Letter: Determination of positions of double bonds in conjugated alkadienes by means of gas chromatography/mass spectrometry of dimethyl disulfide derivatives

Abstract

Dear Sir The addition of dimethyl disulfide (DMDS) to alkadienes has been studied and it has been found that a six-, fiveor four-membered cyclic thioether is synthesized if the number (n) of methylene groups which separate the double bonds is less than 4. A compound containing four methylthio groups is synthesized if n is greater than 3. This addition of DMDS has been used for the determination of the original position of the double bonds of alkadienes by means of gas chromatography/mass spectrometry (GC/MS). It appears, however, that this method for the location of double bonds is not possible with conjugated alkadienes (n = 0) in so far as only one molecule of DMDS is added in the 1,4 position. We have now studied, by means of GC/MS, the products resulting from the reaction between DMDS and several conjugated alkadienes, following a change in the usual experimental procedure, with the intention of being able to locate the double bonds. Electron ionization (EI) mass spectra were obtained using a Nermag R10-10 quadrupole mass spectrometer, coupled to a Varian Model 3400 gas chromatograph. The samples were injected in the septum-equipped programmable injector (SPI) mode. The GC column was a fused-silica capillary (30 m × 0.25 mm i.d., 0.2 μm film thickness) coated with DB5 phase (Chrompack). The column temperature was increased at 5°C min from 100 to 280°C. C NMR spectra were recorded at 50.32 MHz with a Bruker AC 200-Aspect 3000 instrument using CDCl3 as solvent. Each unsaturated compound (50 μg) was treated by the addition of an excess of DMDS (300 μL) and 50 μL of an iodine solution (60 mg iodine in 1 mL diethyl ether). The reaction was carried out in a closed tube for 48 h at 80°C. The excess of iodine was reduced with sodium thiosulfate solution (5% in water). The organic phase was removed and the excess of DMDS evaporated. The dry extract was diluted with 50 μL of hexane and then analyzed by GC/MS. In the present investigation, the behavior of the dimethyl disulfide (DMDS) derivatives of conjugated alkadienes I–III (n = 0), listed in Table 1, was studied by GC/MS. Figure 1 displays the EI mass spectra obtained from the DMDS derivatives of 2,3-dimethyl-1,3-butadiene (I), 1,3-hexadiene (II) and 2,4-octadiene (III).