Cause of m/z = 56 peak in 4-methyl-1-pentanol

This is the mass spectrum for 4-methylpentan-1-ol:


[![mass spectrum for 4-methylpentan-1-ol](https://i.stack.imgur.com/XUDbx.png)](https://i.stack.imgur.com/XUDbx.png)


From its structural formula, what could be the fragmentation which results in a peak at $m/z = 56?$


I was considering a few possibilities; could it be $\ce{C3H4O+}?$




Unfortunately I did not find a high resolution spectrum of this compound, which would have quickly answered your question.


Nevertheless, ionized alcohols and even more primary alcohols have a main fragmentation pathway which is the loss of water to yield an ionized alkene. In this case, the corresponding alkene would be 4-methyl-1-pentene, at *m/z* 84. The peak is present although rather small in the EI spectrum. From there, it is quite obvious, when one looks at the fragmentation spectrum of 4-methyl-1-pentene ([NIST Webbook spectrum](https://webbook.nist.gov/cgi/cbook.cgi?ID=C691372&Units=SI&Mask=200#Mass-Spec)), that it shares a lot of similarities with the 4-methylpentan-1-ol EI spectrum. (See for instance peaks at *m/z* 69, 56, 43, 42, 41.) From there, it is quite likely that the *m/z* 56 peak is ionized isobutene ($\ce{C\_4H\_8}^{\bullet +}$) arising from a retro-ene type rearrangement.


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