Question

When looking at the GC/MS for Allylthiobenzene aka allyl phenyl sulphide, two clear peaks are seen at 91 m/z and 65 m/z, most common for 7 membered and 5 membered carbon rings respectively. The problem is that the substituent group associated to this compound begins with a sulphur, which, when integrated into the ring, does not produce a signal at 91. Is it possible for this to be cleaved and a different carbon (possibly from the tail swinging around) to be integrated?

Answer 1

In GC/MS analysis of Allylthiobenzene, fragmentation can lead to rearrangements, resulting in peaks that correspond to common hydrocarbon rings, even when sulphur is present in the original structure. This process can account for the observed 91 m/z peak.

Step-by-step explanation:

The student is questioning the mass spectrum peaks for Allylthiobenzene (allyl phenyl sulphide) observed at 91 m/z and 65 m/z, which are typically associated with 7-membered and 5-membered carbon rings respectively. Given that the compound in question contains a sulphur, some confusion arises as this element should not yield a signal at 91.

However, in GC/MS analysis, fragmentation can occur leading to rearrangements or loss of substituent groups, resulting in peaks that may not directly reflect the intact structure of the original molecule. Considering the molecule's structure and the standard fragmentation patterns, it is feasible for the sulphur-containing group to cleave and for alternative carbons to form a new ring during fragmentation, which could explain the 91 m/z peak.

While substituents like the sulphur may initially seem to disrupt the expected patterns, these fragmentation processes in GC/MS allow for rearrangements that can create peaks resembling those of more common hydrocarbon rings.