Skeletal and Hydrogen Rearrangements in the Electron Impact Mass Spectrum of Propene

Alex. G. Harrison and Paul P. Dymerski Department of Chemistry, University of Toronto, Toronto, Canada M5S 1Al

The mass spectrum of pr0peoe-2-[~'C] shows 81% retention of C-2 in the [C2H,]+ fragment ion at 70 eV electron energy, decreasing to 75% C-2 retention at low electron energies. The mass spectra of propene-2- d,, propene-1,1,3,3,3-d5, propene-1,1,2-d3 and propene-3,3,3-d3 also have been examined at a resolution sufficient to resolve H2-D doublets. The results at 70 eV electron energy show complete H/D randomization prior to fragmentation to form [C,(H, D),]+ but, in agreement with the "C labelling data, incomplete H/D interchange prior to fragmentation to form [C,(H, D)']'. The results are interpreted in terms of a reversible isomerization of the propene molecular ion to a cyclopropane structure in competition with fragmentation.

The 75 eV mass spectrum of pr0pene-3-['~C] shows' approximately 50% 13C retention in the [C2Hs]' frag- ment ion. This has been interpreted in terms of 1-3 hydrogen shifts in the molecular ion prior to fragmen- tation. Early studies of the mass spectra of deuterium labelled propenes have been variously interpreted as

predominant 1-3 hydrogen interchange or as indicating4 complete randomization of all hyd- rogens prior to fragmentation of the molecular ion. A recent study5 of fr;agmentation reaction (1) occurring in the first drift region of a double

[C3(H, D)61t+ [C3(H, D)51'+ (H, D)' (1)

focusing mass spectrometer showed that complete hydrogen randomization had occurred prior to frag- mentation, supporting the suggestion6 that the in- terpretation of the earlier deuterium labelling results were complicated by H/D isotope effects and interfer- ence from isobaric ions in the mass spectra.

The observation of complete hydrogen randomiza- tion at low internal energies is consistent with the results obtained for labelled b ~ t e n e s ~ - ~ and pen- tenes," where extensive H/D interchange is observed at high internal energies, with essentially complete H/D scrambling at low internal energies. For the butenes,13C labelling879 has shown that extensive skeletal rearrangement also occurs prior to fragmenta- tion. While the deuterium labelling results for the butenes czn be accommodated by a series of 1-2 hydrogen shifts, the 13C labelling data implicate more complicated mechanisms. Both the electron impact ~ t u d i e s ~ ' ~ and a field ionization kinetics study" sup- port an isomerization mechanism involving a sequence of reversible 1-3 ring closures to form methylcyclopropane-type ions for the butene systems.

By analogy with the butenes one might have antici- pated that a similar ring-closing mechanism would lead to skeletal isomerization in the propene system. Therefore, we have re-examined the extent of skeletal isomerization over a range of electron energies using

@ Heyden & Son Ltd, 1977

~ropene -2 - [~~C] . In addition, we have examined the mass spectra of deuterium labelled propenes at a mass resolution sufficient to resolve €3,-D doublets to deter- mine the extent of H/D interchange in molecular ions prior to fragmentation at higher internal energies than those pertaining to the earlier metastable ion study.5 The results show that H/D interchange leads to com- plete randomization before fragmentation to form [C3(H, D)']+ but not before fragmentation to [C2(H, D)J. The 13C labelling results show an energy- dependent extent of skeletal rearrangement consistent with reversible formation of a cyclopropane-type ion prior to fragmentation.

EXPERIMENTAL

The 70eV mass spectra of propene-2-dl, propene- 1,1 ,3,3,3-d5, propene-1,l ,2-d3 and propene-3,3,3-d3 were obtained at a resolution, M/AM = 40 000, suffi- cient to resolve H2-D doublets, using an A.E.I. MS-50 mass spectrometer at a source temperature of 150 "C. Samples were introduced from a room temperature inlet system.

The mass spectrum of ~ r o p e n e - 2 - [ ~ ~ C ] was ex- amined using an A.E.I. MS-902 mass spectrometer operating at a resolution, M/AM = 15 000, sufficient to resolve CH-[13C] doublets. The source was operated at a temperature of 150°C with the sample being introduced from a room temperature inlet system. The ratios ['3cc2H6]'/[c3H6]', ['3CCH3]'/[C2H3]' and [13CCH2]'/[C2H2]' were determined by repeated scanning of the relevant peaks over the electron energy range from 70 eV to -13 eV nominal, the lower limit being determined by the signal-to-noise ratio on the lower intensity fragment ions.

The pr0pene-2-['~C] (-900/, 13C) was obtained from Merck, Sharp and Dohme, Montreal, as were the deuterium labelled propenes. The latter contained up to 4% isotopic impurity as described previously.' No corrections have been applied to the fragment ion intensities where contributions might arise from these impurities.

