15 Worksheet 4
Fragmentation and analysis of organic molecules
Introduction
In the mass spectrometer, organic molecules are first converted into positive ions by bombarding
them with high-energy electrons. These positive ions are molecular ions.
For example, ethanol, C2H5OH, is converted into its molecular ion as follows:
CH3CH2OH + e– → CH3CH2OH+ + 2e–
Because it has absorbed a lot of energy, the molecular ion can then fragment into positive ions and
free radicals.
For example, the positive molecular ion of ethanol can fragment as follows:
CH3CH2OH+ → CH3+ + CH2OH
Only the positive ion is detected. Ethanol can fragment into a number of different ions.
Questions
1
a
Copy and complete the table below which shows the m/e ratios for various peaks obtained
by the fragmentation of the molecular ion of ethanol.
[4]
Peak
2
m/e ratio for
peak
A
15
B
29
Positive ion
responsible
C
CH2OH+
D
CH3CH2O+
b Give equations to show how the positive ions identified in a may be formed from
the molecular ion.
[8]
Ethoxyethane, CH3OCH3, is isomeric with ethanol.
a Give two possible similarities between the fragmentation patterns for both ethoxyethane
and ethanol.
b Give two possible differences between the fragmentation patterns.
[4]
[4]
Total
COAS Chemistry 1 Teacher Resources
21
Score:
Original material © Cambridge University Press 2005, 2008
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