W. H. ~ICFADDEN
1074
Vol. 67
THE MASS SPECTRA OF THREE DEUTERATKD PROPENl3S BYW. 13. ~ I C F A D D I C S Western Regional Resemch Laboratory,' AZbnny, ('difornin
Received Novmber 8, 11162 The mass spectra of 2-Ill-propene, 1,I-Dn-propene, and X,3,8-D3-propme arc prrsentcd for 70 wilt elevt.rons. The data show considerable rearrangement of hydrogen and deutcrium from all positions. 'J'he formation of C3Hs+is due primarily to loss of hydrogen from positions 1 or 3. The ratc of loss of (113, or CI14 to form two carbon ions is sufficiently f:mt to allo\v only n fe\v exchanges of hydrogen and deuterium prior to this breakdown.
I. Introduction I n an attempt to understand simple features of the mass spectra of C10H16terpenes it has been found necessary to study the mass spectra of smaller unsaturated hydrocarbons. This is prompted principally because attempts to correlate the spectral features of terpenes with structure2 are not sufficiently coniplete to be generally applicable and many important exceptions are encountered. The mass spectrum of camphene8-C13has shown that for this compound the methylene does not secm to be lost in forming Cs or C9 ions hut that a general rearrangement and approximate equilibration of the C13 does occur in the formation of C7 The data are not sufficient or smaller fragment for further clarification, however. The reasons for exceptions to simple rules and for our lack of understanding of the mass spectra of these terpenes are partly revealed in the sparse literature on the mass spectra of lower molccular weight labeled olefins and cycloalkanes. Such data indicate the existence of more complex cracking patterns than would be expected. For example, the mass spectra of methylcyclopentane and methylcyclohexane show predominant ions due to loss of CH3. However, the mass spectrum of methyl-C13-cyclopentane shows that only about one-half of the events involve the original methyl group but the mass spectrum of a-deuterated methylcyclohexane shows that the original methyl is lost quantitatively.@ From thc mass spectrum of CD3CH2CHCH2,in addition to other rearrangements, it appears that a t low ionizing voltages the loss of CHB is more probable than loss of CD3.6 However, Katallis has shown that monodeuterocyclopentane gives statistical amounts of CH3+ and CHzDf with electron voltages above 40 e.v. but strongly favors the formation of CH3+at lower voltages.' This is not consistent with the conclusions drawn from the data of CD~CHZCHCH,. Obviously, rearrangements and exchanges occur which are not clearly understood or interpreted with the paucity of existing data and for this reason the mass spectra of the three deuterated propcnes are presented.
II. Experimental The three deuterated propenes were 2-DI-propene, 1,l-Ds-propene and 3.3,3-D3-propene. They were obtained from Merck, Sharp and Dohme of Canada, Limited, and were sold aa 96% isotopically pure but low voltage analysis indicated that isotopic contaminants are considerably in excess of this amount. From the (1) A laboratory of the Western Utilization Rrsearch and Development Division, Agricultural Research Service, U. S. Department of Agriculture. (2) T. Gilchrist and R. I. Reed, Ezperimentio. 16, 131 (1960). (3) L. Friedman and A. P. Wolf, J. A m . Chem. SOC.,80, 2424 (1058). (4) D. P. Stevenson, ibid., 80, 1571 (1958). (5) Seymour Meyeraon, private communication. (6) W. A. Bryce and P. Kebarle, Can. J . Chem., 94, 1240 (1950). (7) P. Natalis, Bull. me. t o y . aci. Lihge, 17, 201(1958).
mass spectral data and high resolution 1i.m.r. analvsis thc isotopic compositions were determined and are presented in Table I.
