Interpreting the mass
spectrum of 2,2,3-trimethylbutane
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Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US
K12 grade 11 grade 12 organic chemistry courses involving molecular
spectroscopy analysing mass spectra of 2,2,3-trimethylbutane
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Mass spectroscopy - spectra index
See also
comparing the
1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16
Mass spectra 2,2,3-trimethylbutane spectra note: Students and teachers please note
my explanation of the mass spectrum of 2,2,3-trimethylbutane is designed for
advanced, but pre-university, chemistry courses. If M represents the
2,2,3-trimethylbutane molecule, the initial ionisation to give the molecular ion is:
M(g) +
high KE e- ==> [M]+(g) + 2e-.
Fragmentation equations assume [M]+ is the start of the
processes and all species are in a gaseous state.
I've not usually shown an unpaired electron on e.g. a non-ionised
alkyl radical R e.g. [M]+ ==> [X]+ + R•,
but you should be aware this is a more accurate depiction of some
processes, but I've used simplified equations to show how some of
the ions are formed in the fragmentation pattern for
2,2,3-trimethylbutane.
Stick diagram and table of m/z ions for the mass spectrum of
2,2,3-trimethylbutane.
2,2,3-trimethylbutane
C7H16
For more
see
The molecular structure,
classification and
naming of alkanes
Interpreting the fragmentation pattern of the mass spectrum of
2,2,3-trimethylbutane
[M]+ is the molecular ion peak (M) with an m/z of
100 corresponding to [C7H16]+, the original
2,2,3-trimethylbutane molecule minus an electron,
[(CH3)3CCH(CH3)2]+.
The molecular ion peak of
2,2,3-trimethylbutane is very small, suggesting it is a very
unstable ion.
You might detect an M+1 peak at m/z 101, corresponds to an ionised
2,2,3-trimethylbutane
molecule with one 13C atom in it i.e. an ionised
2,2,3-trimethylbutane molecule of
formula [13C12C6H16]+
Carbon-13 only accounts for ~1% of all carbon atoms
(12C ~99%), but the more carbon atoms in the molecule,
the greater the probability of observing this 13C M+1
peak.
2,2,3-trimethylbutane has 7 carbon atoms, so on
average, ~1 in 14 molecules will contain a 13C atom.
This sort of argument also applies to fragment ions
from the parent molecular ion of 2,2,3-trimethylbutane - though the ratio will be
greater:
e.g. m/z 44 ion could be [13C12C2H7]+,
m/z 58 ion [13C12C3H9]+,
and m/z 86 ion [13C12C5H13]+
These ions might be more likely than those containing
only 12C isotope atoms
i.e. [C3H8]+,
[C4H10]+ and
[C6H14]+
Either way, for identification
purposes, all these peaks add uniqueness to the fragmentation pattern of the mass spectrum of
2,2,3-trimethylbutane.
The most abundant ion of the molecule under mass
spectrometry investigation (2,2,3-trimethylbutane) is usually given an arbitrary abundance value of
100, called the base ion peak, and all other abundances
('intensities') are measured against it.
The base ion peak for
the mass spectrum of 2,2,3-trimethylbutane is the m/z ion 57
[C4H9]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2,2,3-trimethylbutane.
Unless otherwise indicated, assume the carbon atoms in
heptane are the 12C isotope.
Some of the possible positive ions, [molecular fragment]+,
formed in the mass spectrometry of
2,2,3-trimethylbutane.
The parent molecular ion of 2,3-dimethylpentane m/z 100:
[C7H16]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of
2,2,3-trimethylbutane.
m/z value of
[fragment]+ |
86 |
85 |
69 |
58 |
57 |
56 |
[molecular fragment]+ |
[13C12C5H13]+ |
[C6H13]+ |
[C5H9]+ |
[13C12C3H9]+ |
[C4H9]+ |
[C4H8]+ |
m/z value of
[fragment]+ |
55 |
43 |
41 |
39 |
29 |
27 |
[molecular fragment]+ |
[C4H7]+ |
[C3H7]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2,2,3-trimethylbutane
Examples of possible equations to explain some of the most abundant ion peaks
in the mass spectrum of 2,2,3-trimethylbuttane
Atomic masses: H = 1; C = 12 (13 for ~1
in 100)
Bond enthalpies = kJ/mol: C-C = 348;
C-H = 412
Formation of m/z 85 ion:
[(CH3)3CCH(CH3)2]+ ===> [C6H13]+
+ CH3
C-C bond scission in the parent molecular ion of
2,2,3-trimethylbutane to release a methyl group, Ion mass change = 100 - 15 = 85
(M-15 ion)
Formation of m/z 69 ion:
[?]+ ===> [C5H9]+
+ ?
