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I have understood the mechanism for converting ion 1 (benzyl alcohol) to ion 2 (m/z = 107) but not the mechanism for the conversion of ion 2 to ion 3 (m/z = 79). Please elaborate the mechanism for the conversion of ion 2 -> ion 3.

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This is similar to the phenol fragmentation. In mass spectrometry, benzyl alcohol fragments with two mechanisms, the one in your question and by a simple loss of the OH(91m/z). The 79m/z is the fragment with the most intense peak.

I have added below two possible mechanisms for step 2 to 3.

First option:

enter image description here strong text

Second option:

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EDIT: in the second option, the homolytic cleavage step is meant to be a summary of these steps:

enter image description here

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  • $\begingroup$ The homolytic cleavage step in Option 2 is a doubt. Nevertheless, satisfied with the concerted step in the first option. $\endgroup$ – Abhigyan Aug 26 '19 at 2:49
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    $\begingroup$ Option one feels more natural to me, but I added the second option after reading about a possible radical intermediate in the phenol fragmentation. That homolytic cleavage is in fact 2 steps: 1)hemolytic cleavage with one electron to the carbonyl carbon and one electron to the carbon bound to 2 H. 2)lone pair of the oxygen forms a triple bond, and at the same time the unpaired electron moves to the adjacent carbon with positive charge(+) to avoid a 5 valence state and because it is attracted to the positive site $\endgroup$ – blu potatos Aug 26 '19 at 8:32
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    $\begingroup$ I have added all the homolytic cleavage steps in the edit section at the bottom of my answer $\endgroup$ – blu potatos Aug 26 '19 at 8:50
  • $\begingroup$ Okay.. more understandable now $\endgroup$ – Abhigyan Aug 26 '19 at 15:31
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Interesting question.

The cycloheptatriene - norcaradiene tautomerization is a common equilibium. Usually the norcaradiene is present in small amounts, but as long as some is present the reaction can rapidly proceed through this intermediate.

In the diagram below, the norcaradiene can undergo hydrogen shift with concomitant decarbonylation to produce the m/z = 79 peak in the mass spectrum.

enter image description here

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  • $\begingroup$ Is the mechanism for this reaction the same as what blu potatos has shown in his answer? $\endgroup$ – Tan Yong Boon Sep 19 '19 at 10:02
  • $\begingroup$ @TanYongBoon I don't see anything in his mechanism(s) that looks like a one-step chelotropic extrusion of CO from a norcaradiene-like structure. $\endgroup$ – ron Sep 19 '19 at 13:30
  • $\begingroup$ Oh it is one step! Ok thanks! $\endgroup$ – Tan Yong Boon Sep 20 '19 at 12:09

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