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Topic: Help with fragmentation of EI spectrum  (Read 3320 times)

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Offline Telamond

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Help with fragmentation of EI spectrum
« on: February 04, 2012, 04:03:54 AM »
Hello,
I'm checking through some excercises in EI MS, and I'm supposed to give the mechanism of fragmentation.

From the 1NMR that I was given I deducted that the structure I have has to be 2-phenylethanol. This is in agreement with the 13C-NMR (and DEPT) as well as the molecular weight.

The MS shows the molecular ion to be 122 and the base peak is 91 which is consistent with the tropylium ion (or benzylcation).
If I were to draw the mechanism for the fragmentation, I'd ionize the lonepair on the oxygen, and start the fragmentation from there via a beta-cleavage. This would however give a neutral loss of 91 instead.

So I checked out the lecture hand-outs and I found this image of the fragmentation which is consistent with how to obtain the measured basepeak of 91. Upon a heterolytic cleavage leaving CO, I'd get the benzyl cation. But I don't understand it at all. How can H2 just leave like that? Does this type of fragmentation have a name?

Thankful for any input and guidance.

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