Letter: Determination of double bond position in di-unsaturated alkadienes by means of mass spectrometry of dimethyl disulfide derivatives. Dear Sir.
C. Pepe et al., Eur. Mass Spectrom. 1, 209–211 (1995)
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C. Pepe et al., Eur. Mass Spectrom. 1, 209–211 (1995) Letters
Letter: Determination of double bond position in di-unsaturated alkadienes by means of mass spectrometry of dimethyl disulfide derivatives
Dear Sir The determination of the position of a double bond in unsaturated compounds using mass spectrometric methods after derivatization by dimethyl disulfide (DMDS) has been widely studied in olefins, fatty acids and wax esters.1–6 The use of DMDS addition to locate the double bonds for di-unsaturated compounds is uncommon. This method has been used with acetates, natural long chain alkadienes,7,8 but has never been tried in a systematic way with short chain alkadienes. In this paper, compounds of the type CH3– CH=CH–(CH2)n–CH=CH–CH3 (n varies from zero to four and greater than four) have been studied. The structure of the DMDS adducts has been discussed with respect to the localization of the double bonds, but no consideration of stereochemistry has been given. Electron impact (EI) mass spectra were obtained on an Ion Trap Varian Saturn mass spectrometer coupled to a Varian 3400 gas chromatograph. Chemical ionization (CI) mass spectra were obtained by using ammonia as reagent gas. The samples were injected in the SPI mode. The column employed was a fused silica capillary (30 m long, 0.25 mm i.d.) coated with DB5 phase (Chrompack). The temperature increased at 2°C min–1 from 40°C to 250°C. The di-unsaturated compounds (2-6 octadiene and 2-4 octadiene) were synthesised in our laboratory by classical methods. Hexane and dimethyl disulfide were distilled. Iodine and Na2S2O3 were used as received. Alkadiene (50 µg) was treated in 100 µl of hexane by addition of 100 µl of DMDS and 20 µl of iodine solution (60 mg of iodine in 1 ml of diethyl ether). The reaction was carried out in a closed tube for 48 h at 50°C. The excess of iodine was reduced with Na2S2O3 solution (5% in water). The organic phase was removed and the excess of DMDS was evaporated. The dry extract was diluted with 50 µl of hexane and then analysed by CG/MS. Derivatives were formed by the addition of DMDS (Scheme 1). Depending on the values of n, a mono or a di-addition is observed and the pathway of fragmentation is different depending on the mode of addition.
R1
R2 C C
R3
Scheme 1.
+ CH3 S R4
S
CH3
I2
R1 R3
R2 C
C
R4
SCH3 SCH3
Figure 1. Mass spectra of 2-4 octadiene in EI and CI mode after derivatization by DMDS.
n = 0 (2-4 octadiene) A molecular ion (M+• = 204) is observed, confirmed by the spectrum in CI mode (Figure 1). Only one molecule of DMDS is added in 1-4 position (classical addition for conjugated diene). The allyl position of the sulfur atom only allows us to obtain the usual loss of CH3S or CH3SH from the molecular ion (m/z 157, 156, 109). It is impossible to locate the double bonds, but the spectrum is completely different from a monoalkene spectrum. n = 1 (1-4 hexadiene) The mass spectra of the DMDS derivative (Figure 2) shows a mixture, unseparated by GC, of the two mono-derivatized compounds M+• = 176 proved by CI spectra. On the contrary DMDS adducts of di-unsaturated acetates and alcohols give a di-addition.7 In each spectrum the characteristic cleavage of the bonds between the two carbon atoms linked to the sulfur atom is observed, ions at m/z 61 and 115 for the first and m/z 75, 101 and 161 for the other, allowing us to determine the position of the double bonds. Ions corresponding to the loss of CH3S and CH3SH from the molecular ion (m/z 128, 129) are important.
