6. 7. If acetone is ionized, which electron is most likely to be knocked off? Replace this electron with a + (to indicate the missing electron) on the structure of acetone → :O: The ion you drew above is called the "molecular ion" of acetone. It is identical to the original molecule except that it is missing an electron, has a + charge, and can be accelerated by the mass spectrometer. What is the weight (in amu) of the molecular ion of acetone? (report only two significant figures, e.g., XX amu)

Organic Chemistry: A Guided Inquiry
2nd Edition
ISBN:9780618974122
Author:Andrei Straumanis
Publisher:Andrei Straumanis
Chapter2: Lewis Structures
Section: Chapter Questions
Problem 5CTQ
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Model 3: Ionization and Fragmentation
Most mass spectrometers accelerate a molecule by first turning it into an ion, then using an electric field
to accelerate it toward the detector.
During standard ionization, a molecule loses one electron, the electron that is easiest to
remove. The following is a hierarchy of electrons from easiest to hardest to remove:
Electron in a lone pair (easiest)
Electron that is part of a double bond (pi bond)
Electron in a single bond (hardest)
Knocking off an electron to make a +1 ion can be a harsh process. This harsh treatment often
results in a broken bond, generating two smaller pieces, a +1 ion and a neutral fragment.
(Note that only the ion is accelerated and detected.)
●
7.
●
Critical Thinking Questions
6.
If acetone is ionized, which electron is most likely to be knocked off? Replace this
electron with a + (to indicate the missing electron) on the structure of acetone →
The ion you drew above is called the "molecular ion" of acetone. It is identical to the original
molecule except that it is missing an electron, has a + charge, and can be accelerated by the mass
spectrometer. What is the weight (in amu) of the molecular ion of acetone? (report only two
significant figures, e.g., XX amu)
Model 4: Mass Spectrum of Acetone
The mass spectrum of acetone is a tally of the number of ions of each mass (m/z) that hit the
detector when a large number of acetone molecules are run through the mass spectrometer. The peak
intensity (peak height) tells you the relative number of ions of that weight that hit the detector.
Peak Intensity
m/z
14.0
15.0
26.0
27.0
29.0
38.0
39.0
41.0
42.0
43.0
44.0
58.0
59.0
2.9
23.1
3.5
5.7
3.1
2.2
4.2
2.0
9.1
100.0
3.4
63.8
3.1
Relative Intensity
100-
80-
60
40-
20-
MS-IU-7000
0+
10
15
20
25
+
30
H||||
35
m/z
40
:0:
45
50
55
+
Critical Thinking Questions
8. Label the peak due to the "molecular ion" of acetone with [M]* (symbol for a molecular ion).
9. What is the peak intensity (relative height) of the [M]* peak in this spectrum?
60
Transcribed Image Text:Model 3: Ionization and Fragmentation Most mass spectrometers accelerate a molecule by first turning it into an ion, then using an electric field to accelerate it toward the detector. During standard ionization, a molecule loses one electron, the electron that is easiest to remove. The following is a hierarchy of electrons from easiest to hardest to remove: Electron in a lone pair (easiest) Electron that is part of a double bond (pi bond) Electron in a single bond (hardest) Knocking off an electron to make a +1 ion can be a harsh process. This harsh treatment often results in a broken bond, generating two smaller pieces, a +1 ion and a neutral fragment. (Note that only the ion is accelerated and detected.) ● 7. ● Critical Thinking Questions 6. If acetone is ionized, which electron is most likely to be knocked off? Replace this electron with a + (to indicate the missing electron) on the structure of acetone → The ion you drew above is called the "molecular ion" of acetone. It is identical to the original molecule except that it is missing an electron, has a + charge, and can be accelerated by the mass spectrometer. What is the weight (in amu) of the molecular ion of acetone? (report only two significant figures, e.g., XX amu) Model 4: Mass Spectrum of Acetone The mass spectrum of acetone is a tally of the number of ions of each mass (m/z) that hit the detector when a large number of acetone molecules are run through the mass spectrometer. The peak intensity (peak height) tells you the relative number of ions of that weight that hit the detector. Peak Intensity m/z 14.0 15.0 26.0 27.0 29.0 38.0 39.0 41.0 42.0 43.0 44.0 58.0 59.0 2.9 23.1 3.5 5.7 3.1 2.2 4.2 2.0 9.1 100.0 3.4 63.8 3.1 Relative Intensity 100- 80- 60 40- 20- MS-IU-7000 0+ 10 15 20 25 + 30 H|||| 35 m/z 40 :0: 45 50 55 + Critical Thinking Questions 8. Label the peak due to the "molecular ion" of acetone with [M]* (symbol for a molecular ion). 9. What is the peak intensity (relative height) of the [M]* peak in this spectrum? 60
10. What is the weight (m/z) and intensity (height) of the most common ion detected during this mass
spectrometry experiment? (This will be the tallest peak on the mass spectrum.)
Memorization Task L2.1: Base Peak vs. Molecular lon
Warning! The [M]* peak (molecular ion) is often NOT the same as the base peak (largest peak).
The base peak is easy to identify since it is always the largest peak (by convention set to 100).
The [M] peak (molecular ion) is easy to identify if you know the structure of the analyte (e.g., acetone).
Note: It can be hard to identify the [M] peak on the mass spectrum of an unknown molecule. Much of
this activity will be devoted to figuring out how to identify the [M]* peak on the MS of an unknown.
11. For some spectra, the base peak is the same as the [M]* peak. Is this the case for the spectrum of
acetone shown on the previous page?
12. What is the size (in amu) of the neutral fragment that was lost to give the ion responsible for the
base peak at m/z = 43?
a.
b.
C.
What combination of atoms weighs the amount you reported above, and whose loss could
account for the peak at m/z = 43?
Construct an explanation for why the peak at m/z = 43 is called
the [M-15]* peak.
Using the same naming strategy, name the peak on the mass spectrum at m/z = 15.
13. The major peaks on a mass spectrum representing ions lighter than the molecular ion are called
fragment peaks. Draw an ion that could account for [M-15]* peak in the mass spectrum of acetone.
Transcribed Image Text:10. What is the weight (m/z) and intensity (height) of the most common ion detected during this mass spectrometry experiment? (This will be the tallest peak on the mass spectrum.) Memorization Task L2.1: Base Peak vs. Molecular lon Warning! The [M]* peak (molecular ion) is often NOT the same as the base peak (largest peak). The base peak is easy to identify since it is always the largest peak (by convention set to 100). The [M] peak (molecular ion) is easy to identify if you know the structure of the analyte (e.g., acetone). Note: It can be hard to identify the [M] peak on the mass spectrum of an unknown molecule. Much of this activity will be devoted to figuring out how to identify the [M]* peak on the MS of an unknown. 11. For some spectra, the base peak is the same as the [M]* peak. Is this the case for the spectrum of acetone shown on the previous page? 12. What is the size (in amu) of the neutral fragment that was lost to give the ion responsible for the base peak at m/z = 43? a. b. C. What combination of atoms weighs the amount you reported above, and whose loss could account for the peak at m/z = 43? Construct an explanation for why the peak at m/z = 43 is called the [M-15]* peak. Using the same naming strategy, name the peak on the mass spectrum at m/z = 15. 13. The major peaks on a mass spectrum representing ions lighter than the molecular ion are called fragment peaks. Draw an ion that could account for [M-15]* peak in the mass spectrum of acetone.
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