[Heli0x]

Hi everyone.

First of all I want to point out that I was unsure of whether this falls under o-chem or analytical chem, but seeing as it is a part of my basic o-chem course at college I chose to put it here. (Can mod please move if it is incorrectly placed?)

I guess there is no other way to explain my question than; in EI-MS, when does homolytic cleavage occur, and when does heterolytic cleavage occur? I did some practice problems in my book and I'm totally off..

I'm aware of some of the common fragments, e.g. 29, 77, 91 etc., and that they're resonance stabilized, but it's not really intuitive to me apart from knowing that these compounds readily form thanks to resonance stabilization. I can't really deduce that these fragments are most probable to form just by trying to analyse cleavage patterns. I'm essentially striving to form them just because I'm aware of the carbocation stability and that I've been taught that these are the most common fragments.

I have absolutely NO clue whatsoever when either form of cleavage takes place. I've searched far and wide, checked my books, but I just can't find anything that actually explains when they occur.

Also, when bombarding an carboxylic acid with electrons, do both oxygens become radicals or just the carboxylic oxygen and not the hydroxy oxygen?

Any help is greatly appreciated!

[PhDoc]

Try this:

http://www.chem.umn.edu/groups/harned/classes/8361/lectures/MSFrag.pdf

[Heli0x]

Thanks.

I've actually already read that, but I re-read it a couple of times and I'm still not sure, but I've tried to do some practice problems with a spectrum and given structure. I tried to draw various combinations of fragments that would add up to the m/z in the spec, and came up with 2 heterolytic cleavage-products and 2 homolytic cleavage-products, shown in the image:




According to the book the fragments seem to be correct, but I'm not sure about the actual mechanism as they aren't specified in the key in the book. Any ideas if they are correct?