TABLE I ISOTOPIC ComosiTIox o r DEUTERATED PROPENES Yo I'rraent in 7"Present in yo Piesent Compound
CJb CIIa--CD=CFT* CFI~CI-I-CI~II CHzD-CII=CHp CsH4D2
GHd+ C3H2n4 CzHlIs
CllSCT)=CIT~
8 72 17 3
ciraCII=CI>,
in
CT)s-
c I r:-r II*
12 67 17 4
5 90 4.4 0.3
The spectra of the impure mixtures were used to calculate an approximate spectrum for each of the contaminants and make corrections to the original patterns. As thus presented, the data most likely contain small crrors but these are not expected to cause serious changes in the general conclusi~ins. For example, the corrections for the rather large amount of impurities present in CH3--CH=CD* alter the larger peaks by 1-3$', of the t o t d ionization and the smaller peaks by only 0.2-0.57?. IIaximum weighting of the possible errors in these approximate corrections would not alter the conclusions with respect t o the cracking mechanism. The same arguments are valid for the corrections applied to the other two isotopic propenes. The mass spectra were obtained from a CEC21-620 mass spectrometer modified to give increased resolution and control of the electron ionizing voltage. The samples were introduced through a stainlcss steel inlet system at 80" and a pressure of 40-50 j ~ . The temperature of the ionization chamber wns 105 5'. The sensitivity for the bnse peak of propene-D0 w w 41 div./p.
*
111. Results and Discussion The data obtained with 70-volt ionizing electrons are presented in Table I1 for propene-Do and the three deuterated propenes. The results are expressed as a percentage of the total ionization which was obtained from m/e 13 to the parent mass. Corrections have been made for the isotopic impurities but the data have not been corrected for the natural CI3content. C3 Ions.-The most abundant ion in the mass spcctrum of propene is that due to loss of hydrogen from the parent ion. In the spectrum of CH3CDCH2 the intensity of the ion due to loss of one hydrogen contributes the same percentage (27.7%) to the total ionization as is observed from undeuterated propene. Statistically, one expects about 24%. Even though the isotope effect would enhance the abundance due to loss of hydrogen, one would not expect the contributions from the undeuterated and centrally deuterated propene to be equal and it thus appears that the central deuterium is not important in the formation of the propenyl ion. The data for both the terminally labeled propenes show a significant reduction in the ion due to loss of one hydrogen. This suggests, as would be expected
MASSSPECTRA OF THREE DEUTERATED PROPENES
May, 1963
TABLE I1 THEMAPSSPECTRA OF DEUTERATED PROPENES 4
6
13 14 15 16 17 18 19 19.5 20 20.5 21 21.5 24 25 26 27 28 29 30 31 32 36 37 38 39 40 41 42 43 44 45 46
CHICHCHZ
0.4 1.0 1.3 0.1
CHaCDCHt
0.3 .6 .9
.4
0.7 .4
.4 .5
.5 .1
.4
0.1 0.5 2.7 10.0 0.1 0.3
0.6 3.4 4.9 18.9 7.2 27.7 18.5 0.5 0.1
.4 .1 .1 $2 .9 4.4 7.2 0.3
.4 1.6 2.7 7.6 13.4 7.9 27.7 20.8 0.8
CHsCHCDr
0.2 .6 .7 1.0 .3 .2 .4 .3 .3 .3 .1 .1 .4 1.5 4.2 3.9 4.8 1.1 0.8 .4 1.3 2.7 4.1 11.6 8.4 8.5 22.8 18.2 0.5 0.2
0.3 .6 .2
a'
.7
I-
.1 .4 .3 .2 .2 .2 .1 .4 1.6 4.2 3.9 4.0 I .1 0.3 .1 .4 1.3 2.4 3.3 7.5 9.6 5.1 12.3 18.2 19.3 .6
from previous work,6that exchanges or rearrangements occur to make the hydrogens on positions 1 and 3 approximately equivalent. The ion formed by loss of one deuterium is rather abundant in both cases. The isotope effect is sufficient to make the loss of hydrogen about 3097, greater than the value expected by statistical rearrangement of H and D on all sites. This is inconsistent with the fact that bonds alpha to the double bond are preferentially broken, unless an exchange equilibrium of the type CH3&CH-CH2+ +--+ CH2-CH-CH3+ occurs a t a rate greater than the rate of loss of hydrogen. The different behavior of the hydrogen on the central atom is also indicated in Fig. 1 which shows I' as a function of per cent deuterium. r is defined by the equation
