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Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: gk000 on October 03, 2016, 12:22:57 AM

Title: cyclopropyl phenylmethanol:mass spectrum
Post by: gk000 on October 03, 2016, 12:22:57 AM
Hi
Undergrad organic chem
Ive been struggling with working out the fragmentation pattern of my unknown molecule. I definitely know its cyclopropyl phenylmethanol. Just cant figure out some of the fragmentation

Peak    Intensity    Assignment
149   5   C10H12O
131   100   [C10H10]+
120   20   ----
107   7   ------
91   10   [C7H6]+
71   61   

I've calculated the losses from each peak. Between peaks 131 to 120, the loss is 11 which I'm really confused about what would break off. I know m/z 91 is a tropylium ion which would generally break down to a benzyl ion. Not sure if I'm just going about it the wrong way.
Structure image attached
Title: Re: cyclopropyl phenylmethanol:mass spectrum
Post by: MOTOBALL on October 05, 2016, 03:43:51 PM
You have started this problem the right way, by tabulating the ions & intensities....BUT note,

1. Highest mass ion is M+. which is at m/z 148 for Molecular formula C10H12O1
2. m/z 149 is due to the 13C and 2H isotopic contributions----ignore it !!
3. your tabulation is now
             m/z
              148   C10H12O1
              131
              120 etc...
                91 is C7H7+ not C7H6

Obviously, m/z 120 is not derived from 131, and 107 is not derived from 120 etc.  In these situations, go to the next higher m/z value----is m/z 120 from m/z 148 ?? Is m/z 107 from 131 or 148 ?

You are almost there, stay with it and it will fall out !!!!!

NB I have assumed electron ionization (which would give M+. at m/z 148; positive chemical ionization WOULD give [M+H]+ at m/z 149) check with prof. or the TA to find out what ionization method was used.

Regards
Title: Re: cyclopropyl phenylmethanol:mass spectrum
Post by: AWK on October 05, 2016, 04:37:12 PM
Quote
2. m/z 149 is due to the 13C and 2H isotopic contributions----ignore it !!
?
why m/z 148 and 130 are absent?
Title: Re: cyclopropyl phenylmethanol:mass spectrum
Post by: MOTOBALL on October 05, 2016, 10:06:15 PM
why m/z 148 and 130 are absent?

They are absent because it has to be a positive CI mass spectrum. I had initially assumed an EI spectrum, because that is what is almost always presented to undergrad organic students.
That led me to put in the NB, because I started to get second thoughts about the spectrum that he was working with.

Regards
Title: Re: cyclopropyl phenylmethanol:mass spectrum
Post by: AWK on October 06, 2016, 02:06:24 AM
Spectrum presented by gk000 is pure (without isotopic contributions). Alcohols very often give m/z M+1 (MH+). This ion eliminates water.
130 = M+-28(C2H4)

My printing error - should be 120
Title: Re: cyclopropyl phenylmethanol:mass spectrum
Post by: MOTOBALL on October 06, 2016, 02:15:46 PM
AWK,

I have not run an EI spectrum in more than 25 years, so will take your word for it.
However, we don't want to confuse gk000 with statements like
m/z 130 is M+ - C2H4

Regards
Title: Re: cyclopropyl phenylmethanol:mass spectrum
Post by: AWK on October 06, 2016, 02:26:00 PM
Elimination of ethylene is typical for cyclopropane derivatives.
See, eg ethylcyclopropane
http://webbook.nist.gov/cgi/cbook.cgi?ID=C1191964&Mask=200
Title: Re: cyclopropyl phenylmethanol:mass spectrum
Post by: MOTOBALL on October 06, 2016, 05:45:28 PM
Yes, but MW is not 158 is it ?

Regards
Title: Re: cyclopropyl phenylmethanol:mass spectrum
Post by: AWK on October 07, 2016, 02:26:26 AM
Quote from: MOTOBALL on October 06, 2016, 05:45:28 PM
Yes, but MW is not 158 is it ?

Regards
Sorry for my previous fault, but 158 is a back interpretation of this printing error.