R/IAss
SPECTRA OF
THE
THREE MONODEUTERATED PROPENES
moved by collisional de-excitation. Furthermore, “discharged” nitrogen almost certainly contains very low concentrations of any excited species relative to the concentration of ground state atomic nitrogen. 1 2 , 1 9 Therefore, the fact that the rate constants for the reactions in the argon-oxygen mixtures, and in oxygennitrogen mixtures made up from oxygen which had not passed througfh the discharge, were high relative to the constants whert nitrogen was the carrier gas indicates that ground state molecular oxygen (3&-) when present, plays an important role in the atomic oxygenmethane reaction. I t is not necessary to invoke excited molecular oxygen to explain the results. The role played by molecular oxygen is not known. Judging from the stoichiometry, a chain mechanism
1615
must occur which probably involves OH and OzH radicals. Any methyl radicals produced in the primary reaction will rapidly react with molecular oxygen to form CH302 radicals, which are unstable under many conditions, forming formaldehyde and OH. This enhanced production of OH could explain the effect of molecular oxygen, but numerous other explanations can be formulated. Acknowledgment. This work was supported in part by Grant No. APOO 343-01, Division of Air Pollution, Bureau of State Services, U. S. Public Health Service. (12) D . S. Jackson and H. I. Schiff, J. Chem. Phys., 2 5 , 2333 (1955). (13) J. Berkowitz, W. A. Chupka, and G. B. Kistiakowsky, ibid., 2 5 , 457 (1956).
The Mass Spectra of the Three Monodeuterated Propenes
by S. R. Smith,lagbR. Schor,lc and W. P. Norrislb University of Connecticut, Storra, Connecticut, and U . S. Naval Ordnance Teat Station, Chino Lake, California (Receized November 9, 1964)
The mass spectra of propene-1-d1, propene-&d1, and propene-3-dl are presented for 50-v. electrons. The spectra are strikingly similar and show considerable rearrangement of the hydrogens and deuterium. A tentative explanation of the spectra is given on the basis of almost complete equilibration of the deuterated propene structures.
Introduction The mass spectra of deuterated hydrocarbons have been investigated in order to gain an insight into the mechanism of the reactions of these molecules under electron impact. Stevenson and Wagnerzahave studied a series of nionodeuterated alkanes, RiIcFadden and WahrhaftigZbhave studied four deuterated butanes, Bryce and Kebarle3 have studied 1-butene-4-dr, and RlcFadden4 has worked with the series of propenes, propene-l,l-dz, propene-3,3,3-d3 and propene-2-dl. All of these studies have indicated that there is a migration of the deuterium from the carbon atom to which it was originally attached prior to the carbon-carbon bond rupture to form the fragmented portions of the cracking
pattern. We are presenting a unique system that shows the effects of these exchanges on the mass spectra of the monodeuterated propenes.
Experimental Propene-14. This compound was prepared as described in the literature.6 (1) (a) Chemistry Department, University of Connecticut; (b) Research Department, U. S. Naval Ordnance Test Station: (c) Physics Department, University of Connecticut. (2) (a) D. 1’. Stevenson and C. D. Wagner, J . Chem. Phya., 19, 11 (1951); (b) W. H. McFadden and A. L. Wahrhaftig, J. A m . Chem. SOC.,7 8 , 1572 (1956). (3) W. A. Bryce and P. Kebarle, Can. J . Rea., 34, 1249 (1956). (4) W. H . McFadden, J. Phga. Chem., 6 7 , 1074 (1963).
