J. L. Holmes, J. K. Terlouw, and F. P. Lossing
2860 For a pertinent recent review, see D. G. I. Kingston, J. T. Bursey, and M. M. Bursey. Chem. Rev., 74, 215 (1974). G. Splteller, M. Spitelier-Friedmann.and R. Houriet, Monatsh. Chem., 97, 121 (1966); see also D. H. Hunneman and W. J. Richter, Org. Mass Spectrom., 6, 909 (1972). C. G. Macdonald, J. S. Shannon, and G. Sugowdz, Tetrahedron Lett., 807 (1963); H. Budzikiewicz, 2 . Pelah, and C. Djerassi, Monatsh. Chem., 95, 158 (1964); M. M. Green and J. Schwab, Tetrahedron Lett., 2955 (1968); R. S. Ward and D. H. Williams, J. Org. Chem., 34,3373 (1969); M. M. Green, R. J. Cook, J. M. Schwab, and R. B. Roy, J. Am. Chem. SOC., 92, 3076 (1970); J. L. Holmes, D. McGillivray, and R. T. B. Rye, Org. Mass Spectrom., 7, 347 (1973). J. H. Beynon and A. E. Williams, "Mass and Abundance Tables for Use in Mass Spectrometry", Elsevier. New York, N.Y. 1963. J. H. Futreli, K. R. Ryan, and L. W. Sieck, J. Chem. Phys., 43, 1832 (1965); K. R. Jennings, ibid., 43, 4176 (1965); S. Meyerson, R. W. Vander Haar, and E. K. Fields, J. Org. Chem., 37, 4114 (1972). M. M. Green, J. M. Moldowan, M. W. Armstrong, T. L. Thompson, K. J. Sprague, A. J. Hass, and J. J. Artus, J. Am. Chem. SOC.,in press. R. J. Liedtke and C. Djerassi, J. Am. Chem. SOC., 91, 6814 (1969); K. Christiansen, V. Mahadevan, C. V. Viswanathan, and R. T. Hoiman, Lipids, 4, 421 (1969); A. G. Harrison, Org. Mass Spectrom., 3, 549 (1970). J. Cable and C. Djerassi, J. Am. Chem. SOC.,93, 3905 (1971). For a closely analogous photochemical example, see R. Breslow and M. A. Winnik, J. Am. Chem. SOC.,91,3083 (1969); R. Breslow and P. C. Scholi, ibid, 93, 2331 (1971).
(15) See, for example, F. 0. Rice and T A. Vanderslice. J. Am. Chem. SOC., 80, 29 (1958); A. A. Zavitsas. ibid., 94, 2779 (1972); A. A. Zavitsas and A. A. Meiikian, ibid., 97, 2757 (1975). (16) W. Carpenter, A. M. Duffieid, and C. Djerassi, J. Am. Chem. SOC.,90, 160 (1968). (17) N. C. Rol, Red. Trav. Chim. Pays-Bas, 84, 413 (1965). (18) A. Beugelmans, D. H. Williams, H. Budzikiewicz, and C. Djerassi, J. Am. Chem. SOC.,86, 1386 (1964); W. Carpenter, Y. M. Sheikh, A. M. Duffieid, and C. Djerassi, Org. Mass Spectrom., 1, 3 (1968). (19) M. A. Winnik, C. K. Lee, and P. T. Y. Kwong, J. Am. Chem. Soc., 96,2901 (1974). (20) Cf. P. J. Derrick, A. M. Falick, A. L. Burlingame, and C. Djerassi, J. Am. Chem. SOC.,96, 1054(1974); R. P. MorganandP.J. Derrick, J. Chem. SOC., Chem. Commun., 836 (1974). (21) See, for example, W. J. Baumann, A. J. Aasen, J. K. G. Kramer, and R. T. Holman, J. Org. Chem., 38, 3767 (1973). (22) F. W. McLafferty, Anal. Chem., 29, 1782 (1957); J. H. Beynon, "Mass Spectrometry and Its Applications to Organic Chemistry", Elsevier, New York, N.Y., 1960, pp 275 ff; K. Biemann, "Mass spectrometry. Organic Chemical Appfications", McGraw-Hili, New York, N.Y., 1962, p 55. (23) W. E. van Heyningen. D. Rittenberg, and R. Schoenheimer, J. Bioi. Chem., 125, 495 (1938). (24) "Beilstein's Handbuch der Organischen Chemie", Vol. 2, Ill, SpringerVerlag, West Berlin, 1960, p 991. (25) "Beilstein's Handbuch der Organischen Chemie". Vol. 1, Ill, SpringerVerlag. West Berlin, 1958, p 1834.