ITD-
( P - H)D ( P - H ) H
r=
nH
where (P - H) is the ionization due to loss of hydrogen from the deuterated or undeuterated molecule, respectively, and n is the number of hydrogens in the molecule. Since the data are reported here as per cent of total ionization this is slightly different from the usual r calculated from relative abundances. However, the difference is not important, unless the distribution of deuterated ions affects the base peak in which case meaningful values of I' can only be obtained from data expressed as per cent of total ionization. I n Fig. 1 it is seen that the two points for the terminally labeled propenes at 33.3 and 50% can be regarded as falling on a straight line from the origin
1
II
CDaCHCNz
.5 .6
I
1.61
1075 1
I
1
II
1
/ /
1.4
0
o
a
LL
I .2
k
1.0
1
I
I
I
20
40
60
80
1
100
% DEUTERIUM. Fig. 1.-The
r
factor calculated from the mass spectra of three deuterated propenes.
which indicates that the C-H bonds of carbon 1 and carbon 3 dissociate a t equivalent rates.8 The point for the centrally labeled propene is notably higher which indicates that the hydrogen on this position is less labile. An alternative interpretation of the data in Fig. 1 is suggested by the dotted lines. The deviation of the points for carbons one and three from the solid line may indicate that the hydrogens of all three positions are different in their lability hinting that a small fraction of the hydrogen is lost from position three before exchanges can occur. However, the data axe not accurate enough for a firm conclusion and more points must be obtained. CzIons.-The two-carbon region of the propene mass spectrum is due chiefly to the ion C2&+ (10%) and CzHz+ (2.7%). The data for the three deuterated propenes indicate rearrangements and exchanges but show that the loss of the CHB to form the vinyl ion occurs fast enough to prevent a statistical distribution of the H and D. Thus, from CHZCDCHZ, loss of CHo without rearrangement mould give a mass 28 intensity of about 9.5% or with complete randomization the intensity would be about 4.8%. The observed value is 7.1%. Similarly, from CH3CHCD2or CD3CHCHzthe ions CHCDz+or CHCH2+are also intermediate between the values expected for no rearrangement and for random arrangement. This indicates that the reaction CHCHz+ must occur at a CH,-CHCHz+ + .CH3 rate which is of the same order of magnitude as the exchange reactions between hydrogen and deuterium. A similar conclusion niay be deducted from the deuterated butene data of Bryce6 for the reaction CH3CHzCHCH,++. CH3 CH2CHCH2+. From an estimat,ion of the contribution of the ions CZHZ+and CzHDt to the mass spectrum of CH3CDCH2 one may conclude that only about 20% of the events involve exchange of deuterium to the lost carbon fragment. This percentage is observed for loss of CHI to give the C2H3+ ion (or deuterated equivalent) and for loss of CH, to give the CzH2+ion. The estimated ionization for the various deuterated vinyl ions is compared with the statistical values in Table 111. For each of the labeled compounds the amount of unrearranged vinyl ion is in excess of the statistical value. This indicates that loss of the CH8 must occur considerably faster than equilibration of (8) J. G. Burr, J. M. Scarborough, and R. H. Shudde, J . Phys. Chehsm., 04,
+
+
1358 (1960).
W. H. MCFADDEN
1076
.l.O[
I
In
I
I
3
5
7
I
9
Number O f H-D E x c h a n g e s .
