Lecture 23
Quiz 6 Average = 72% (36 out of 50)
Mass Spectrometry
This week in lab:
• Ch 9 Final Report Due
• Synthetic #2 PreLab Due
Next week in lab:
• Synthetic #1 Final Report Due
Molecular Ions
Molecular Ions = sum of atomic weight(s) of all isotopes of all elements
in a molecule. If you had 10,000 atoms of each type:






C:
H:
O:
N:
Cl:
Br:
9890 12C atoms
110 13C atoms
9999 1H atoms
1 2H atoms
9985 16O atoms, 4 17O atoms, 20 18O atoms
9963 14N atoms
37 15N atoms
7577 35Cl atoms
2423 37Cl atoms (3:1 35Cl:37Cl)
5069 79Br atoms
4931 81Br atoms (1:1 79Br:81Br)
Molecular Ions: Cl & Br
Compounds - Isotopes
Significant M+2 peak => Cl or Br
• Cl is 3 parts 35Cl and 1 part 37Cl
• Br is 1:1 79Br and 81Br
MS of Chlorobenzene
M+
Cl
C6H535Cl = m/z 112
C6H537Cl = m/z 114
Note ratio:
m/z 112:114
is 3:1
M+2
MS of Bromobenzene
C6H579Br = m/z 156
Br
C6H581Br = m/z 158
M+ = 156
Note ratio:
156:158
1:1
M + 2 = 158
Mass Spec Correlation Chart #1
Common fragments ions and/or neutral losses:
•15 can only be methyl
•17 usually OH
•18 always H2O
•28 CO or CH2=CH2
•29 CH3CH2 or CHO
•31 CH3O
•35/37 Cl (special isotope pattern too!)
•42 CH2=C=O
•43 CH3CO or C3H7
•45 CH3CH2O or COOH
•79/81 Br (special isotope pattern too!)
Mass Spec Correlation Chart #2
Weights of common fragments or neutral losses:
O
15 = CH3 18 = H2O 29 = C2H5 43 = C3H7 or
O
31 = CH3O
O
57 = C4H9 or
71 = C5H11 or
45 = CH3CH2O
O
79Br 81Br
85 = C6H13 or 35Cl
77 =
O
C
CH2
91 =
105 =
37Cl
Lab Guide Problems
Problem 15.3
Mass spec shown in class (Figure 15.11)
Account for the peaks at m/z values of 88 (M+) , 73, 70, 59,
and 55.
Lab Guide Problems
Problem 15.5
Account for the peaks in the mass spectrum of 2-octanone
at m/z values of 129 (M+1), 128 (M+), 113, 85, and 43.
Lab Guide Problems
Problem 19.13
The most intense peak in the mass spectrum of
2,2-dimethylbutane is at m/z = 57. Propose a structure of
this fragment ion (a.k.a. daughter ion).
Nutmeg Oil GC-MS Data
Mass Spectrum of A
M+ at 192
Compound is:
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
OCH3
O
O
Allyl methoxybenzodioxole
FW = 192
Nutmeg Oil GC-MS Data
Mass Spectrum of B
M+ at 208
Compound is:
OCH3
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
H3CO
OCH3
Allyl trimethoxybenzene
FW = 208