[wildbiologist2]

Hi everyone. I am currently identifying an unknown using EA, IR, HNMR, CNMR, and mass spec. EA and mass spec suggests that the molecular formula is C11H14O2, Thus, there are 5 unsaturations. IR shows evidence of an aromatic ring and a C=O bond, so all unsaturations are accounted for. H-NMR shows a very broad, deshielded signal, often characteristic of a carboxylic acid. There is also a relatively shielded singlet representative of 9 H, probably a tert-butyl group.

So I have a disubstituted aromatic with a carboxylic acid and a tert-butyl group. I feel HNMR splitting patterns and CNMR suggest that it is para-tert butyl benzoic acid, but I have one issue...

The M+ on mass spec is 178 m/z with lower, (noteably abundant compared to other signals) at 163 m/z, 135 m/z and 91 m/z. I am very confused as to why these splitting values exist. For example, I can see the CO2H substitutent fragmenting to yield a respective signal at 133 m/z but there is a slight descrepency in the data. Why does it show a fragment at 135 m/z and not at 133 m/z which is what one would expect?

[Irlanur]

178 -> 163,  loss of a methyl group
163 -> 135, loss of CO

91: tropyllium cation

[wildbiologist2]

Explain how it could be a tropyllium cation if para-tertbutyl benzoic acid is a 6-membered aromatic ring?

If the CO fragments off, then doesnt the O-H of the acid fragment off as well or does it "attach" to the aromatic? Sorry for the confusion...

Why is it I do not see a signal representative of the tert-butyl group fragmenting off to produce a signal at 121 m/z? I have a VERY small signal at 123...

[MOTOBALL]

I'm afraid that you are falling into the classic trap (and we have all been there) of getting strong evidence from one or two spectroscopic methods (here IR, H-NMR) and trying to make the evidence from the third method (here MS) fit the preconceived answer.

As Irlanur says, m/z 91 is the classic C7H7+ tropylium ion, derived from CH3-C6H4-R.

You might write out the rings, functional groups, substituents etc. suggested by each INDIVIDUAL method and then arrange to get a structure(s) that is/are consistent with all spectroscopic data.

For instance, you propose a -COOH group; does the IR support an -OH present ?

[The Guy]

Can I ask you what do you mean by EA??

[wildbiologist2]

By EA, I mean elemental analysis through percent composition.

IR showed evidence of a para disubstituted ring (with sp2 C-H bonds), a C=O bond, an O-H carboxylic acid bond, and sp3 alkyl C-H bonds.

H-NMR showed that there were four non-equivalent hydrogen groups with one being a singlet with an integration reflective of nine protons. hmmm must be a tert-butyl group. C-NMR showed 7 unique carbon types; 1 from a carboxylic acid, 4 aromatic, and 2 alkyl.

The only slight discrepancy is from the MS I talked about earlier. I am 100% that I ID'd it correctly, I am just kind of wondering why some values don't match up.

As for the rearrangement to form a tropylium ion at 91m/z, I understand that it is part of the "aromatic cluster" of signals in mass spectra of benzene-containing compounds, but I can see how a tropylium ion can form from a mono-substituted aromatic, but I am wondering how a tropylium ion can from from a di- or tri-substituted aromatic.

[MOTOBALL]

Your full explanation of the IR/NMR data is persuasive enough to move the proposed structure from "preconceived" to "well-conceived".

As is often the case, the MS data alone is insufficient to ID a complete unknown; the best that can be said is that the MS is consistent with the proposed structure (successive losses of methyl radical and CH2=CH2 from molecular ion are characteristic of t-butyl group; m/z 91 = as you described).