Fig. 2.-Rate
of equilibration of hydrogen and deuterium in CDaCHCHz.
the hydrogen and deuterium. One notes further that if one assumes a hydrogen (or deuterium) from carbon 3 to be transferred to carbon 1, the vinyl ion expected occurs in an amount comparable to the statistical value (CZH3+ from CH3CHCDzand C2HD2+from CDICHCHZ) even though only a first step towards equilibrium has been achieved. TABLE I11 ESTIMATED FRACTIONS OF VARIOUS VINYLIONS CHaCHCHz
CzHa CzHzD CzHDz CzDo
1.00
CHaCDCHz Obsd. Stat.
0 . 2 6 0.5 0.74 0 . 5
’
CHsCHCDa Obsd. Stat.
CDaCHCHz Obsd. Stat.
0 . 2 2 0.20 .34 .60 .44 .20
0.29 0.05 .26 .45 .36 .45 .09 -05
It is seen from the data of 2-D1-propene that the fraction 0.74 of the vinyl ions involve the deuterium of carbon 2. It is therefore interesting to compare this value with the sum of the vinyl ion formed without rearrangement and the vinyl ion formed by transfer of one hydrogen (or deuterium) from carbon 3 to carbon 1 as suggested above. The values are 0.66 for 1,lD2-propene and 0.65 for 3,3,3-D3-propene. The difference (0.10) may be regarded as a measure of the fraction of the ions that undergo further exchanges prior to breakdown. Rate of H-D Exchange.-Because of the loss of CH, and the H-D exchange occur a t a comparable rate, it is desirable to attempt to estimate the number of exchanges that correspond to the observed ratios of various deuterated vinyl ions. The actual molecular processes giving rise to these exchanges may involve such structures as CH3CHCH2+, CH3CCH3+, H(CH3)CHCH+, and others, but it is not possible to apply a weighting factor to account for the importance of each, nor is it practical to estimate the influence of the hydrogen-deuterium isotope effect. Furthermore, all ions will not experience the same number of exchanges, and the observed results will be averages of the various deuterated structures that can occur. Nevertheless, a useful estimation of the probable number of exchanges occurring can be obtained from a simplified model. An exchange mechanism is considered in which a hydrogen (or deuterium) from position three is transferred to positions one or two, and a statistically selected hydrogen (or deuterium) from those positions returned back t o position three. Thus, for CDzCHCH2, the exchanges may be represented by CD3(CzH3)+ CDO(C2HdD)+ tf CD2H(CzH2D)+ etc. and the rear-
-
-
Vol. 67
ranged ion CDzH(C2H2C)+ would lead to CDHz(CzHD2)+ and CH3(C2D3) + in subsequent exchanges. The results of applying this model to CDaCHCHz without consideration for the isotope effect and simply giving a statistical weight to the removal of hydrogel1 or deuterium are presented in Fig. 2. It should be recognized that if the isotope effect were considered, then the approach to equilibrium would be faster than is illustrated. This results, of course, from the fact that the deuterium is originally on one carbon only. In the first transfer, deuterium must be exchanged but in subsequent transfers the lighter isotope will be favored. In addition, the model proposed actually involves two rearrangement reactions for each exchange process. This is done to take account of such structures as CDICDHCHz+ but in actuality the most probably first intermediate would be CDaCHCHzD+ which could be readily expected to lose CHzD without the necessity for subsequent rearrangements. The most interesting feature of this model, as illustrated by Fig. 2, is the rapid approach to equilibrium. Thus, for all practical considerations, equilibrium values could be expected in three or four exchanges. The second feature to be noted is that if it were possible for one or two exchanges of this type to predominate, then C2HzD+would be the most abundant vinyl species contrary to observations. As expected, the model is overly simplified but the estimation of the number of exchanges required to reach equilibrium is valid and similar estimations are obtained for the other two deuterated propenes from the same model. It is concluded that most vinyl ions are formed with only a small number (0-5) of hydrogendeuterium exchanges. These conclusions place a considerable emphasis on the equilibrium CH3-CH~CH2+tfCH2-CH-CH3+ and imply that it is the important mechanism controlling the distribution of hydrogen and deuterium. Such a suggestion is in accordance with the observed values for loss of H or D as discussed in the section on D3 ions and in accordance with data obtained in 3-Cx3propene by Stevenson and I3’agner.O The data they obtained for that isotopic compound indicated only about a 501, preference to form CzHa+ over C13CH3+. This is consistent with the present observations because equalization of carbons one and three can occur with very few