The Thermochemistry of C2H40*+Ions John L. Holmes,' J. K. Terlouw,' Chemistry Department, University of Ottawa, Ottawa, Canada K I N EN5
and F. P. Losing+ Division of Chemistry, National Research Council of Canada, Ottawa, Canada K I A OR6 (Received May 1 I , 1976) Publication costs assisted by the National Research Council of Canada
The heat of formation of the molecular ion of vinyl alcohol has been measured by an electron impact method to be 181kcal/mol. The heat of formation of the transient neutral molecule is estimated to be -26.5 kcal/mol, whence the ionization potential is 9.0 f 0.15 eV. The significance of these results is discussed with particular regard to some recent metastable ion studies and to the generation of CzH40.+ ions in the mass spectra of aliphatic aldehydes.
Introduction
In a recent paper2 it was shown that the three isomeric CzHdO.+ ions (1-111) can unequivocally be identified by the CH2-CH21t
\0/
CH,CHO 1t I1
CH,=CHOHl? I11
I
characteristic metastable peaks accompanying their fragmentation by H atom loss:
-
C*HdO*+
C2H30+
+ H.
Two further forms of C&140-+were observed in these experiments but their structures were not confirmed. Although the ionic heats of formation of I and I1 are well-established, there is no reliable value for I11 since the parent neutral molecule cannot be isolated. However, the unequivocal participation of I11 in certain ionic fragmentations2 provides a means of deriving its heat of formation as a fragment ion. The Journal of Physical Chemistry, Vol. 80, No. 26, 1976
In this paper we report measurements of the heats of formation of C*H40.+ ions I and I1 by direct ionization of parent molecules, and I11 as a primary fragment ion in decompositions of known mechanism. Experimental Section
All compounds were of research grade purity or were purified by glc. Metastable peak measurements were performed on a G.E.C.-A.E.I. MS902S mass spectrometer. The electron impact apparatus comprising an electrostatic electron monochromator3 together with a quadrupole mass spectrometer and minicomputer data system4 has been described elsewhere. Results and Discussion The ionization potentials (IP) of ethylene oxide and acetaldehyde measured in this work are in close agreement with those obtained in photoionization s t ~ d i e sThese . ~ IP and the derived heats of formation for structures I and I1 are given in
Thermochemistry of C2H40-+ Ions
2861
TABLE I: Heat of Formation of C,H,O-+ Molecular Ions --
Precursor molecule Structure AH,, kcal/mol
AP( c,H,o.+), eV
Neutral fragment
CH -CH,
fa/
CH,CHO
-39.70
CH,-CHOH
I
I
-32.2b
c*H4
U
f (C Z H D +1, kcal /mol
10.57
231
10.23
196
9.87
182
CI-12-CH,
CH, =C HOC, H, CH,(CH,),CHO (CH,),CHCH,CHO CH,CH,OH
-33.60 -48.9a -56.5C -56.20
CH,-CHCH,OH
\o/
0
0
C2H.l C3H6 H2
10.19 10.52 10.57 Q 10.45d
-59e
CH,O
10.30
204.5
-71.10
CH, 0
10.87
205.5
2'
H4
189 181
182
< 185d
0 Reference 6. b Derived from AHf(cyc1obutane) - 6.78 kcal/mo16 and the average of the differences AHf(cyc1opentane) iWf(cyclopentana1) = 39.5 kcalimol and AHf(cyc1ohexane) - AHf(cyc1ohexanal) = 38.9 kcal/mol. c AHf(isovalera1dehyde) taken as AHf(va1eraldehyde) - 2 kcal/mol.6 d Long-tailed curve, suggesting upper limit only. e From AlJf(1iquid) = -71.3 kcal/mo16 and estimated AH(vapor) = +12 kcalimol.
