# Why does 2-chloropropane's mass spectrum have a peak at 27?

I have noticed that all halogenoalkanes of the form 2-Xpropane seem to have a mass spec peak at 27, implying that a $\ce{C2H3+}$ ion is being formed.

Forming a $\ce{C2H3+}$ ion, however, would mean breaking three bonds and, given its abundance, that wouldn't make sense - especially in the case of 2-chloropropane where it has an abundance of 25.36 according to this website.

Why exactly does the $\ce{C2H3+}$ ion form?

EDIT: You can also see this peak in the mass spectrum of 2-bromopropane, 2-iodopropane and 2-fluoropropane, but it is less pronounced.

EDIT 2: as pointed out by @PLD, it appears that other molecules of this form have peaks at 27 for a different reason to that of 2-chloropropane, so my previous edit is irrelevant.

• Maybe alpha cleavage with H2 elimination or whatever; can't see unlikeliness in here. May 4, 2017 at 21:49
• Nice question. I am not sure that 2-bromopropane and 2-iodopropane m/z 27 share the same originating process though. Referring to Rosenstock et al. (pubs.acs.org/doi/pdf/10.1021/ja00373a001) and Ikuta et al. ( journal.csj.jp/doi/pdf/10.1246/bcsj.49.66) the m/z 27 ion is described as a fragment of either m/z 41 (allyl cation) or m/z 29 (ethyl cation), but neither of these ions do appear in the fragmentation spectrum of 2-Chloropropane, nor do these two articles reference 2-chloropropane. I did not find any studies specifically on this issue.
– PLD
May 6, 2017 at 7:35
• The m/z 42, 41, 39, 27 pattern looks like that of the EI spectrum of propene / cyclopropane. This suggests loss of HCl as first step, followed by fragmentation of m/z 42. Why so abundant? According to Maccoll & Mature (onlinelibrary.wiley.com.proxy.scd.u-psud.fr/doi/10.1002/…) : the appearance energy for m/z 42 (ionized cyclopropane or propene) from 1-chloropropane is lower (about 2.7 eV) than that for 2-chloropropane. Thus the m/z 42 ion would be formed with much higher energy, leading to more fragmentation in the latter case. I am not completely convinced though.
– PLD
May 11, 2017 at 7:13