Solanesol Pyrolyzes Via C 10 Biradical Philip Morris research group confirms new pathway of breakdown of tobacco component In addition to the two previously known pathways of forming terpenoid hydrocarbons by the pyrolysis of solanesol, a third—through the forma tion of a C 10 biradical—has been found by Dr. J. D. Grossman, Dr. R. M. lkeda, Dr. E. Deszyck, and Dr. A. Bavley of the Philip Morris Research Center, Richmond, λ a. Dr. Deszyck described the work at the Interna tional Union of Pure and Applied Chemistry meeting in London. Solanesol, a C4r, polyisoprenoid pri mary alcohol, is a major constituent of tobacco resin. Pyrolysis of it or its acetate yields isoprene and terpenoid compounds—important factors in the flavor of tobacco smoke. Earlier work by the Philip Morris group showed that solanesol and solanesyl acetate are precursors of the dipentene found in tobacco smoke. Dipentene is
formed both by direct breakdown of solanesol and by dimerization of the isoprene formed in pyrolysis. Evidence of the formation of a C 1 0 biradical is based on differences in yields of various C 1 0 hydrocarbons formed by pyrolyzing solanesol in hy drogen, as opposed to those formed in nitrogen. For example, pyrolysis at 500° C. gave terpene fractions with the following distributions: Fraction
Pyronenes Dimethyloctadiene Dimethyl vinyl cyclohexene A'-Menthene Dipentene
r /c Total terpenes Hydrogen Nitrogen atmosphere atmosphere
4.17 19.12
7.13 11.82
7.01 8.54 52.72
12.20 12.84 45.31
Recently, Dr. R. A. W. Johnstone and Dr. R. M. Quan of the Medical
Biradical mechanism confirmed by different yields of various terpene hydrocarbons on pyrolysis of solanesol in hydrogen vs. nitrogen atmospheres
38
C&EN
JULY
2 2,
1963
Research Council's unit at the Uni versity of Exeter, England [/. Chem. Soc, 2221 (1963) ] proposed a mecha nism for the formation of a C 1 0 biradi cal during the pyrolysis of solanesol, with subsequent cyclization to give products such as 1,3-dimethy 1-1 -vinyl cyclohexene and dipentene. R. A. Meyer and J. G. Burr of the North American Aviation Science Center, Canoga Park, Calif., recently found that the free radicals formed in the pyrolysis of toluenes can be termi nated by hydrogen [JACS, 85, 478 (1963)]. This led the Philip Morris group to believe that hydrogen might also terminate C1() biradicals formed in the pyrolysis of solanesol. Their work has confirmed that solanesol breaks up through the formation of a C 1 0 biradical as well as through direct breakdown to give dipentene. The considerable increase in the amount of dimethyloctadiene formed during pyrolysis of solanesol in a hy drogen atmosphere along with the de crease in pyronenes, dimethyl vinyl cyclohexene, and A'-menthene con firms that a dimethyloctadiene biradi cal is formed and terminates by cap turing two hydrogens, forming di methyloctadiene, the Philip Morris group says. Thus a lower yield of dimethyl vinyl cyclohexene and other products formed through the inter mediate biradical is obtained. The pyrolysis experiments were car ried out at 450° C. to 750° C. Sol anesol was passed under nitrogen or hydrogen through a 30-cm. silica tube filled with porcelain chips heated to the desired temperature. Material was added to the apparatus at a con stant dropping rate. The pyrolysis products were collected in a trap cooled with a dry ice-acetone mixture. The total pyrolysis product was sepa rated by distillation at 40° C. into the volatile pyrolysis components. The remainder of the mixture was then separated by fractionation on silica gel into a hydrogen and an oxygenated fraction. The hydrocarbon fraction was then separated into various com ponents by gas chromatography. Temperature is a critical factor in the production of the hydrocarbon components. Up to 550° C , the hy drocarbon fraction is composed mainly of monoterpenoids with traces of aromatic products. At higher tem peratures, aromatization is favored. Above 650° C , no terpenoids form from solanesol. Instead, various aro matic compounds are formed.