ORGANIC MASS SPECTROMETRY, VOL. 12, NO. 11, 1977 703

A. G. HARRISON AND P. P. DYMERSKI

RESULTS AND DISCUSSION

Fragmentation reaction [C3(H, D)6t-+ [C3(H, D)& + (H, D)'

Table 1 records the [M-H]'/[M-D]+ ratios ob- tained from the high resolution mass spectra of the complimentary labelled propenes, propene-2-dl- propene-1,1,3,3,3-d5 and propene-1,1,2-d3, propene- 3,3,3-d3, at 70 eV ionizing electron energy. Following standard the following expressions may be written for the intensity ratios

[M-H]* I ( l - a2 ) i Propene-2-d, : -- [M - D]' a 2

[M-H]+ - a2i Propene- 1,l ,3,3,3-d5: -~

[M-D]+ I - a z [M- HIt - a3i Propene-1,l ,2-d3: -- [M-D]+ l -a3 [M - HI' - (1 - a3) i Propene-3,3,3- d3: -- [M - D]+ a3

Where a2 represents the fraction of H lost from the 2 position, a3 the fraction lost from the 3 position, and i represents an average isotope effect, kH/kD, for the four compounds studied. From the experimental in- tensity ratios we obtain the values of a2, a3 and i recorded in the ffinal two columns of Table 1.

The results in Table 1 indicate an average isotope effect of 1.7-2.0 favouring H loss over D loss, substan- tially lower than the isotope effect observed5 for [C3(H, D)Jt ions fragmenting in the first drift region. This result is in general agreement with the observed effect of internal energy on H/D isotope e f f e~ t s . ' ~ The results also indicate that at 70eV the relative prob- abilities of H loss from each position in propene ((~1:c~2:(~3 = 0.33:0.19:0.48) are in satisfactory agree- ment with the results obtained5 at low internal ener- gies ( ( ~ 1 : ( ~ z : ( ~ 3 = 0.28:0.24:0.48) and in good agree- ment with the relative probabilities calculated assum- ing complete equivalence (scrambling) of the hydrogens prior to fragmentation (a 1 : a2 : a 3 = 0.33 : 0.17 : 0.50).

The present results along with the earlier metastable ion studies'' show that complete hydrogen randomiza- tion occurs in the molecular ion of propene prior to fragmentation by loss of a hydrogen atom at all inter- nal energies. These observations are not consistent with 1-3 hydrogen shifts only. Although they could be explained by a series of 1-2 hydrogen shifts the 13C results discussed below indicate that isomerization of the carbon skeleton also occurs, leading to hydrogen randomizalion.

Table 1. Formation of [C,(H, D)J+ in mass spectra of label- led propenes

Propene [M-HI'/[M-D]+ i=k" Fraction of H loss kD

3,3,3-d3 1,1,2-d3

1 .72 a* = 0.19

1 .9, a3 = 0.48

2-d3 7.25

2.09 1.82

1,1,3,3,3-d5 0.406

Table 2. Abundance ratios and % C-2 retention in mass spectrum of pr0pene-2-['~C]'

% C-2 Retention in Electron energy ["CCH31+ I"CCHZI+ IC~HJ+I ICzHzI+ [CzH,l+ [CzH21+ (eV nominal)

70 2.788 4.43, 81 .3 go.,

12.8 2.15, - 75.3 -

18.0 2.676 2.19, 80.4 75.8 15.0 2.663 1.38, 80.3 63.8

Fragmentation reaction [CH2=13CHCH3]'+ [(13C, C),Hj]+ + (13C, C)H3'

As discussed above the 75eV mass spectrum of ~ r o p e n e - 3 - [ ~ ~ C ] showed 50% I3C retention in the vinyl ion suggesting equilibration of C-1 and C-3 by 1-3 hydrogen shifts in the molecular ion prior to fragmentation. A more sensitive test of carbon skeletal isomerization can be obtained by examining the mass spectrum of propene-2-[13C] since if only 1-3 and 1-2 hydrogen shifts occur the vinyl ion should retain quan- titatively C-2.

Table 2 records the ['3CCH3]'/[C2H3]' and ['3CCH2]+/[C,H2]' intensity ratios observed as a func- tion of electron energy for a sample of propene-2- ["C] with a ['3CC2H6J'/[C3H6]' ratio of 9.174. From these data one may calculate by standard proceduresg the O/O retention of C-2 in the [C2HJ and [C2H2JC fragment ions. These are given in the final two col- umns in Table 2. At 70eV electron energy the [C2H3]+ ion shows 81% C-2 retention decreasing to 75% retention at the lowest electron energy (12.8 eV nominal) at which measurements could be made. Re- tention of the intact carbon skeleton would lead to 100% C-2 retention regardless of the extent of H interchange by 1-2 or 1-3 hydrogen shifts while com- plete carbon randomization would lead to 67% C-2 retention. In contrast to the interpretation' of the results for propene-3-[13C], the results for propene-2- ["C] clearly show incomplete carbon skeleton isomerization at 70 eV with the extent of isomeriza- tion increasing with decreasing ion internal energy. The 50% 13C retention observed in the vinyl ion produced from propene-3-[13C] presumably results from fragmentation without hydrogen or carbon rear- rangement (0% 13c retention) as well as fragmenta- tion from a precursor where skeletal isomerization has occurred (67% 13C retention), although some equilib- ration of C-1 and C-3 through 1-3 hydrogen shifts cannot be excluded.

