spectrum of 3-methylpentane
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 3-methylpentane
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Mass spectroscopy - spectra index
comparing infrared, mass, 1H NMR and 13C NMR
spectra of the structural alkane isomers of C6H14
see The molecular structure,
naming of alkanes
Interpreting the fragmentation pattern of the mass spectrum of
[M]+ is the molecular ion peak (M) with an m/z of
86 corresponding to [C6H14]+, the original 3-methylpentane molecule minus an electron,
The small M+1 peak at m/z 87, corresponds to an ionised
molecule with one 13C atom in it i.e. an ionised 3-methylpentane molecule of
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
3-methylpentane 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 3-methylpentane - though
the ratio will be greater e.g. the m/z 58 ion.
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 3-methylpentane.
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
|m/z value of
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 3-methylpentane
Atomic masses: H = 1; C = 12 (13 for ~1
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 3-methylpentane
Formation of m/z 71 ion:
[(CH3CH2)2CHCH3]+ ===> [(CH3CH2)2CH]+
C-C bond scission in the parent molecular ion,
leading to loss of a methyl group from the parent molecular ion
via C-C bond fission.
Mass change: 86 - 15 = 71, to give a secondary carbocation.
I've quoted a possible structure of the [C5H11]+
Formation of m/z 57 ion:
[(CH3CH2)2CHCH3]+ ===> [CH3CH2CHCH3]+
C-C bond scission of the parent molecular ion,
leading to loss of an ethyl group from the parent molecular
Mass change = 86 - 29 = 57, to give a secondary carbocation.
The m/z 57 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
Formation of m/z 29 ion:
[(CH3CH2)2CHCH3]+ ===> [CH3CH2]+
or from: [CH3CH2CHCH3]+ ===> [CH3CH2]+
C-C bond scission of the parent molecular ion or
larger fragment ion.
Mass change = 86 - 57 = 29 or 57 - 28 = 29
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.
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
exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower
aliphatic alkanes (non-cyclic alkanes).
Infrared spectra below.
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
Infrared spectra above, mass spectra below.
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
2,3-dimethylbutane: m/z 43, 41, 42 and 71
Mass spectra above, 1H NMR spectra below.
1H NMR SPECTRA: They can
all be distinguished by their different integrated proton ratios -
need very high resolution.
δ shifts, H ratio 3:2:2 (6:4:4 in formula)
δ shifts, H ratio 6:3:2:2:1
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)
1H NMR spectra above, 13C NMR spectra below.
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
(ii) 2-methylpentane: 5 13C
(iii) 3-methylpentane: 4 13C
(iv) 2,2-dimethylbutane: 4 13C
(v) 2,3-dimethylbutane: 2 13C
13C NMR spectra above.
Key words & phrases: image diagram on how to interpret and explain the mass spectrum of
3-methylpentane m/z m/e base peaks, image and diagram of the mass spectrum of
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mass spectrum of 3-methylpentane, how to construct the mass spectrum diagram for abundance
of fragmentation ions in the mass spectrum of 3-methylpentane, how to analyse the mass
spectrum of 3-methylpentane, how to describe explain the formation of fragmented ions in the
mass spectra of 3-methylpentane equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 3-methylpentane recognising the
base ion peak of 3-methylpentane interpreting
interpretation the mass spectrum of 3-methylpentane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
3-methylpentane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 3-methylpentane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 3-methylpentane. The m/e m/z value of the base ion peak in the
mass spectrum of 3-methylpentane. Possible examples of equations showing the formation
of the ionised fragments in 3-methylpentane. Revision notes on the mass spectrum of
Matching and deducing the structure of the 3-methylpentane molecule from its mass
spectrum. Mass spectroscopy of
mass spectra of 3-methylpentane, a structural isomer of molecular formula C6H14
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum of
The H-1 NMR spectrum of
The C-13 NMR spectrum of
Mass spectroscopy index
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