Formation of m/z 57 ion:
[(CH3)3CCH(CH3)2]+ ===> [(CH3)3C]+
+ C3H7
C-C bond scission in the parent molecular ion,
mass change 100 - 43 = 57 (M-43 ion)
The m/z 57 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
The m/z 57 ion can also be formed from the
elimination of ethene from the m/z 85 ion.
[(CH3)3C]+
===> [C4H9]+
+ C2H4
Mass change = 85 - 28 = 57
Many other fragments are formed by hydrogen
atom/molecule loss, so
you get m/z ion sequences like 71 ==>70, 57 ==> 56 ==> 55, 43 ==> 39 and 29 ==>
27 etc. (see examples below).
Formation of m/z 56 ion:
[C6H13]+ ===> [C4H8]+
+ C2H5
Perhaps C-C bond scission and proton rearrangement?
85 - 29 = 56, or the other
fragment could be ionised to give the m/z 29 ion.
[C6H13]+ ===> [C2H5]+
+ C4H8
Formation of m/z 43 ion:
[(CH3)3CCH(CH3)2]+ ===> [CH(CH3)2]+
+ (CH3)3C
C-C bond scission in the parent molecular ion.
Ion mass change: 100 - 57 = 43
(M-57 ion)
Note this is the same C-C bond fission as the m/z 57
ion formation, but the other fragment is ionised.
Formation of m/z 41 ion:
[?]+ ===> [C3H5]+
+ ?
Comparing the
1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16
You can distinguish all 9 isomers from a data combination of their number of
1H NMR
chemical shifts,
and their resulting integrated 1H proton ratios, plus, their number of
13C
chemical shifts. |
Name of the alkane structural isomer of molecular
formula C7H16 |
Abbreviated structural formulae
of the nine isomers of molecular formula C7H16 (interpretation complications with 3-methylhexane and
2,3-dimethylpentane because they exhibit R/S isomerism due to a
chiral carbon) |
Skeletal formula of the
nine
alkane isomers of
molecular formula C7H16 |
Number of 1H NMR chemical shifts (δ) and
proton ratio (links
to spectrum) |
Number of 13C chemical shifts (δ)
(links
to spectrum) |
heptane |
|
|
4 δ: proton ratio: 3:2:2:1 (6:4:4:2 in the molecule) |
4 δ shifts |
2-methylhexane |
|
|
6 δ: proton ratio :
6:3:2:2:2:1 |
6 δ shifts |
3-methylhexane |
|
|
7 δ: proton ratio:
3:3:3:2:2:2:1 (simplification) ! |
7
δ shifts |
3-ethylpentane |
|
|
3 δ: proton ratio:
9:6:1 |
3 δ
shifts |
2,2-dimethylpentane |
|
|
4 δ: proton ratio:
9:3:2:2 |
5 δ shifts |
2,3-dimethylpentane |
|
|
6 δ: proton ratio:
6:3:3:2:1:1 (simplification) ! |
6 δ
shifts (simplification) !!! |
2,4-dimethylpentane |
|
|
3 δ: proton ratio:
12:2:2 |
3 δ
shifts |
3,3-dimethylpentane |
|
|
3 δ: proton ratio:
3:3:2 (6:4:4 in the molecule) |
4 δ
shifts |
2,2,3-trimethylbutane |
|
|
3 δ: proton ratio:
9:6:1 |
4 δ shifts |
Key words & phrases: C7H16 image diagram on how to interpret and explain the mass spectrum of
2,2,3-trimethylbutane m/z m/e base peaks, image and diagram of the mass spectrum of
2,2,3-trimethylbutane, details of the mass spectroscopy of 2,2,3-trimethylbutane, low and high resolution mass
spectrum of 2,2,3-trimethylbutane, prominent m/z peaks in the mass spectrum of
2,2,3-trimethylbutane, comparative
mass spectra of 2,2,3-trimethylbutane, the molecular ion peak in the mass spectrum of
2,2,3-trimethylbutane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2,2,3-trimethylbutane, characteristic pattern of peaks in the mass spectrum of
2,2,3-trimethylbutane, relative
abundance of mass ion peaks in the mass spectrum of 2,2,3-trimethylbutane, revising the mass
spectrum of 2,2,3-trimethylbutane, revision of mass spectroscopy of
2,2,3-trimethylbutane, most abundant ions in the
mass spectrum of 2,2,3-trimethylbutane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2,2,3-trimethylbutane, how to analyse the mass
spectrum of 2,2,3-trimethylbutane, how to describe explain the formation of fragmented ions in the
mass spectra of 2,2,3-trimethylbutane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2,2,3-trimethylbutane
recognising the base ion peak of 2,2,3-trimethylbutane
interpreting interpretation the mass spectrum of 2,2,3-trimethylbutane
(CH3)3CCH(CH3)2
(H3C)3CCH(CH3)2 Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2,2,3-trimethylbutane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2,2,3-trimethylbutane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2,2,3-trimethylbutane. The m/e m/z value of the base ion peak in the
mass spectrum of 2,2,3-trimethylbutane. Possible examples of equations showing the formation
of the ionised fragments in 2,2,3-trimethylbutane. Revision notes on the mass spectrum of
2,2,3-trimethylbutane.