Volume 69, Number 6 May 1966
S.R. SMITH,R. SCHOR, AND W. P.NORRIS
1616
Propene-2-dl. Pure 2-bromopropene, b.p. 45-46’ (705 mm.), was prepared in 75oj, yield by treating a methacrylic acid dibromide with one equivalent of NaHC03 in dry dimethylformamide at 100’. Propene-2-dl was then prepared in 75% yield from the 2bromoproperie using the same procedure as used for the preparation of p r ~ p e n e - l - d ~ In . ~ addition, the propene-2-dl was passed through a trap a t -48” to remove traces of higher boiling substances. Propene-%&. Allylmagnesium bromide6was treated with DzO and the product was worked up in the same manner as for pr~pene-l-dl.~ The n.m.r. spectra were run on liquefied samples of the propenes and in each case the spectrum agreed with the struzture assignment. In the case of propene1-&, the n.ni.r. spectrum indicated a cis-trans ratio of 60 :40. All of the samples were purified by gas chromatography using a 2.5-m. x 0.6-cm. column packed with 1.5% squalane on 30-60 mesh Pelletex and programmed from liquid nitrogen temperatures. The mass spectra were obtained with a Consolidated Electrodynaniics Corp. RIodel 21-103B mass spectrometer operated at an ionizing potential of 50 e.v. The mass spectra of the deuterated compounds and the comparison normal propene were corrected for normal CI3 content and are presented as percentage of total ionization in ‘Fable I.
Results and Iliscussion An examin:%tionof the mass spectra of the four propenes in Table I indicates that within experimental error the percentage of the total fragments in the C3+,C2+, C32+,Cl+, and hydrogen regions is constant and that the introduction of a deuterium into the various positions of the propene molecule does not affect either the probability of C-C bond rupture or the probabi1it.y of double ionization. C3 Ions. The C3 ions account for approximately 81% of the ;otal ions in each of the propene mass spectra. The parent molecule ion accounts for 1718% of the ions and is almost identical for propene and the nionodeuterated propenes. The peak due to the loss of 1 hydrogen ( m / e 42) in monodeuterated propene accounts for 24y0 of the ions and is the same for all the nionodeuterated propenes. The corresponding peak in normal propene is 27y0. The probability of the loss of a hydrogen from a deuterated molecule compared to the loss of a hydrogen from a nondeuterated molecule is expressed as I’, which is defined as I’=
The Journal of Physicdl Chemistry
Table I : Mass Spectra of the Propenes Propene-24 (CH3CDCHz) %Z
0’2710.34
2
3
0.07
1
12 13 14 15 16 17 I8 19 19.5 20 20.5 21 24 25 26 27 28 29
0.34 0.29,
1.37
0.30 0.08 0.09 0.52
i::;
0.94
I
14.53
2.00 ) 14.21 5.75 5.94
0.48 2.32 7.15 4.42
’ 14’60
36 37 38 39 40 41 42 43
where (P - H) is the ionization due to loss of hydrogen from the deuterated or undeuterated molecule and n is the number of hydrogens in the molecule. Our data lead to a value of r = 1.1 for all three deuterated propenes, a value which plots consistently as a function of percentage deuteration with AfcFadden’s data4 for the r calculated from propene-1 ,l-& and propene3,3,3-d3. The linear increase of I‘ with percentage deuteration in the various positions indicates a rapid equilibration of structures and a linear effect of the deuterium on the increased probability of the loss of hydrogen regardless of the initial positioning of the deuterium. The remaining ions in the C3 group from m/e 41 to 36 are due to the loss of a combination of additional hydrogens and/or deuterium. The ions make up 39% of the spectrum and the intensity at each of the niasses is almost identical in the three monodeuterated propenes. This detailed identity can most easily be explained if it is assumed that within the time it takes for the excited molecule ion to decompose to form the C3 ion fragments, complete or aliiiost coni~~
(5) W. P. Norris, J . Org. Chem., 24, 1579 (1959). (6) N. Rabjohn, “Organic Syntheses,” Coll. 1‘01. IV, John Wiley and Sons, Inc., New York, N. Y . , 1963, p. 749.
