Interaction of Dichloromethane Solvent with n-Alkylamines Analyzed by Electron Ionization GC–MS

Mar 01, 2010

Results and Discussion

Figure 1
The relative amounts of the rearrangement/reaction products varied from compound to compound as did the ability to separate the individual compounds using the chromatographic conditions employed in this study (Figure 1).

Due to the fact that the mass spectra of all the reaction products indicated that the same type of compounds (differing only by the number of carbon atoms that corresponded to the number of carbon atoms in the amine) were formed, a thorough analysis was carried out only of stearylamine. In order to show that the reaction took place in solution prior to injection, the solutions were subjected to TLC, which clearly showed two components in all cases. The APCI data alone would not have been conclusive because the solutions are subjected to the same high temperatures encountered in the GC injection port.

Figure 2
An examination of the EI mass spectrum of the rearrangement/reaction product associated with stearylamine (Figure 2) prior to obtaining any other data (accurate mass, APCI, etc.) showed that the highest m/z value peak (that was not an isotope peak or due to background) was at m/z 280. This peak probably did not represent a molecular ion M+• because the next lowest m/z value peak was at m/z 266; a difference of 14, which is not a logical loss. The fact that there are peaks with significant intensity every 14 m/z units between m/z 280 and m/z 42 indicates that straight-chain aliphatic ions are formed retaining a single atom of nitrogen and differ by one methylene unit. The intensities of the peaks at m/z 84 (the base peak in the spectrum) and m/z 126 are greater than would be expected for a homologue series. The intensity of the peak m/z 85 is far greater than can be rationalized as being due to the presence of a 13C isotope relative to the peak at m/z 84.

Peaks in the mass spectrum of stearylamine representing ions that have retained the nitrogen atom and were formed by the loss of an aliphatic radical do not have the same m/z values observed in the mass spectrum of the unknown — that is, the mass spectrum of stearylamine (nominal mass 269) has peaks at m/z 254, the loss of a CH3 radical, a peak at m/z 240, the loss of a C2H5 radical; m/z 226 loss of a C3H7 radical; m/z 212 loss of a C4H9 radical; and so forth. What appears to be corresponding peaks in the mass spectrum of the unknown are 2 m/z units less than those in the spectrum of stearylamine; that is, 252 instead of 254, 238 instead of 240, 224 instead of 226, and so forth. This would mean that if an alkyl radical is lost to form the ion that retains the nitrogen atom, then the analyte is not a primary, secondary, or tertiary amine.

Another very important point indicating that the unknown is not a primary amine is that there is no peak at m/z 30. Because there is no peak at m/z 30, it is also not possible for the unknown to be a secondary amine with the smaller of the two substituents allowing for the hydride-shift rearrangement to produce the primary immonium ion with m/z 30. The lack of the peak at m/z 30 also means that there are not two substituents, in addition to the main aliphatic substituent, that could undergo two sequential hydride-shift rearrangement fragmentations to produce the primary immonium ion.

Figure 3
One possibility would be that the unknown is an isocyanate (R–N=C=O) with a C-18 carbon chain (R=C18). The nominal mass of this compound is 295 Da. The peak at m/z 280 would represent the loss of a CH3 radical from the M+•, which is consistent with the spectrum of stearylamine (Figure 3). The intensities of the peaks at m/z 84 and m/z 126 are not explained by such a structure, nor does this structure readily explain the presence of peaks at m/z 57 and m/z 85 in the mass spectrum of the unknown. It is possible that the peak at m/z 84 represents a methyl substituted pyrrolidine fragment formed through a rearrangement, and the peak at m/z 85 represents a methyl substituted pyrrolidine molecular ion. Another possibility would be the formation of a C-18 ester of cyanic acid (R–O–C≡N). Again, this structure does not readily provide an explanation of the intensities of the peaks at m/z 84 and m/z 120 or the presence of the peaks at m/z 57 and m/z 85.

Figure 4
Based on a ball-and-stick model, through the shift of a pair of electrons on the nitrogen atom in response to the charge site on the carbon of the +CH2CH2CH2N=C=O ion, the ion shown on the left side in Figure 4 would form and could explain the high intensity of the peak at m/z 84 in the spectrum of the unknown. Unfortunately, a similar ball-and-stick model for the +CH2CH2CH2O–C≡N ion indicated that there would be any similar stabilizing factor for this ion involving the oxygen atom (right side of Figure 4). This means that the structure of the unknown is still ambiguous.

Figure 5
Both of the two proposed structures have the elemental composition of C19H37NO. Even though the spectrum of the unknown had been submitted for a Spectral Identity Search against the NIST/EPA/NIH Mass Spectral Database, NIST08, of 191,436 compounds and had resulted in no hits with a better than 700 Match Factor for the best Match and the spectrum represented by that Hit had a base peak at m/z 56 compared to the base peak of the unknown at m/z 84, it was deemed to be prudent to search the proposed formula against the database. The Formula Search of the NIST08 Database resulted in 24 Hits, none having a mass spectrum with a base peak at m/z 84; however, there was a mass spectrum for octadecyl isocyanate, one of the two candidate structures. This spectrum (Figure 5) was in no way a match for the spectrum of the unknown (Figure 2). This meant there was now only one proposed structure, and this structure was predicated on a nominal mass of 295 Da for the unknown, which was highly speculative at this point.

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