The formation of [C2H2] shows 90% C-2 retention at 70eV decreasing to 64% at low internal energies indicating, for this process as well, extensive carbon skeleton isomerization prior to fragmentation of the molecular ion.

Fragmentation reaction [C3(H, D)6]f -+ [C,(H, D)J'+ C(H, D)-3

Table 3 presents the relative intensities of the [C2(H, D)J' ions observed in the 70 eV mass spectra

704 ORGANIC MASS SPECTROMETRY, VOL. 12, NO. 11,1977

SKELETAL AND HYDROGEN REARRANGEMENTS

Table 3. Distribution of [C,(H, D)J+ ions from labelled propenes'

Propene m/e Formula 3,3,3-d5 1,1,2-d3 2-d, 1.1.3.3.3.-d6 30 tC2D31+ 0.082 0.230 - 0.395 29 [CZDzHI+ 0.358 0.315 - 0.605 28 [CzDH2]+ o.3lO o.373 0.63, - 27 IC2H31' 0.24, 0.082 0.363 -

~~~~~ ~~

a Intensities expressed as fraction of total [C,(H, D),]+ signal.

of the four labelled propenes examined. The com- plementary pairs, propene-1,1,2-d3-propene-3,3,3-d3 and propene-2-dl-propene- 1,1 ,3,3,3-d5 give distribu- tions in good agreement with each other. For complete H/D randomization the d3 labelled propenes should give

[C,D,]':[ C2D,H]+ :[ C2DHJ: [ C2HJ =

0.05 :0.45:0.45:0.05.

The large relative intensity of [C2H3]" in the mass spectrum of propene-3,3,3-d3 and of [C2D3]' in the mass spectrum of propene- 1 , l ,2-d3 indicates signific- ant fragmentation to form the vinyl ion prior to any H/D interchange, although the large relative inten- sities for [C2D2H]' and [C2DH2]+ in both spectra indicate extensive interchange prior to fragmentation. Although the formation of these ions could be exp- lained by a series of 1-3 hydrogen shifts, the observa- tion of [C2D3]' in the spectrum of propene-3,3,3-d3 and of [C2H3]+ in the spectrum of propene-1,1,2-d3 indicates more complex H/D interchange processes. This is confirmed by the large [C2H3]' intensity in the mass spectrum of propene-2-dl and the large intensity of [C2D3]' in the mass spectrum of propene-1,1,3,3,3- ds , neither of which would be formed if only 1-3 hydrogen shifts were occurring. It is not clear whether the H/D interchange involving the H/D bonded to C-2 is solely the result of carbon skeletal isomerization or also indicates 1-2 hydrogen shifts.

In summary, the labelling results indicate complete hydrogen randomization prior to fragmentation of the propene molecular ion by loss of a hydrogen atom at all internal energies of the molecular ion sampled by

conventional electron impact mass spectrometry. By contrast, the 13C labelling data indicate incomplete carbon skeletal isomerization prior to fragmentation to[C2H3]+ + CH3'. The deuterium labelling results for the same reaction show incomplete hydrogen randomi- zation with a substantial contribution to [C2H3]+ forma- tion arising by simple C-C bond cleavage prior to any interchange reactions. Thus, one must conclude that hydrogen randomization in the propene molecular ion is fast compared with fragmentation by H atom loss, but that both H randomization and carbon skeletal randomization are, relatively speaking, slow compared with fragmentation to [C2H3]++ 'CH3. In terms of energetics, the cyclopropane molecular ion (AHf = 245 kcal mol-') is only slightly higher in energy than the propene molecular ion (AHf = 229 kcal mol-') and substantially lower in energy than either [C3H5]'+ H'

AH, = 278 kcal mol-l) or [C2H3]+ + 'CH3 (C AHf = 303 kcal mol-'). Thus, hydrogen and carbon randomi- zation in the propene molecular ion is possible through reversible ring closure to the cyclopropane structure. Indeed, as has been dem~nst ra ted , '~ the two forms of the [C3H6]' ion cannot be distinguished by metastable ion properties. Thus, it is not surprising that complete H randomization occurs prior to frag- mentation to [C3H5]' + H'. However, the activation energy to form [C2H3]+ + 'CH3 is considerably higher and this fragmentation reaction will be observed only for ions of high internal energy where the lifetime with respect to isomerization will be short, thus leading to incomplete randomization of the carbon skeleton, as observed. It should be noted that there is an apparent activation energy for isomerization of the cyclop- ropane molecular ion to thegropene structure since there is substantial evidence l6 that at low internal energies the cyclopropane molecular ion maintains its structural identity.

Acknowledgements

The authors are indebted to the National Research Council of Canada for financial support. A. G. H. gratefully acknowledges the kind hospitality of Professor K. R. Jennings during a sabbatical leave which provided access to the MS-50 mass spectrometer.

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Received 18 February 1977; accepted 2 May 1977

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ORGANIC MASS SPECTROMETRY, VOL. 12, NO. 11, 1977 705