I have this example exam question for which I don't have the answer. I have been working on it the whole morning but I can't figure it out. The question suggests three molecules, only one of which corresponds to a given fragmentation table and mass spectrum following EI-based MS.

The molecules are 4-(p-hexyl phenyl) butyric acid:

hexylphenyl butyric acid

and two variants where the phenyl is shifted one to the left (pentylphenyl pentanoic acid) or one to the right (heptylphenyl propanoic acid).

The table and EI-MS spectrum are unfortunately not of ideal quality, but they are as follows:


The 177 and 188 were given to both be directly stemming from the molecular ion, and with one of them having an even and the other an odd mass, this suggests a simple cleavage and a cleavage following rearrangement.

With pentylphenyl pentanoic acid I got close, but the fragment following rearrangement is 192 Da instead of the 188 Da seen in the spectrum.


UPDATE - Let me add two other examples of what I have tried so far. Maybe there is something wrong with my approach or someone can see the obvious change that would get to an observed peak at 188 m/z from here. The fragmentation on the left (below), is where there actually are 188 and 60 fragments, but the 60 fragment carries the charge (this peak at 60 m/z can be seen in the spectrum). On the right, I tried to get to a similar fragmentation while starting by putting the charge on the phenyl.


  • $\begingroup$ On an unrelated note, please always upload images to the SE imgur using the create image button in the editor. (You can enter a link from the web there.) I know it is sometimes a pain, but it preserves the images in case their original source (here: a probably personal Dropbox, unlikely to withstand the test of time) gets deleted. We want questions and answers to be helpful to future visitors, that’s why =) $\endgroup$
    – Jan
    Commented Feb 6, 2016 at 13:00
  • $\begingroup$ Presumably this is electron ionization (EI)-based mass spectrometry? You may want to edit the question to add this detail. $\endgroup$
    – Curt F.
    Commented Feb 6, 2016 at 15:09
  • $\begingroup$ The question doesn't specify it, but I suppose you are correct. I edited the question. May I ask how you recognised it, just for my learning process :) $\endgroup$
    – Mårten
    Commented Feb 6, 2016 at 15:18

1 Answer 1


In your update you nearly got to the point: the McLafferty rearrangement from the carboxylic acid can lead to two products, either the ionized acetic acid (ML product) or the ionized alkene (ML'). In this case, ionization energy of the enol tautomer of acetic acid is probably much larger than that of alkenyl substituted benzenes (for instance the closest I could find in structure is o-Allyl-Toluene for which the IE is 7.78 eV). Thus from application of the Stevenson-Audier rule, the charge should remain on the moiety with the lowest ionization energy, leading to the formation of the m/z 188 moiety (ionized p-pentyl-allylbenzene).

This hypothesis is further reinforced as it also explains the formation of m/z 117 from a oss of 71 ($C_5H_{11}^\bullet$) from fragmentation of the m/z 188 radical cation, through a pathway similar to that leading to m/z 177 from the molecular ion.

(This is likely oversimplified mechanism interpretation: I strongly suspect that some other rearrangements or cyclisation likely occur in the process. I did not run a complete database search for other EI spectra of similar molecules, but for instance 10-phenyldecanoic acid leads to only a very minor loss of 60. Thus I would suggest that the m/z 188 probably undergoes cyclization and / or hydrogen migration processes leading other products.)

  • $\begingroup$ I didn't think of inductive cleavage after the rearrangement (rather than the alpha cleavage I drew) but clearly, if the ionisation energy is lower it is the favoured option. All clear :) $\endgroup$
    – Mårten
    Commented Feb 16, 2016 at 13:12

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