[Jackie884652]

Can someone please explain how to come up with the molecular formula of this unknown mass spectrum.


I know the formula is C4H7BrO2 but I do not know the steps to get there, can you please explain. Thank you.

[discodermolide]

Why are there 2 signals for the M⁺ ion? What does this tell you?

[Jackie884652]

Why are there 2 signals for the M⁺ ion? What does this tell you?

this is where I am having trouble. Do I subtract 166 from 87 and that is the M+?

This is what I have so far and idk if its even right
The mass spectrum is presented as a vertical bar graph, in which each bar represents an ion having a specific mass-to-change
 ratio and length of the bar indicated the relative abundance of that ion. I used the mass spectrum to identify the formula and the formula will give me the degree of unsaturation(DU).  I first started by looking at the base peak which is the tallest peak (100%). In the above mass spectra for unknown one the
 base peak is 87(M). The M+1 value is m/z= 88 (4.7%).The M+1 value is 88 since we know that the definition of M+1 is a molecular ion whose
mass is one amu higher than M. 88 amu is one amu higher than the m/z of M so it must be the M+1 value. Dividing the relative abundance of M+1 (4.7%) by 1.1% gives 4.27 and using the rounding rules it becomes 4. 4 carbons are in the analyte molecule.

M+2 shows whether or not sulfur, chlorine or bromine is present, because these are the only isotopes that have significant peaks. By scaling M to 100% and finding the relative abundances of sulfur, chlorine, and bromine through their natural abundances of M versus M+2, we can determine the intensity ratio of M / (M+2). I learned the following information to help identify sulfur, chlorine and bromine, “The intensity ratio of M compared to M+2 for sulfur is 100:4.4. Thus the M+2 intensity ratio for a compound containing sulfur will be about 4.4%. The same rule can be applied to chlorine and bromine which yield intensity ratios of about 31.9% and 97.2% respectively.”

[discodermolide]

That text you posted is straight out of a book.
The M⁺ is actually written on the spectrum. You can see immediately something, what?
Also the other peaks lower in mass have the same pattern, what is this?

[orgopete]

Generally, problems like this can be solved with trial and error. If you subtract the bromine from the MW, how many carbons are possible? What if there is oxygen(s)?

If you want to see the number of possible answers, try searching molecular formula calculators. You will find several online versions.