exchanges; for such an observation, it is only necessary that the number of molecules that lose a methyl group after an even number of rearrangements be balanced by the number that have undergone an odd number of rearrangements. I n addition their data suggests that a cyclopropane intermediate is not a n important contributor t o the many structures that may exist. The data presented here also indicate, as would be expected, that other structures which require exchange of the hydrogen on carbon 2 must also be involved. An estimation of the importance of each would require additional data from other labeled olefins and in addition, a much more refined knowledge of the isotope effect than is currently possessed. C1 and Doubly Charged Ions.-The ionization due to doubly charged C3 fragment and due to C1 fragments (9) D. P. Stevenson and C . Wagner, private aommunication,
May, 1963
DISSOCIATION PRESSURE OF AMMONIUM PERCHLORATE
does not constitute an important part of the total. There is obviously considerable scrambling of the hydrogens and deuteriums and for the most part they appear to be statistically arranged. However, the observed values and the statistical values arie small so that a close comparison is not warranted. Acknowledgment.-Miss Mary Sawyer ran the mass spectra and tabulated the data. ‘Dr. It. E. Lundin
1077
performed the n.m.r. analyses which were so necessary in correcting the spectra for the isotopic impurities. Dr. C. Wagner very kindly made available the unpublished data on 3-C13-propene. Reference to a company or product name does not imply approval or recommendation of the product by the U. 5. Department of Agriculture to the exclusion of others that may be suitable,
DISSOCIATIOK PRESSURE OF AMRIONIURI PERC’HLORATEI BY S. HENRY IKAMI, WILLISA. ROSSER,AND HENRYWISE Stanford Research Institute, Menlo Park, California: Received November 8, 1968 The dissociation pressure of ammonium perchlorate has been measured in the temperature range of 520-620’ K. by the transpiration method. The data indicate that ammonium perchlorate sublimes by the dissociation process NH4C10Qs) = NHa(g) HC104(g). The heat of dissociation has been found to be 58 f 2 kcal./mole in the cited temperature range.
+
Introduction During the past several years some efforts have been directed to elucidating the mechanism of the decomposition2-’ and combustion*~9of ammonium perchlorate (AP). Bircumshaw and Newman2 found that the decomposition of AP below 513OK. in vacuo left a residue after 20-30% of the salt had decomposed. This residue had a different crystalline appearance from the original material. However, it was confirmed to be AP by analytical and X-ray methods. Friedman and Levylo in their preliminary study of the reaction between NH3 and HC104 reported that solid formation was observed in the mixing chamber under certain experimental conditions which gave a crude measure of the dissociation pressure of 4P. This observation suggests that a dissociative process is involved in the sublimation of AP. It is apparent that the dissociation pressure of AP represents an important parameter in the anal.ysis of the combustion mechanism of solid propellants based on this oxidizer.
composition and dissociation products of AP were carried away in a helium gas stream passing in an upward direction through the glass frit and the powdered sample. The volumetric flow rate of helium was determined by suitable calibrated flowmeters. All experimental measurements were carried out on that portion of AP which remains after decomposition of 20 to 30% of the original sample.2 Before the dissociation pressure measurement, the sample was heated for approximately an hour. During the initial stage of decomposition a rapid exothermic reaction occurs which may elevate the temperature of the powder bed as much as 100°K. above the bath temperature with the evolution of brownish-yellow vapors. The residual material was a porous, amorphous solid, which crumbled easily. At this time a water-cooled cold finger was quickly placed in position in the transpiration cell (Fig. 1). The distance between the cold finger and the sample was about 2 cm. in most experiments; in some cases it was increased to 4 cm. The temperature of the water in the cold finger was maintained a t approximately 340 f IO’K. to prevent the condensation of H20. After a suitable reaction time, the finger was removed carefully and the white sublimate washed into a flask with about 50 ml. of distilled water. The solution was analyzed primarily for NH4+, Clod-, and C1- in some ca~es.12-1~The dissociation pressure of AP may be readily deduced from the flow rate of helium and the quantity of AP deposited on the finger.
