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The mass
spectrum of 2,3-dimethylbutane
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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 2,3-dimethylbutane
See also
comparing infrared, mass, 1H NMR & 13C NMR
spectra of the structural alkane isomers of C6H14
2,3-dimethylbutane C6H14
 ,
,
For more
see The molecular structure,
classification and
naming of alkanes
Interpreting the fragmentation pattern of the mass spectrum of
2,3-dimethylbutane
[M]+ is the molecular ion peak (M) with an m/z of
86 corresponding to [C6H14]+, the original 2,3-dimethylbutane molecule minus an electron,
[(CH3)2CHCH(CH3)2]+.
The minute M+1 peak at m/z 87, corresponds to an ionised
2,3-dimethylbutane
molecule with one 13C atom in it i.e. an ionised
2,3-dimethylbutane molecule of
formula 13C12C5H14
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.
2,3-dimethylbutane has 6 carbon atoms, so on
average, ~1 in 17 molecules of will contain a 13C atom.
A similar argument applies to fragment ions from the
breakdown of the parent molecular ion of 2,3-dimethylbutane - though
the ratio will be greater e.g. the m/z 44 ion.
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2,3-dimethylbutane.
The most abundant ion of the molecule under mass
spectrometry investigation is usually given an arbitrary abundance value of
100, called the base ion peak, and all other abundances
('intensities') are measured against it.
| m/z value of
[fragment]+ |
71 |
57 |
55 |
44 |
43 |
| [molecular fragment]+ |
[(CH3)2CHCHCH3]+ |
[C4H9]+ |
[C4H7]+ |
13C12C2H7 |
[(CH3)2CH]+ |
| m/z value of
[fragment]+ |
42 |
41 |
39 |
29 |
27 |
| [molecular fragment]+ |
[C3H6]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2,3-dimethylbutane
Atomic masses: H = 1; C = 12 (13 for ~1
in 100)
Bond enthalpies = kJ/mol: C-C = 348;
C-H = 412
Examples of possible equations to explain some of the most abundant ion peaks
in the mass spectrum of
2,3-dimethylbutane
Formation of m/z 71 ion:
[(CH3)2CHCH(CH3)2]+ ===> [(CH3)2CHCHCH3]+
+ CH3
C-C bond chain scission in the parent molecular ion, methyl fragment lost from
parent molecular ion,
mass change = 86 - 15 = 71.
I've quoted a possible structure of the [C5H11]+
ion.
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 43 ion:
[(CH3)2CHCH(CH3)2]+ ===> [(CH3)2CH]+
+ (CH3)2CH
C-C bond chain scission in the parent molecular ion,
mass change = 86 -43 = 43.
The m/z 43 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
m/z sequences like 43, 42, 41 or 29, 28, 27, indicate
successive hydrogen atom loss.
Sequences including m/z values of 42, 41, 40, 39 or
28, 28, 27, 26, indicate
successive hydrogen atom/molecule loss from the m/z 43 or 29 ions.
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Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the five structural alkane isomers of C6H14
NOTE: The images are linked to their
original detailed spectral analysis pages AND can be doubled in
size with touch screens to
increase the definition to the original hexane,
2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and
2,3-dimethylbutane image sizes. These five molecules
are structural isomers of saturated alkanes of molecular formula C6H14
and
exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower
aliphatic alkanes (non-cyclic alkanes). |
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Infrared spectra below. |
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INFRARED SPECTRA:
Apart from the significant differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, there are no other
great striking differences, but each could be identified from
its infrared spectrum. All the absorption
bands are typical of molecules containing saturated alkyl structure and
there are no characteristic infrared absorptions due to a specific
functional group. |
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Infrared spectra above, mass spectra below. |
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MASS SPECTRA: Base ion
peaks plus m/z comments.
Hexane: m/z 57, 42 and 56 prominent
2-methylpentane: m/z 43, 42 and 71 prominent
3-methylpentane: m/z 57, 41 and 56 prominent
2,2-dimethylbutane: m/z 43, 41, 57 and 71
prominent
2,3-dimethylbutane: m/z 43, 41, 42 and 71
prominent |
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Mass spectra above, 1H NMR spectra below. |
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1H NMR SPECTRA: They can
all be distinguished by their different integrated proton ratios -
need very high resolution.
Hexane:
3 1H
δ shifts, H ratio 3:2:2 (6:4:4 in formula)
2-methylpentane:
5 1H
δ shifts, H ratio 6:3:2:2:1
3-methylpentane:
4 1H
δ shifts, H ratio 6:4:3:1
2,2-dimethylbutane: 3 1H
δ shifts, H ratio 9:3:2
2,3-dimethylbutane: 2 1H
δ shifts, H ratio 6:1 (12:2 in formula) |
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1H NMR spectra above, 13C NMR spectra below. |
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13C NMR SPECTRA: From the
number of shifts, you can't distinguish (iii) and (iv) but you can
distinguish them from (i), (ii) and (v). (i) Hexane: 3 13C
δ shifts
(ii) 2-methylpentane: 5 13C
δ shifts
(iii) 3-methylpentane: 4 13C
δ shifts
(iv) 2,2-dimethylbutane: 4 13C
δ shifts
(v) 2,3-dimethylbutane: 2 13C
δ shifts |
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13C NMR spectra above. |
Key words & phrases: image diagram on how to interpret and explain the mass spectrum of
2,3-dimethylbutane m/z m/e base peaks, image and diagram of the mass spectrum of
2,3-dimethylbutane, details of the mass spectroscopy of 2,3-dimethylbutane, low and high resolution mass
spectrum of 2,3-dimethylbutane, prominent m/z peaks in the mass spectrum of
2,3-dimethylbutane, comparative
mass spectra of 2,3-dimethylbutane, the molecular ion peak in the mass spectrum of
2,3-dimethylbutane,
analysing and understanding the fragmentation pattern of the mass spectrum
of 2,3-dimethylbutane, characteristic pattern of peaks in the mass spectrum of
2,3-dimethylbutane, relative
abundance of mass ion peaks in the mass spectrum of 2,3-dimethylbutane, revising the mass
spectrum of 2,3-dimethylbutane, revision of mass spectroscopy of
2,3-dimethylbutane, most abundant ions in the
mass spectrum of 2,3-dimethylbutane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 2,3-dimethylbutane, how to analyse the mass
spectrum of 2,3-dimethylbutane, how to describe explain the formation of fragmented ions in the
mass spectra of 2,3-dimethylbutane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2,3-dimethylbutane recognising
the base ion peak of 2,3-dimethylbutane interpreting
interpretation the mass spectrum of 2,3-dimethylbutane
Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
2,3-dimethylbutane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 2,3-dimethylbutane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 2,3-dimethylbutane. The m/e m/z value of the base ion peak in the
mass spectrum of 2,3-dimethylbutane. Possible examples of equations showing the formation
of the ionised fragments in 2,3-dimethylbutane. Revision notes on the mass spectrum of
2,3-dimethylbutane.
Matching and deducing the structure of the 2,3-dimethylbutane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic alkanes,
mass spectra of 2,3-dimethylbutane, a structural isomer of molecular formula C6H14
Links associated
with 2,3-dimethylbutane
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum of
2,3-dimethylbutane
The H-1 NMR spectrum of
2,3-dimethylbutane
The C-13 NMR spectrum of
2,3-dimethylbutane
Mass spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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