I have a Q, do carbonium ions like the propyl cation at 43 and the butyl at 57...can these primary carbocations rearrange to more stable sec carbonium ions?
They can. So an n-propyl cation can rearrange to isopropyl. In terms of the MS, the m/z value is the same. A particularly famous rearrangement is of the benzyl cation (PhCH2+) to a tropylium ion (a fully aromatic cation with a 7-membered ring structure). See this video for a discussion (but not the mechanism) - th-cam.com/video/m3OjQPHcFqc/w-d-xo.html.
@@ChemHelpASAP So from ur explanation, i take it that these carbocations could very well rearrange in an MS set up. So, for example, the n-propyl peak that u see at 43 could very well be also due to the same n-propyl cation rearranged to isopropyl cation as soon as it is formed by fragmentation and picked up by the senser as a peak at 43.. right?
@@starman923 Correct. Also, since these fragment cations are generated in the gas phase, the odds of them colliding with something else is very low. Intramolecular processes (like a carbocation rearrangement) are one of the only reaction options that are available.
For 2-hexanone, the main fragmentation pathways should be around the carbonyl. MW is 100 g/mol, so look for a parent ion at m/z 100. You can lose the methyl on one side - m/z 85 and/or 15. You can lose the butyl on the other side - m/z 57 or 43. Finally, you'd expect a McLaffery rearrangement to give a peak at m/z 58.
I have a Q, do carbonium ions like the propyl cation at 43 and the butyl at 57...can these primary carbocations rearrange to more stable sec carbonium ions?
They can. So an n-propyl cation can rearrange to isopropyl. In terms of the MS, the m/z value is the same. A particularly famous rearrangement is of the benzyl cation (PhCH2+) to a tropylium ion (a fully aromatic cation with a 7-membered ring structure). See this video for a discussion (but not the mechanism) - th-cam.com/video/m3OjQPHcFqc/w-d-xo.html.
@@ChemHelpASAP Thank you very much for your response
@@ChemHelpASAP So from ur explanation, i take it that these carbocations could very well rearrange in an MS set up. So, for example, the n-propyl peak that u see at 43 could very well be also due to the same n-propyl cation rearranged to isopropyl cation as soon as it is formed by fragmentation and picked up by the senser as a peak at 43.. right?
@@starman923 Correct. Also, since these fragment cations are generated in the gas phase, the odds of them colliding with something else is very low. Intramolecular processes (like a carbocation rearrangement) are one of the only reaction options that are available.
Plzz explain the mass spectrum of 2 hexanone
For 2-hexanone, the main fragmentation pathways should be around the carbonyl. MW is 100 g/mol, so look for a parent ion at m/z 100. You can lose the methyl on one side - m/z 85 and/or 15. You can lose the butyl on the other side - m/z 57 or 43. Finally, you'd expect a McLaffery rearrangement to give a peak at m/z 58.