[jamaica]

Which of the following moleculars ion is more stable?
This question is related to mass spectrometry.

http://www.freeimagehosting.net/ztf95

The image is clear in the above link.

[discodermolide]

I think you need to make your image much much larger. I can't see anything.

[Arkcon]

Also, try to give us some more specifics on the mass spec analysis ... what sort of ionization is being used, and positive or negative ion mode, are useful bits of info.  Although we might have been able to guess, if we could see the charges on the molecules in your images.  (I tried zooming in on the image, but it would be better if you made a bigger image.)

[jamaica]

I just edit a new link where the image is clearly seen.


Unfortunately no further information is given.

[Arkcon]

Great.  Now, we have a bunch of positive ions being generated, and we'd like to see them in the mass detector.  Question for you is -- what structures do they have that can stabilize the charge so the ion survives long enough to be seen?

*[EDIT]*

What I mean to say is -- what would ruin a positive charge, something ... fundamental.  And how will you keep it away from the positive charge.  Or maybe, move the positive charge around, so its not such a target.  Maybe that last hint gives too much away.  Anyway, you have to write a good explanation on the exam on this topic, so start think up an explanation you can write when the time comes.

[jamaica]

Maybe the answer to this question has to do with the resonance structures of the molecular ions. The more resonance structures the more stable the cation is.
But who can draw the resoance structures

[Arkcon]

I see you've already started to figure it out.  Good.  Can you try to draw some possible resonance structures?  Consider the structures, they're very similar, what slight difference makes the resonance structure impossible or less likely?

[jamaica]

In the first compound the positive charge moves through the oxygen to the aromatic ring and where is delocalized and the sum of 5 resonanse structures are possible.

In the second and third compound only two resonance stuctures are possible, so these moleculars ions are less stable.

Is that the difference thats makes the first molecular ion more stable?

[orgopete]

I thought I knew the answer, but not wanting to make a (bigger) fool of myself, I looked up the MS spectra in SDBS. I was wrong. I also thought this would be a tough question. If you think you know the answer, look them up.