[Sophia7X]

How would you do this?

Predict the relative heights of the peaks at M, M+2, and M+4 of CH2BrCl. I know how to do this if it were two halogens atoms that are of the same identity (binomial expansion), so how do you do this if the halogen atoms are not the same?


Another question, why is chloramphenicol's M/M+2/M+4 ratio not 9:6:1?

[Arkcon]

Another question, why is chloramphenicol's M/M+2/M+4 ratio not 9:6:1?

Let's start with this one.  Why should it be?  And why would it not be?  Consider: does nitrophenol have a perfect ratio?  Is there something special about chlorine's isotope distribution?  Then you can try part 1.

[Altered State]

This is what I have from my lectures, guess it may help

[Dan]

Predict the relative heights of the peaks at M, M+2, and M+4 of CH2BrCl. I know how to do this if it were two halogens atoms that are of the same identity (binomial expansion), so how do you do this if the halogen atoms are not the same?

You can write out all the possible combinations of Br/Cl isotopes and scale the peaks to the relative abundance:

e.g. if we say ⁷⁹Br:⁸¹Br is 0.5:0.5 and ³⁵Cl:³⁷Cl is 0.75:0.25, it follows that:

⁷⁹Br:⁸¹Br:³⁵Cl:³⁷Cl is 50:50:0.75:0.25

In the molecule CH₂BrCl, there are only 4 combinations. Write them out and use the ratio of the isotopes to work out the ratio of the peaks. This is thinking in terms of probability, i.e. the probability of getting ⁷⁹Br³⁵Cl is 0.5 x 0.75 = 0.375. Repeat for every combination and you have the ratio of the peaks.

Another question, why is chloramphenicol's M/M+2/M+4 ratio not 9:6:1?

Who says it's not? Based on the Cl alone it should be 9:6:1 as far as I can see.

[Sophia7X]

Thanks Dan.

For the chloramphenicol problem, my solutions manual says this:





I don't understand why the M+2 peak is so close to the M peak.

[Dan]

I don't understand why the M+2 peak is so close to the M peak.

Nor do I.

[Arkcon]

This web widget http://www.sisweb.com/mstools/isotope.htm likewise gives (approximately) a 9:3:1 ratio for C₁₁H₁₂Cl₂N₂O₅ (chloramphenocol).  The image 15.64 sure looks odd.  In the reference I have, it looks like a molecule with a single bromine.  Maybe the image is a typo, mislabeled from another problem in the text?

[MOTOBALL]

There seem to be some transcription errors; SIS gives 100/66.5/11,6 as expected

[orgopete]

I don't understand why the M+2 peak is so close to the M peak.

Nor do I.

I would not either, however I am cynical about mass spectra. I searched for "chloramphenicol mass spectrum" and I scanned through several of the results. The NIST MS did not include the parent ions, presumably due to low abundance from fragmentation. The chemical ionization spectra did show the P+1 peaks, but the ratios were closer to the predicted values.

I have no reason to doubt the peak ratios of the posted problem, but I don't have any reason to think it is representative of the spectrum you may actually find in the lab under normal conditions (or similar to the published spectra).