The molecule has two methyl groups. The methyl next to the carbonyl is the one that is more likely to be lost during the cleavage. The radical forms on the carbonyl oxygen, and the group closer to the radical is more likely to be lost. Maybe the other methyl could be lost, but I wouldn't be able to draw a mechanism for it.
@@ChemHelpASAP So, at 86 m/z, it is the whole 2-pentanone molecule: CH3 CO CH2 CH2 CH3 Then, at 71 m/z, CH3 has broken off and it is now: CH3 CO CH2 CH2 (This makes sense to me because CH3 CO CH2 CH2 has a mass of 71 - same as the m/z. And 86-71=15 - same as the mass of the broken off CH3) But I don't understand what exact part of the molecule has broken off and what is left at both 58 m/z and 43 m/z. For 58 m/z, I can't think of what fragment could have a mass of 58 and have lost a fragment of the mass 13. (Can you lose just CH?) For 43 m/z, I can't think of what fragment could have a mass of 43 and have lost a fragment of the mass 15. Should there be CO here because that's the strongest bond? Or do the fragment masses not necessarily always match up with the m/z? Thank you so much for your help and reply!!!
@@Gabi-jy8ih In *general*, in MS, the main peaks that you see arise from fragmentation of the parent ion. Remember that there are millions (billions?) of parent ions. Some of those parent ions never fragment and are detected intact (m/z 86). Some of the parent ions lose a methyl and are detected as the m/z 71 fragment. Some of the parent ions lose a propyl and are detected as the m/z 43. So, you do not normally see the outcome of one piece falling off and then another (lose a methyl on one side and then a propyl from the other). Every peak is *normally* a single fragment lost from the parent ion. The fragment at m/z 58 is different. That arises from the McLafferty rearrangement, which is a complex fragmentation pathway (covered in this video). Regardless, the McLafferty is still a single fragmentation pathway from the parent ion.
ive seen people attribute the m/z 71 to two kinds of methyl losses. is there a clear answer to which is "correct"?
The molecule has two methyl groups. The methyl next to the carbonyl is the one that is more likely to be lost during the cleavage. The radical forms on the carbonyl oxygen, and the group closer to the radical is more likely to be lost. Maybe the other methyl could be lost, but I wouldn't be able to draw a mechanism for it.
I need help with this ASAP please reply to my comment
What is your question Gabi?
@@ChemHelpASAP
So, at 86 m/z, it is the whole 2-pentanone molecule: CH3 CO CH2 CH2 CH3
Then, at 71 m/z, CH3 has broken off and it is now: CH3 CO CH2 CH2
(This makes sense to me because CH3 CO CH2 CH2 has a mass of 71 - same as the m/z. And 86-71=15 - same as the mass of the broken off CH3)
But I don't understand what exact part of the molecule has broken off and what is left at both 58 m/z and 43 m/z.
For 58 m/z, I can't think of what fragment could have a mass of 58 and have lost a fragment of the mass 13. (Can you lose just CH?)
For 43 m/z, I can't think of what fragment could have a mass of 43 and have lost a fragment of the mass 15. Should there be CO here because that's the strongest bond?
Or do the fragment masses not necessarily always match up with the m/z?
Thank you so much for your help and reply!!!
@@Gabi-jy8ih In *general*, in MS, the main peaks that you see arise from fragmentation of the parent ion. Remember that there are millions (billions?) of parent ions. Some of those parent ions never fragment and are detected intact (m/z 86). Some of the parent ions lose a methyl and are detected as the m/z 71 fragment. Some of the parent ions lose a propyl and are detected as the m/z 43.
So, you do not normally see the outcome of one piece falling off and then another (lose a methyl on one side and then a propyl from the other). Every peak is *normally* a single fragment lost from the parent ion.
The fragment at m/z 58 is different. That arises from the McLafferty rearrangement, which is a complex fragmentation pathway (covered in this video). Regardless, the McLafferty is still a single fragmentation pathway from the parent ion.