Table I. Also given are the appearance potentials (AP) and derived heats of formation for CzH40.+ ions formed as a fragment in the elimination of a neutral olefin (or Hz) from five compounds in which the metastable peak characteristics have indicated that the vinyl alcohol ion was produced.2 The data show that the C2H40.+ion formed in these processes is nearly 50 kcal/mol more stable than I and 14 kcal/mol more stable than 11. Since the AP may include some kinetic energy, the preferred value for AHf(CzH4O.+) will lie a t or below the lower end of the range shown in Table I. We propose therefore that AHf(viny1 alcohol.+) = 181 kcal/mol. Although neutral vinyl alcohol cannot be isolated, its heat of formation can be assessed using simple additivity. For example, the effect on AHf of substituting vinyl for ethyl in straight-chain compounds is +30.0 f 2.0 kcal/mol$ thus going from ethanol (AHf = -56.2 kcal/mol) to vinyl alcohol leads to AHffor the latter = -26.2 f 2.0 kcal/mol. Similarly, the effect of substituting hydroxyl for methyl in n-alkanes is -31.5 f 0.2 kcal/mol; thus going from propene (AHf = -4.9 kcal/mol) to vinyl alcohol, AHf for the latter = -26.6 f 0.2 kcal/mol. A reasonable value for AHdvinyl alcohol) is therefore -26.5 f 2.0 kcal/mol. From AHf(viny1alcohol.+) = 181kcd/mol, the IP for this compound is derived to be 9.0 f 0.15 eV. Note that the last two compounds in Table I produce CzH4O-+ ions with AHf's which are above those of the vinyl alcohol and acetaldehyde molecular ions, but below that for ethylene oxide. The metastable peaks for H atom loss for these two ions were clearly both of the vinyl alcohol type.2 The metastable peaks for their formation from their precursor molecular ions were of Gaussian shape, showing that no significant reverse activation energy was involved. We tentatively propose therefore that the CzH40.+ ion in these two cases may be a fourth structural species, which however decomposes over the same potential energy surface as does the vinyl alcohol ion and thus produces a similar metastable peak for H atom
neutral CzH40.The reaction is believed to proceed via a sixmembered intermediate: OHlt "CHR I d U - P C,H3R 'CH,
$.
I
" HLi
-
CzH40
+
+ CLHSRl?
The most recent experiments are a field ionization kinetic (FIK)study by Morgan and D e r r i ~ kThese . ~ authors showed that at times in excess of 20 ps (following ionization) CzH40.+ was the favored product in the y-hydrogen rearrangement of 1-hexanal, whereas for 3-methylpentanal the C4Hg+ ion predominated. For both molecules the CzH40-+/C4Hg+ ratio fell steeply between 20 ps and the microsecond region, more sharply in the case of the branched aldehyde. It was argued that the suppression of CzH4O.+ in 3-methylpentanal supported the proposed mechanism in which y-hydrogen transfer and cleavage occur as discrete steps, not as a concerted reaction. They also proposed that on p cleavage the charge is retained by the product of lowest ionization potential. Two estimates for the ionization potential of I11 were then available, namely, 9.5 eV by Meyerson and McCallums and a more recent estimate of 9.25 eV by Bentley and Johnstone.9 Thus, in the case of 3-methylpentanal, Morgan and Derrick suggested that vinyl alcohol and but-2-ene (IP 9.13 eV)5 compete for the charge with the latter succeeding. Note that for this molecule the hydrocarbon can be produced directly, without hydrogen rearrangement:
loss.
The present result for the IP of vinyl alcohol has particular relevance to the long-standing controversy concerning charge location and the mechanism of ion rearrangement in olefin elimination from the molecular ion of aliphatic aldehydes. This reaction generates two pairs of partners: CzH4O.+ of structure I11 plus a neutral olefin, and an olefin ion plus a
H2
In contrast, 1-hexand without hydrogen rearrangement would give but-1-ene, whose IP, 9.58 eV, exceeded the estimates for vinyl alcohol. Hence the larger ratio for CzH40-+/C4Hgf compared with that for the branched aldehyde would be exThe Journal of Physical Chemisfry, Voi. 80, No. 26, 1976
2862
J. L. Holmes, J. K. Terlouw, and F. P. Lossing
plained. For 1-hexanal, however, the ratio falls appreciably below a value of unity at times exceeding -100 ps and this was interpreted as an indication that hydrogen rearrangement yielded but-2-ene at these longer times. From the previous2 and present results there can be little doubt that the charged C2H40 species is indeed ion 111. This ion is not converted into any of its isomeric forms up to its threshold for H atom loss. Much of the argument of Morgan and Derrick rests on the assumption that the ionization potential for vinyl alcohol is larger than that of but-2-ene. This is not in agreement with the present findings. However, their observations can be better analyzed by consideration of the thermochemistry of the possible pairs of products, rather than the ionization potential alone. The pairs are listed below, with individual heats of formation, and the sum for the product pairs. Reactions involving the more energetic ionic forms I and I1 are clearly not competitive, and have not been included. SSlf(products), kcallmol CGH120*+
+
-+
+
CHzCHOH*+ CHz=CHCHzCH3 (181) (0) CHzCHOH.'
