[Lazerus]

Hey all,
Any help on this would be appreciated since i'm truely stumped. The peaks on my mass spec are at 27,41,43(base peak), 107,109 and 122,124.
The 122,124 i know is from the bromine isotope and is the parent ion, likewise i know the 107,109 is again due to the isotope and an alpha cleavage removing a methyl group.
The base peak at 43 is bromine in an outright cleavage from the propyl chain.

My problem comes from identifying how the peaks at 27 and 41 come about (they are relatively abundant, slightly larger then the parent ion, about 25-30%ish judging from the spectrum)
My first thought is a HBr removal, this would give a peak at 42 not 41 though. Then maybe an alpha cleavage of a methyl group (CH3-CH=CH2+) to leave me with C2H3+ which matches the peak at 27.

Any help on trying to explain where the peak at 41 is coming from or where my missing peak at 42 (extremely quick breakdown?) has went is appreciated.

Thanks

[Albert]

This is what I think to be a reasonable fragmentation pathway.

Edit: I deleted the image. My new painting   is large enough and I don't want to waste too much of space...not sure about it...I mean: maybe the server will appreciate...but I might be wrong: I don't know anything about computers

[Lazerus]

It seems quite unfavourable for the CH₄ to leave the isopropyl cation though, can you explain your mechanism/reason for this occuring to me ?

cheers for the reply

[Albert]

I hope to have done better this time.   I tried to consider every kind of fragmentations.

A methane that leaves a cation is not impossible, although, I must admit, is something not very frequent to be seen. Once I had an alcohol as a product of a fragmentation.
However, I agree with you: this time it sounds a little bit incorrect.

Looking at the pathway, I think you'll easily understand why a 42 peak is hard to see.

[Lazerus]

Thanks for the continued interest.
I'm not sure if i'm following the breakdown patterns i've been given too closely and that mass spec is somewhat more chaotic but when the methyl is alpha cleaved from the original molecule (the arrow going left) wouldn't the Br carry the +ve charge and also be double bonded to the carbon ? Because from that point i'm at a loss trying to draw out the mechanism as to how H-Br would break off.
Also forgive my ignorance since organic mechanisms were never my strongpoint but why are the hydrogens leaving ?  

[Albert]

when the methyl is alpha cleaved from the original molecule (the arrow going left) wouldn't the Br carry the +ve charge and also be double bonded to the carbon ?

Also forgive my ignorance since organic mechanisms were never my strongpoint but why are the hydrogens leaving ?

Ok, for what concerns the first question the answer is NO. The phenomenon you describe is common with O, N, S, ect., but absolutely uncommon with halogens. So, if I were you, on this occasion, I'd forget about it.  

The latter is a little bit unclear to me. Which fragmentation are you referring to? I mean, is H2 or H. that puzzles you?

[Lazerus]

Hmm an example i was given involved a halogen and it ended with a double bond.

So if the methyl takes 1 electron from that bond, as i can see since it's a radical, how does the other transfer along to the bromine ? (I'm assuming it does as it now seems to have lost its initial charge caused by an electron being knocked off)

Both cases of the H loss really   I can't really think of a clear idea to explain how they would lose the hydrogens nor why either would transform to the alkene cation as my first thoughts of hyperconjugation and such don't seem to fit.

[Albert]

Ok, first of all: uncommon doesn't mean 'never'. I also saw that kind of example.

However, generally speaking, there are cations which are extremely stable:

CH2=CH+                 CH2=CH-CH2+            CH2=C+-CH3

So, every reaction leading to these fragments is very frequent.

Moreover, there is also the so-called 'driving force', which is function to the great stability of neutral molecules (HBr, H2, ect.) as fragments, that balances the yield of unstable cations.
To sum up, every time there's a distinct possibility that a certain pathway produces a neutral molecule, that will be the real one.

[Lazerus]

Could you explain in abit more mechanistic detail how the hydrogens (both routes) actually break off from the molecule allowing the more stable cation to form ??

ps, Just to clear something up in my head aswell, is the m/z 42 this CH₃-CH=CH₂ with the .+ delocalised in the double bond ?

*sigh* i shoulda stuck with maths lol

[Albert]

First of all, peak at 42 you may be right, but this is a very reactive fragment and I daresay it constantly rearranges.

For what concerns hydrogens, I don't know the exact mechanism because it's not very important: fragmentations are not like syntheses. Rearrangments are of paramount importance and extremely common.
You always have to consider that not only is there organic chemistry behind fragmentation pathways, but also thermodynamics. Fragmentation pathways are the display of reaction mechanisms.

However, H2 is formed because of the already-mentioned driving force, which often leads to neutral molecules.
H. leaves because this produces a stable fragment, or, at least, a more stable one. So, I think we can say it is again because of a driving force (though a less strong one).
Remember that, in this context, because of the great energies involved in the fragmentation process, reactions which seems hard to take place, can actually happen.

Sorry, but this is everything I can give you as explaination.