Matching and deducing the structure of the 2,2,3-trimethylbutane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic alkanes,
mass spectra of 2,2,3-trimethylbutane, a structural isomer of molecular formula
C7H16
How do you interpret the mass spectrum of
2,2,3-trimethylbutane How to interpret
the mass spectrum of 2,2,3-trimethylbutane Explanatory diagram of the mass spectrum of the
2,2,3-trimethylbutane molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2,2,3-trimethylbutane. How to explain the mass spectrum of
2,2,3-trimethylbutane. The m/z value of the
molecular ion peak in the mass spectrum of
2,2,3-trimethylbutane. Identifying 2,2,3-trimethylbutane from
its mass spectrum pattern. The m/z m/e peak analysis interpretation
diagram of the mass
spectrum of the 2,2,3-trimethylbutane molecule. The uses of the mass spectrum of the
2,2,3-trimethylbutane molecule. The distinctive features of the mass spectrum of
the 2,2,3-trimethylbutane molecule explained. explaining the fragmentation pattern of the mass spectrum of
2,2,3-trimethylbutane equations showing the
formation of the ionised fragments in the mass spectrum of
2,2,3-trimethylbutane
what does the mass spectrum tell you about the structure and
properties of the 2,2,3-trimethylbutane molecule? Data table of ionised fragments in
the mass spectrum of 2,2,3-trimethylbutane and equations for their formation in the
fragmentation of 2,2,3-trimethylbutane molecules
Links associated
with
2,2,3-trimethylbutane
The infrared spectrum
of 2,2,3-trimethylbutane
The H-1
NMR spectrum of 2,2,3-trimethylbutane
The
C-13 NMR spectrum of 2,2,3-trimethylbutane
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Infrared spectra of the isomers of C7H16
The infrared
spectrum of heptane
The
infrared spectrum of 2-methylhexane
The
infrared spectrum of 3-methylhexane
The
infrared spectrum of 3-ethylpentane
The infrared spectrum of
2,2-dimethylpentane
The infrared spectrum of
2,3-dimethylpentane
The infrared spectrum of
2,4-dimethylpentane
The infrared spectrum of
3,3-dimethylpentane
The infrared spectrum
of 2,2,3-trimethylbutane
|
Mass spectra of the isomers of C7H16
The mass
spectrum of heptane
The mass
spectrum of 2-methylhexane
The mass
spectrum of 3-methylhexane
The
mass spectrum of 3-ethylpentane
The mass spectrum of
2,2-dimethylpentane
The mass spectrum of
2,3-dimethylpentane
The mass spectrum of
2,4-dimethylpentane
The mass spectrum of
3,3-dimethylpentane
The mass
spectrum of 2,2,3-trimethylbutane
|
1H NMR spectra of the isomers of C7H16
The H-1 NMR
spectrum of heptane
The
H-1 NMR spectrum of 2-methylhexane
The
H-1 NMR spectrum of 3-methylhexane
The
H-1 NMR spectrum of 3-ethylpentane
The H-1 NMR spectrum of
2,2-dimethylpentane
The H-1 NMR spectrum of
2,3-dimethylpentane
The H-1 NMR spectrum of
2,4-dimethylpentane
The H-1 NMR spectrum of
3,3-dimethylpentane
The H-1
NMR spectrum of 2,2,3-trimethylbutane
|
13C NMR spectra of the isomers of C7H16
The C-13 NMR
spectrum of heptane
The
C-13 NMR spectrum of 2-methylhexane
The
C-13 NMR spectrum of 3-methylhexane
The
C-13 NMR spectrum of 3-ethylpentane
The C-13 NMR spectrum of
2,2-dimethylpentane
The C-13 NMR spectrum of
2,3-dimethylpentane
The C-13 NMR spectrum of
2,4-dimethylpentane
The C-13 NMR spectrum of
3,3-dimethylpentane
The
C-13 NMR spectrum of 2,2,3-trimethylbutane
|
|