Apparatus and Procedure.-The reagent grade AP from Matheson, Coleman and Bell Company was fractionated mechanically into two particle size ranges, 43-61 U, and > 61 p. I n EL typical experiment a weighed sample of fractionated AP was placed in a glass-fritted cell (Fig. 1) and brought to the desired temperature by immersing the cell in a bath of molten salt.ll The temperature of the molten bath was carefully regulated and measured with a glass-sheathed iron-constantan thermocouple. The de-
Results and Discussions I n a number of runs the following major parameters were varied: (1) fl~owrate of helium gas, (2) reaction temperature, (3) initial mass of AP, (4)initial particle size, and ( 5 ) partial pressure of NH3 added to carricr gas. As shown in Table I the sublimates analyzed were found to contain equimolar quantities of NH4+ and Clod-; therefore, the results may be interpreted in terms of an equilibrium djssociation of “&IO4
(1) This work was supported b y the Office of Naval Research, Department, of the Navy. (2) L. L. Bircumshaw and B. H. Kewman, Proc. Rov. SOC.(London), 8241,115 (1954); A221, 228 (1955). (3) L. L. Bircumshaw and T. R. Phillips, J . Chem. Sue., 4741 (1957). (4) A. K. Galwey and P. W. M. Jacobs, ibid., 837 (1959); 5031 (1960). ( 5 ) A. K. Galwey a n d P. W. M. Jacobs, Trans. Faraday Sac., 65, 1166 (1959); 66, 581 (1960). ( 6 ) A. K. Galwey and P. W. M. Jacobs, Proc. R o y . Soc. (London), 8254, 455 (1960). (7) P. W. M. Jacobs and A. R. Tariq Kureishy, “Eighth Symposium (International) on Combustion,’’ The Williams and Wilkins Company, Baltimore, Maryland, 1962, p. 672. (8) R . Friedman, R . G. Nugent, X. E . Rumbel, and A. S. Scurlock, “Sixth Symposium ([nternational) on Combustion,” Reinhold Publ. Corp. Kew York. N. Y., 1957, p. 612. (9) J. B. Levy and R. Friedman, “Eighth Symposium (International) on Combustion,” The Williams a n d Wilkins Company, Baltimore, Maryland, 1962, p. 663. (10) R. Friedman and J. B. Levy, Final Technical Report AFOSR 2005, Atlantic Research Corporation, Alexandria, Virginia (1961). (11) J. A. Beattie, Rev. Sci. Inrfr., 2, 458 (1931).
SH4C104(s)
=
NH3(g)
+ HC104(g)
(1)
Since equivalent amounts of KH3 and HClO, are praduced, the dissociation pressure P d is equal t o the sunn of P N Hand ~ P H C provided ~ O ~ there is no excess of either gas initially. The calculated dissociation pressures based on the analyses of NH4+ and C104- are in satisfactory agreement. The small variations observed are within the precision of the analytical techniques estimated to be &lo%:, for NH4+ and C104-. Based on (12) F. D. Snell a n d C. T. Snell, “Colorimetric Methods of Analysis,” 3rd. Ed., Vol. 11, D. van Noatrand Co., Inc., New York, N. Y., 1949, p. 816. (13) E. A. Burns a n d R. F. Muraca, A n a l . Chem., 82, 1316 (1980). (14) D. M. Coulson a n d L. A. Cavanagh, abod., 82, 1246 (1960).
