Organic Chemistry Semester 1 LABORATORY MANUAL - Moravian ...

Mass Spectrometry Appendix D6 Part V. Techniques and Theory
e. Breakage of Unsaturated Compounds with Rearrangement:
e.g. The mass spectra of (E)3-methyl-2-hexene, (3-methylbutyl)benzene and 3-
nonanone show a prominent peaks at 70, 92 and 72 respectively due to the
breakages indicated below.
H
H
a
m/z = 70
CH 2
b
H
H
a
m/z = 70
CH 2
b CH 2
m = 28 m/z = 92
H
"
!
H
"
m/z = 92 m = 56
O
H
H
C
O
H
m/z = 72
m/z = 72 m = 70
Molecular Formula from the Mass Spectrum.
You may have noted that there are often some small peaks at higher mass than expected for the
molecular ion. These peaks usually appear at masses of M + 1 and M + 2. They are caused by
the presence of heavy isotopes of carbon, nitrogen and oxygen in some of the molecules. It is
possible to use the intensities of the M + 1 and M + 2 to determine the molecular formula of a
compound from the mass spectrum (see CGWW pp. 50-56). The in-lab questions for experiment 8
will explore this aspect of mass spectrometry.
Additional Out of Class Applications:
Now use the reference MS information in Padías pp. 99-105 and the activities on pp. D1→D6, to
devise responses to the following:
a. Predicting Mass Spectra from Structures:
Propose structures for the ions that most likely produced the numbered peaks in each of the
following spectra.
(1.)
Intensity
41
43
69
CH 3
87
H
CH 3 O
CH CH 2 CH
111
CH 3
CH 2
CH CH 3
126
M +
144
m/z
(2.)
29
57
Intensity
O
M +
m/z