A comprehensive in-depth, specification led, revision booklet of the 2015 spec chemistry AS level course. Written during AS studies by an AS student, meaning it is written in an easily comprehensible stance for any student wishing to find extra resources to revise from.
A Cook 2016
Chapter 7 - Mass Spec in Organic Compounds
Topic 7A: Mass spectrometry
1. Be able to use data from a mass spectrometer to:
i Determine the relative molecular mass of an organic compound from the
molecular ion peak
ii Suggest possible structures of a simple organic compound from the m/z of the molecular
ion and fragmentation patterns
Background to mass spectrometry
The mass spectrum of an element often appears as very simple, with small vertical lines, called
peaks, each representing a different isotope of the element
In contrast, the masse spectrum of an organic compound often appears complex, with a large
number of peaks. If all the peaks are included,m there may be more peaks than there are atoms in
a molecule of the compound, so something different is happening.
The molecular Ion peak
The peak furthest to the right with the highest (m/z) value, is the molecular ion peak. This peak is
the molecular ion peak - the result of the organic molecule losing an electron in the mass
spectrometer.
The equation for this process, using butane as an example is:
C4H10 + e- —> C4H10+ + 2e-
An electron collides with a butane molecule and knocks out an electron, so forming a positive ion
form the molecule. The m/z value of this peak (58) indicates the relative molecule mass of butane.
Other peaks
The spectrum may show a very small peak to the right of the molecular ion peak (M+1 peak).
Caused by the presence of naturally occurring isotope (13C instead of the normal 12C) in the
molecule. Approximately 99% of all carbon atoms are 12C, with the remaining 1% being 13C.
The peaks with smaller m/z values, are a result of the fragmentation in the mass spectrometer,
rearrangement reactions and the loss of more than one electron.
Fragmentation in hydrocarbons:
Fragmentation is very common, and can be used to work out the structure of an organic
compound:
Consider the breaking of a carbon-carbon bond, in the molecular ion formed from a hydrocarbon.
Two species are formed. They are:
1. Another positive ion
2. A neutral species (usually a radical)
Examples of fragmentation
A very simple example of fragmentation is the molecular ion of ethane, which can fragment to form
a methyl cation and a methyl radical:
, A Cook 2016
(CH3-CH3)+ —> CH3+ + CH3
The ethane molecule is symmetrical, and there is only one carbon-carbon bond. So the second
methyl group is just as likely to become a positive ion, as the left one was. This would be the
equation for that to happen:
(CH3-CH3)+ —> CH3 + CH3+
You can see that the products produced are identical so there will be a peak at m/z = 15.
Sometimes there is a radical formed which is shown the a dot (representing the unpaired electron),
and the molecular ion is shown with a dot as well as a positive charge, as in this example:
(CH3-CH3)+. —> CH3. + CH3+
Now consider propene’s fragmentation:
There are 2 carbon - carbon bonds, but their are equivalent - they can both be described as the
bond between the central carbon and one of the two terminal carbons. However, there are now two
possible fragment ions the can form:
(CH3-CH2-CH3)+ —> CH3+ + CH2-CH3
(CH3-CH2-CH3)+ —> CH3 + (CH2-CH3)+
You would therefore see peaks at m/z = 15 (methyl) and m/z = 29 (ethyl) in its spectrum.
Radicals are not detected in a mass spectrometer so only the positive ions will be detected.
Mass spectrum
The peak with the largest m/z value is the molecular ion peak - caused by the organic molecule
losing one electron in the mass spec. It is the molecular mass of the compound too.
The peak which is the highest is called the base peak and is caused by the ion which has the
highest abundance
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