay be meiitioned that the isomerization of butadiene-1,2 to --+
Sept., 1963
SOLUTION FORMlED B E T W E E N
butyne-2 is favored thermodynamically over that to butyne-1.16 The formation of cis-butene-2 at! 77 OK. is most unusual. All other straight chain fourcarbon olefins yield trans-butene-2 with no cis-butene-2 observed. Ilowever, with butadiene-1,2, the ratio of cis- to trans-butene-2 was found to be about one. Disproportionation (14 and 17) yields cis-butene-2 (as well as Irans) because of the rigidit-y of the double bond in the radical, and either the cis- or the trans-butene-2 will be formed with equal probability. If the postulated mechanism for H atom addition to butadiene-1 ,% is correct, the. only radical expected from the primary reaction is CH3C=CHCH3. Although thjs radical possesses resonance, the two forms would be distinguishable only by isotope labeling. The secbutyl radical is formed from subsequent reactions and three distinct eight-carbon compounds are expected. These are
c-c=c--c I c-c=c--c,
c-c-c-c
c--.c=c-c
I
a,nd
c-c-c-c,
1
c-c-c-c
(16) E. J. Prosen, F. W. Maron, and F. D. Rossini, J . Res. Y a l l . Bur. Std., 46, 106 (1951).
HC1 AND PYRIDINIUM CHLORIDE
1.877
Further hydrogenation does not lead to new compounds as jn the case of butadiene-1,3. Three product peaks corresponding to eight-carbon molecules were found with gas chromatography. Summary The general feature of hydrogen atom addition to four-carbon olefins, as with others investigated, is the terminal hydrogen addition to terminal double bonds. The resulting radicals dimerize and disproportionate to give products. Elutene-1, cis-butene-2, arid butadiene1,3 give trans-butene-2 as the only butene-2 isomer. Butadiene-1,2 gives a mixture of cis- and trans-butene-2. Butadiene-1,2 alone forms butyne-2 as a major Rroduct. Butadiene-1,3 gives a large number of dimers, in'payl as a consequence of the two resonance structures of the primary radical formed. Finally, isobutene is unique in the absence of the formatjon of an eight-carbon compound from the t-butyl radicals produced by the addition of H atoms at 77 OK. Acknowledgment.-The capable experimental work of Mr. Richard Kelly and M r . Richard Stern is gratefully acknowledged.
THE KATURE OF THE SOLUTION FORMED BE'TWEEN HYDROGES CHLORIDE AND PYRIDINIUM CHLORIDE BY
M A R T I N GOFFML4iY1AND
GEORGEw.H-4RRINGTON2
Department of Chemistry, Temple University, Philadelphiz Z.8, Pa. Received April 13, 1963 A solution, melting a t SO", was found to form between anhydrous HCI and pyridinium chloride. The nature of this solution was investigated by solubility and electrical conductivity measurements. These properties were studied as a function of pressure and temperature. The infrared spectrum of the solid was also investigated. The results of the various measurements suggest R solution having a dual character which is very temperature dependent. A model based on the formation of a T-complex is proposed to explain the observed results.
Introduction Fused pyridinium chloride has been used frequently as a solvent. Audrieth and Long3 found that, certain metallic oxides dissolve to form chlorides amd free pyridine. Starke4 used the dissolving capability for many metal analyses. Displacement reactions were studied by Audrieth3 and &Is0 by Scott and Coe.6 Gruen6 used molten pyridinium chloride as a solvent to investigate complexes of Co(I1) spectroscopically. He later found that the salt has art appreciable vapor pressure just above the melting point.' In these earlier studies the molten salt was not kept in the fused state for long periods of time. I n this Laboratory, however, it was desired to mainf,ain the pyridinium chloride as a melt for several hours and perhaps even days. It 17-as found virtually impossible to maintain the salt in this condition for long periods of time without appreciable decomposition. Attempts to hold the melt in an inert atmosphere were of no avail. (1) Part of the work submitted by AI. Goffman t o Temple University in partial fulfillment of the requirements for the degree of Master of Arts. (2) Author t o whom inquiries should be sent. (3) L. F. Audrieth and A. Long, Trans. Illznozs State Acad. Sa.,28, 121 (1935). (4) K. Starke, Can. J . Res., 28,225 (1950). (5) R . Scott a n d C. S. Coe, J . Am. Chew. Soc , 59, 1876 (1937). (6) D. M. Gruen, .I. Inorg. Nucl. Chem., 4, 73 (1956) (7) D. hl. Giuen and R. L. MoUeth, zbzd., 9, 290 (19SQ).
Finally, melting in an atmosphere of dry HCl was attempted. In this case, however, a yellow liquid was t the application of heat. This liquid was later found to freeze sharply at 50'. This investigation waa undertaken to study the nature of the solution thus formed. Experimental Reagents.-The yridinium chloride was prepared by two different methods. $he first consisted of bubbling dry HCI into an anhydrous ethyl ether solution of pyridine. The pyridinium chloride that precipitated was filtered, washed with cold, dry ether, and recrystallized twice from 2-propanol. The salt was dried in a vacuum desiccator until all excess solvent was removed. The second method of preparation consisted of distillation of practical grade pyridinium chloride in air. The melting point, 144.5', of the salt obtained by either method agreed with the literature Identical experimental results were obtained with samples from either method of preparation. The HC1 gas used was anhydrous reagent grade. The manufacturer's analysis !specified less than 10 p.p.m. water. The gas was further dried by passage through a 1-m. long drying tower containing anhydrous magnesium perchlorate. Apparatus.-The conductivity cell was similar in design to the type M cell described by Jones and Bollinger.8 The body of the cell was constructed (of 22-mm. 0.d. Pyrex tubing. The electrodes were platinized platinum. The filling tubes of the type M cell were used as gas inlet and outlet tubes. A third tube was attached t o the body of the cell for filling purposes. The diam(8) G. Jones and G. M. Bollinger, J . Am. Chem. Soc., 63, 441 (1931).