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Chemists Study High KE Ion Origin Electron bombardment of polyatomic organic molecules indicates high kinetic energy ions come from fragmentation of singly-charged ions Findings by three chemists at the Uni versity of California's Lawrence Radi ation Laboratory, Berkeley, could lead to some changes in the quasi-equilibrium theory of mass spectra. The find ings could also change application of both theory and mass spectral data to processes of radiation chemistry. Dr. John Olmsted III, Dr. Kenneth Street, Jr., and Dr. Amos S. Newton find that high kinetic energy ions (two to three electron volts), formed by electron bombardment of polyatomic organic molecules in a mass spectrom eter, are much more abundant than previously expected. They feel these ions come mostly from fragmentation of excited, singly-charged ions, rather than doubly-charged ions as previously postulated. Neither abundance nor origin can be determined directly with present mass spectrometric equipment because of "discrimination" present in ion sources. Discrimination is a catch-all term covering whatever processes might allow ions to be formed and not collected. There is a gap between the time of first ion formation in a mass spectrometer's ion source and the time the ions leave the source on the way to the detector. Ion formation takes about 10 1Γ) second, but it takes about 10~7 second for the ion to leave the ion source. During this period, the singly- or doubly-charged, positive par ent ion which is formed first breaks down into secondary or fragmentary ions. Some of these secondary ions have high kinetic energy. It then takes the ions another 10 ° second to reach the collector. A major reason for discrimination is that kinetic energy imparted to some fragment ions gives them trajectories which cause them to miss the col lector. The quasi-equilibrium theory, developed to explain observations of spectra accumulated over many years, does not consider the relative amount of high kinetic energy ions, principally because these did not appear to be significant at the time. Furthermore, scientists generally have assumed that these ions arise from a repulsive
breakup into two charged fragments of high kinetic energy. Abundance and Origin. Speaking to a symposium sponsored by the Di vision of Physical Chemistry at the University of Utah, Dr. Olmsted took issue with these assumptions. De tailed data on kinetic energies and appearance potentials, he said, had been collected by the California group for methyl and other fragment ions possessing excess kinetic energy. These were formed from methyl chlor ide, methylamine, and propane. Ap plying laws of conservation of mo mentum and energy to these data in dicates that all the high kinetic energy ions could not have been formed through fragmentation of doublycharged parent ions. Most of the fragments must have come from singlycharged parent ions, he believes. Repulsive singly-charged ionic states have not been used to explain the origin of high kinetic energy fragments from polyatomic organic molecules. In 1930, Dr. W. Bleakney at the Uni versity of Minnesota found that such a state gives rise to hydrogen ions having excess kinetic energy. To determine the relative abundance of methyl ions having excess kinetic energy, the three California workers first assumed that an ion peak recorded in a mass spectrometer is the sum of thermal and excess energy components. They then determined the thermal component by measuring the variation in height of a methyl ion peak under varying accelerating voltages. The curve was normalized to fit the thermal ion component at low accelerating voltages (where thermal and excess kinetic energy curves are resolved). The two curves were then separated at higher accelerating voltages by sub tracting the normalized thermal curve. Analysis of 15 compounds shows an abundance of excess kinetic energy methyl ions of up to 3% of the total ions formed, Dr. Olmsted says. But, due to discrimination, he feels these values underestimate the amounts of these ions that are formed. A better estimate comes from the extent of this
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discrimination. To find this, the California workers used an equation for collection efficiency developed by Dr. C. E. Berry at Consolidated Electrodynamics Corp. in 1950. This allowed them to estimate the abun dance of excess kinetic energy ions in the ion source. For the compounds tested, the amount of excess kinetic energy ions was found to be between 8 to 2 5 % of the total ions produced. An internal check of this method appears to substantiate it. Dr. Berry's equation also allows calculating varia tion of collecting efficiency with vary ing accelerating voltages. This pro cedure adequately reproduces the ex perimental curve for the methyl ion peak from benzene, composed almost entirely of high kinetic energy ions, Dr. Olmsted says. Differing Views. The assertions of a higher abundance of high kinetic energy ions along with the proposal of origin from singly-charged parent ions did not evoke complete agree ment at Salt Lake City. For example, these assertions did not agree with results presented by Dr. R. Taubert of Physikalisch-Technische Bundesanstalt, Braunschweig, West Germany (the German Bureau of Standards). Dr. Taubert has also made measure ments of the amount of kinetic energy following unimolecular decomposition of paraffin ions that yield both ther mal and high kinetic energy ions. Kinetic energies and abundances of both one- and two-carbon fragments from propane and one- and threecarbon fragments from butane fit the concept of origin from a doublycharged parent ion, he says. In both instances, laws of conservation of mo mentum and of matter hold with "ap proximate validity . . . too good to be accidental," according to Dr. Taubert. Discussion of the findings is certain to stimulate more work from both groups of investigators as well as others. The evidence points to a higher abundance of high kinetic energy ions than previously considered, both Dr. Olmsted and Dr. Taubert agree. The two groups differ on the absolute amount of these ions and their origin. At present, several work ers agree, there seems to be no way to rationalize these two points or to estab lish a clear superiority for either ex perimental method. The immediate goal is to fix accurately whether the origin is via singly- or doubly-charged parent ions and just how abundant fragment ions are.