MASSSPECTRA OF
THE
THREE MONODEUTERATED PROPENES
plete exchange occurs leading to an equilibration of structures. C2 Ions. The C2 ions account for 14-15% of the ions in all of the propene mass spectra. Although the total number of C2 ions is the same in all the spectra, detailed differences exist in this region. These differences can be related to different rates of exchange between the 1,3 and 2,3 positions of the molecule. On the basis of the 27 arid 26 peaks in normal propene, we have corrected the mass 27 peak for C2HD contribution and calculated the intensity of CzHzD+ and C2&+ in each of the deuterated propenes. These values are given in Table 11.
Table I1 CZHJ
Ratio, exchanged: unexcbanged
?& equi-
CZHZD
5.94 7.09 4.42
4.44 3.91 6.17
4.44: 5.94 3.91 :7.09 4.42:6.17
75 55 72
+
CHaCHCHD CHaCDCH2 CHzDCHCH2
+
librium
Examination of data in Table I1 shows that for the 1-dl and 3-dl nioriopropene approxiniately 757@equilibration is attained. However, in the case of the 2-dl propene, equilibration occurs to only about 55%. If the niethyl group is lost in the fornietion of the C2 ion, these data indicate that exchange between the 2 and 3 positions or exchange between the 1 and 2 positions and subsequent transfer to the 3 position is a slower process than the rupture of a C-C bond. C1 Ions. The C1 ions constitute less thari 4% of the ions in the propene spectra. One should expect that the exchanges observed in the Cz region should be reflected in the C1 ions. On the basis of the dissociation of normal propene we have corrected the mass 15 peak in the deuterated propenes to calculate the intensities of CH2D+ and CH3+. We have calculated the ratio of ions due to exchange and the data are listed in Table 111. These data agree for the percentage of exchange and rearrangeiiient found in the C2 region of the 1-d1 and 2-dl propene, but there is a discrepancy in the case of 3-dl propene. It is apparent from these data that the iiiethyl ion is fornied more rapidly than the time necessary to achieve a 3,l or 3,2 transfer or exchange statistical equilibrium.
1617
Table 111
CHaCHzCHU CH&I>CH* CHzlXHCH2
CHs+
Ratio, exchanged: unexchanged
%, equi-
CHzD+
0 61 0 53 1 08
1 28 0 85 0 36
0 61:l 28 0 .i3:0 85 0 3 6 : l 08
48 62 33
librium
Voge, Wagner, and Stevenson' have analyzed the mass spectrum of propene-3-C13 arid have pointed out the result of a 3,l hydrogen transfer on the ion fragnients in the C2 and C1 mass ranges. Their results indicate that equilibrium between the structures kH3CHCH2+ eH&HCH3+ is complete before decomposition occurs to form the C2H3+ type ion and 60% coniplete before the formation of the CH3+ type ion. These results with C13 as a marker on the propene are higher thari when deuterium is used (as in CHzD). This difference is to be expected since in C13H3CHCH2any 3,l hydrogen transfer will give a new isomeric structure that can be detected by the mass spectrometer, whereas in CH2DCHCH2the deuterium niust undergo transfer to give a detectable new structure.
Conclusion The inass spectra of the l-dl, 2-dl, and 3-dl nionopropenes have been presented and an explanation of the distribution of ion intensities has been given on the basis of almost complete equilibration of deuterated propene structures. This rapid equilibration leads to an alniost indistinguishable fragnientation in the C3 region. The ion fragments in the C2 region are explained on the basis of 1,3 and 2,3 transfer or exchange of deuterium and hydrogen. The intensities of the niethyl ion fragnients are riot completely accounted for by this model. Acknowledgment. We wish to acknowledge the assistance of Mr, Joseph H. Johnson, who carried out soiiie of the gas chromatographic purifications and operated the mass spectrometer. Nrs, Helen R. Young assisted i n the data reduction. We are grateful to I I r . D. W. ;\loore for the n.11i.r. runs and interpretation. (7) €1. H. Voge, C. D. Wagner, and D P. Stevenson, J . Catalysis, 2 , 58 (1962).
Volume 69. Sumber 6
M a y 1966
