The Carbon-13 NMR spectrum of
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 C-13 NMR spectra of 3-methylpentane
comparing infrared, mass, 1H NMR & 13C NMR
spectra of the structural alkane isomers of C6H14
TMS is the acronym for tetramethylsilane, formula Si(CH3)4,
whose 13C atoms are arbitrarily given a chemical shift of 0.0
ppm. This is the 'standard' in 13C NMR spectroscopy and all other
13C shifts, called chemical shifts, depend on the
individual (electronic) chemical environment of the 13C atoms in
an organic molecule - 3-methylpentane here.
see The molecular structure,
naming of alkanes
Interpreting the C-13 NMR spectrum of 3-methylpentane
As you can see from the diagram above there are
4 different chemical shift lines in the C-13 NMR spectrum of
indicating 4 different chemical environments of the carbon atoms.
(Note the 4 colours indicating the
4 different chemical environments of the six carbon atoms in
Note the symmetry of the molecule - six carbon atoms,
but only four different chemical environments for the in the
The end methyl group carbon atoms are equivalent to
each other, as are the pair of CH2 group carbon atoms
- same chemical environments.
The carbon-13 NMR spectra a provides direct evidence of
carbon atom environments in the 3-methylpentane molecule from 4 different chemical
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: Interpreting the C-13 NMR spectra of 3-methylpentane, C-13 nmr spectrum of
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Molecular structure diagram of the
carbon-13 NMR diagram for the 13C NMR spectrum of 3-methylpentane. Deducing the number
of different chemical environments of the carbon atoms in the
from the 13C chemical shifts in the carbon-13 NMR spectrum of
notes on the carbon-13 NMR spectrum of 3-methylpentane. Matching and deducing the
structure of the 3-methylpentane molecule from its 13C NMR spectrum.
Carbon-13 NMR spectroscopy of aliphatic
13C NMR spectra of 3-methylpentane, a structural isomer of molecular formula C6H14
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum of
The mass spectrum of
The H-1 NMR spectrum of
NMR spectroscopy index
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