2.8: Mass Spectrometry Problems
- Page ID
- 432166
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The following figure shows the mass spectrum of a saturated hydrocarbon (containing only carbon and hydrogen with only single bonds between carbons, not double bonds).
- Draw five different structures that would have the molecular weight of this compound.
- Choose three smaller m/z values from the spectrum and draw one structure for each of them. Note that these fragments will not have complete Lewis structures.
- Answer
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1. Molecular Weight = 114, which cooresponds to a C8H18 hydrocarbon. There is the possibility of 18 isomers, but here are a few isomers:
2. m/z = 57; [CH3CH2CH2CH2]+
m/z = 43; [CH3CH2CH2]+
m/z = 29; [CH3CH2]+
Caffeine has a mass of 194.19 amu, determined by mass spectrometry, and contains C, N, H, O. What is a molecular formula for this molecule?
- Answer
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C8H10N4O2
C = 12 × 8 = 96
N = 14 × 4 = 56
H = 1 × 10 = 10
O = 2 × 16 = 32
96+56+10+32 = 194 g/mol
The following are the spectra for 2-methyl-2-hexene and 2-heptene, which spectra belongs to the correct molecule. Explain.
A:
B:
Source: SDBSWeb : http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, 2 December 2016)
- Answer
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The (A) spectrum is 2-methyl-2-hexene and the (B) spectrum is 2-heptene. Looking at (A) the peak at 68 m/z is the fractioned molecule with just the tri-substituted alkene present. While (B) has a strong peak around the 56 m/z, which in this case is the di-substituted alkene left behind from the linear heptene.
What are the masses of all the components in the following fragmentations?
- Answer
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The first undergoes an alpha cleavage. The second undergoes a dehydration. The final one goes througha McLafferty rearrangement.
5-Chloro-2-pentanone has the mass spectrum shown. Which peak represents the M+? Which is the base peak? Why is there a peak at 122? Explain what the fragment for the base peak would be.
Source: SDBSWeb : https://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_frame_disp.cgi?sdbsno=10178 (National Institute of Advanced Industrial Science and Technology, 16 August 2022)
- Answer
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M+ = 120
base peak = 43
The m/z peak at 122 is the M + 2 peak. It occurs because chlorine has two isotopes 35C and 37C in a 3:1 ratio.
The m/z = 43 occurs due to the alpha cleavage. The acylium ion has an m/z of 43. This fragment is particularly stable due to resonance.