+ CH3CH=CHCH3
181
(1)
179
(2)
(-2) -+
CHZCHOH (-26.5)
-*CHzCHOH
+ CH*=CHCH&H3.+
194.5 (3)
(221)
+ CH3CH=CHCH3.+
181.5 (4)
(208) +
-
+
181
(5)
+ CH3CH=CHCH3*+
168
(6)
CH3CHO CH2=CHCH2CH3.+ (-39.7) CH3CHO
((AHf(g) = -59.4 kcal/mol)l'J was measured to be 9.89 eV, compared with the calculated heat of reaction 6,9.87 eV. Since the calculated appearance potential for the other reactions generating C*Hs.+ (reactions 3 , 4 , and 5) are in excess of 10.4 eV, it appears that there is no significant impediment to the occurrence of reaction 6 on the time scale of these measurements (residence time in the ion source 30 ks).l1This is confirmed by the presence of a weak metastable peak of Gaussian shape having a small To.5 = 0.019 eV, indicating the absence of any reverse activation energy. The appearance potential of CzH40.+ ion from 1-hexanal was found to be about 10.7 eV, the uncertainty arising from its rather gradual onset. The calculated appearance pot,ential for reactions 1and 2 are -10.4 eV. Formation of acetaldehyde or ethylene oxide ions would require an AP of 11.0 and 12.5 eV, respectively. No metastable peak could be discerned for this fragmentation in the first field free region indicating that this reaction does not occur to any appreciable extent on the microsecond time scale. This observation is in keeping with the appearance potential measurements. Very similar observations were recorded for 3-methylpentanal. (AP m/e 44 = 10.88 eV, calcd for [vinyl alcohol.+] and but-2-ene = 10.42 f 0.1 eV; AP mle 56 = 9.86 eV, calcd for [but-Z-ene.+]and CH3CHO = 9.96 f 0.1 eV.)12 Again the formation of m/e 56 was accompanied by a metastable peak (To5 = 0.012 eV) and none was observed for CzH40.+ production.
Reactions 1-4 are those considered by Morgan and Derrick.; However, if the hydrogen rearrangement generating but-2-ene from 1-hexanal is a reasonable proposition, one should also allow the analogous rearrangement to give the most stable neutral C2H40species, acetaldehyde, as in reactions 5 and 6. It can be seen that reaction 6 is much less endothermic than reactions 1-5, and under conditions where sufficient time is available for rearrangement it will predominate. Thus for 1-hexanal, reaction 1 will give way to reactions 2 or 5 and finally to reaction 6 at increasing times. Similarly, for 3methylpentanal reaction 4 competes favorably with reaction 2 at short times, but will be overtaken by reaction 6 at longer times. The great decrease in the C2H40.+/C4H8sf ratio with time for both molecules is thereby explained. It is necessary to show that there is no significant barrier, such as a reverse activation energy, to the occurrence of reaction 6. The appearance potential for CdHg+ from 1-hexanal
The Journal of Physical Chemistry, Vol. 80,No. 26, 1976
Acknowledgments. J.L.H. wishes to thank the National Research Council of Canada for continuing support of this research. J.K.T. thanks the Netherlands Organization for the Advancement of Pure Research (Z.W.O.) for a Fellowship during the tenure of which this work was initiated. References and Notes Permanent address: Analytical Chemistry Laboratory, University of Utrecht. Croesestraat 77A, Utrecht, The Netherlands. J. L. Holmes and J. K. Terlouw, Can. J. Chem., 53, 2076 (1975). K. Maeda, G. P. Semeiuk, and F. P. Lossing, lnt. J. Mass Spectrom. /on Phys., 1, 395 (1968). (4) F. P. Losslng and J. C. Traeger. lnt. J. Mass Spectrom. /on Phys., 19, 9 (1976). (5) J. L. Franklin, J. G. Diliard, H. M. Rosenstock, J. T. Herron, K . Draxl, and F. H. Field, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 26, (1969). (6) J. D. Cox and G. Pilcher, "Thermochemistry of Organic and Organometallic Compounds," Academic Press, New York, N.Y., 1970. (7) R. P. Morgan and P. J. Derrick, Chem. Commun., 836 (1974). (8)S. Meverson and J. D. McCollum, Adv. Anal. Chem. Instrum., 2, 179 ( I 963j. (9) T. W. Bentley and R. A. W. Johnstone, Adv. Phys. Org. Chem., 8, 242 (1970). (IO) D. R. Stuli, E. F. Westrum, Jr., and G. C. Sinke, "The Chemical Thermodynamics of Organic Compounds," Wiley, New York, N.Y., 1969. (11) The alternative fragmentation yielding C3H60.' and & t i 6 has a minimum (calcd) AP of 10.6 eV. (12) The uncertainty in the calculated AP lies In the estimate for A/+,(3methylpentanal) = -61 f 2 kcal mol-'.