© IM Publications 1995, ISSN 1356-1049
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Letters
Figure 2. Mass spectra of 1-4 hexadiene in EI and CI mode after derivatization by DMDS.
n = 2 (2-6 octadiene) The molecular peak of the di-adduct is not present in the EI and CI spectra. Only a cyclic ion is generated according to Scheme 2 confirmed by EI and CI spectra. From this ion (m/z 236), the classical cleavage is observed (m/z 161) and the loss of CH3SH from the key fragment (m/z 113) allows us to locate double bonds (Figure 3) in this symmetrical molecule. Loss of CH3S from the molecular ion is always the most important fragmentation (m/z 189). Figure 4. Mass spectra of 1-6 heptadiene in EI and CI mode after derivatization by DMDS. CH3 CH CH CH2 CH2 CH CH CH3
n = 3 (1-6 heptadiene) I2
CH3
CH SCH3
DMDS
S
CH CH3 SCH3
Scheme 2.
The two mechanisms described for n = 1 and n = 2 coexist and two chromatogram peaks are observed. One is constituted by the monoderivatized compound (M+• = 190) and the second by the cyclic thioether adduct (M+• = 222). Chemical ionisation confirms these results. Fragment ions from the mono-derivatized compound (m/z 129) and from the cyclic adduct (m/z 161, 61) indicate the original position of the double bonds (Figure 4). Ions at m/z 143, 142 and 175 correspond to the loss of CH3S or CH3SH from the molecular ion. Ions at m/z 95 and 127 correspond to the loss of CH3SH from ions at m/z 143 and 175. n = 4 (1-7 octadiene)
Figure 3. Mass spectra of 2-6 octadiene in EI and CI mode after derivatization by DMDS.
The molecular peak exists in EI mode (M+• = 298), the cyclic ion is not generated. Classical fragmentations are observed (m/z 61, 237) allowing us to locate the double bonds as shown in Figure 5. Once again, ions corresponding to the loss of CH3S and CH3SH from the molecular ion have the most important relative intensity (m/z 251, 203). When n is higher than four, the results have been reported recently,8 and are equivalent to n = 4.
C. Pepe et al., Eur. Mass Spectrom. 1, 209–211 (1995)
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2. 3. 4. 5. 6. 7. 8.
Figure 5. Mass spectra of 1-7 octadiene after derivatization by DMDS.
The behaviour of the DMDS derivatives of alkadienes by mass spectrometry depends on the number of carbon atoms lying between the two double bonds. The present derivatization method was applied to a small number of di-unsaturated compounds. Nevertheless it is clear that their behaviour is different from other classes of compounds such as acetates or alcohols where the double bonds are separated by one, two or three methylene groups. Except when the double bonds are conjugated, it is always possible to determine the location of double bonds. References 1.
G.W. Francis and K.N. Veland, J. Chromatogr. 219, 379 (1981).
M.C. Caserio, C.L. Fisher and J.K. Kim, J. Org. Chem. 50, 4390 (1985). H.R. Buser, H. Arn, P. Guerlain and S. Rauscher, Anal. Chem. 55, 818 (1983). P. Scribe, J. Guezennec, J. Dagaut, C. Pepe and A. Saliot, Anal. Chem. 60, 928 (1988). C. Pepe, J. Dagaut, P. Scribe and A. Saliot, Org. Mass Spectrom. 28, 1365 (1993). P. Scribe, C. Pepe, A. Barouxis, C.H. Fuche, J. Dagaut and A. Saliot, Analusis 18, 284 (1990). M. Vincenti, G. Guglielmetti, G. Cassani and C. Tonini, Anal. Chem. 59, 694 (1987). D.A. Carlson, C. Roan, R.A. Yost and J. Hector, Anal. Chem. 61, 1564 (1989).
C. Pepe*, P. Dizabo Laboratoire de Spectrochimie Moléculaire, Université Pierre et Marie Curie, Boîte 49, 4 Place Jussieu, 75252 Paris cedex 05, France. J. Dagaut, N. Balcar Laboratoire de Physique et Chimie Marines, UA CNRS 5353, Université Pierre et Marie Curie, Boîte 134, 4 place Jussieu, 75252 Paris cedex 05, France. M.F. Lautier LASIR, CNRS, 2 rue Henri Dunan, BP 28, 94230 Thiais, France. Received: 23 January 1995 Accepted: 18 February 1995
