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Interpreting the
13C NMR spectrum of
2-methylpentane
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Dr
Phil Brown PhD:
Doc Brown's advanced level organic chemistry exam revision notes
suitable for students of UK A level chemistry courses & US K12 grade
11, grade 12 and AP honors chemistry courses: Molecular
spectroscopy - analysing the
C-13 NMR spectrum of
2-methylpentane
[spectra
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CH3CH2CH2CH(CH3)2
Links associated
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The chemistry of ALKANES and the petrochemical
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C-13
NMR spectroscopy - spectra index
See also
comparing infrared, mass, 1H NMR & 13C NMR
spectra of the structural alkane isomers of C6H14
Introductory note on the 13C NMR spectrum of 2-methylpentane
Students and teachers please note that my explanation of the
carbon-13 NMR spectrum of 2-methylpentane is designed for advanced, but
pre-university, chemistry courses.
The description does not involve
the chemical shift δ
spin-spin coupling effects for 2-methylpentane and the relative size of the carbon-13
NMR shifts does not give the ratio of the carbon atoms in the
different non-equivalent chemical environments of the
2-methylpentane molecule.
The most common solvent used for investigating the C13 NMR
spectrum of compounds like 2-methylpentane, is CDCl3 and other
deuterated solvents.
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 - 2-methylpentane here.
2-methylpentane C6H14,
 ,
,
For more
see The molecular structure,
classification and
naming of alkanes
Interpreting the C-13 NMR spectrum of 2-methylpentane
As you can see from the diagram above there are
5 different chemical shift lines in the C-13 NMR spectrum of
2-methylpentane
indicating 5 different chemical environments of the six carbon atoms.
Marked as five 13C NMR shifts
(a) to (e)
on the diagram above.
(CH3)2CHCH2CH2CH3
(Note the 5 colours indicating the
5 different chemical environments of the carbon atoms in
2-methylpentane).
The molecular symmetry of the 'left-hand' methyl
groups of carbon atoms make them equivalent to each other
- same chemical environment, so an identical chemical shift
a (22.7 ppm).
The carbon-13 NMR spectra a provides direct evidence of
5 different
carbon atom environments in the 2-methylpentane molecule from 5 different chemical
shifts (ppm).
Summary of the 13C NMR spectrum of
2-methylpentane
Note that in 2-methylpentane, the two methyl groups
attached to C2 are chemically equivalent due to their identical spatial and
electronic environments.
This symmetry reduces the number of unique carbon environments in the ¹³C
NMR spectrum to five, not six.
¹³C NMR Breakdown for 2-Methylpentane
| Chemical
Shift (δ, ppm) |
Carbon
Type |
Environment |
Notes |
| ~8–10,
14.4 ppm |
CH3 |
Terminal methyl (C5) |
Least field influenced C atom. |
| ~11–13?,
22.7 ppm |
2 x CH3 |
Branched methyls (C6 and C6′ on
C2) |
Equivalent, appear as one signal |
| ~22–25,
20.6 ppm |
CH2 |
Methylene (C4) |
Equivalent or nearly so |
| ~27–30,
27.9 ppm |
CH |
Methine (C2, attached to 2×CH3
and CH2) |
Most downfield due to branching |
| ~35–40,
41.6 ppm |
CH2 |
Slightly deshielded CH2
(C3) |
May appear separately if
resolution allows |
In high-resolution spectra, C3 and C4 may split into two signals due to
subtle differences in their environment, but for most A-level and AP
contexts, they’re treated as equivalent.
Exam Tip Update
- Signal count ≠ atom count: Always consider symmetry.
Equivalent carbons give one signal.
- Draw and label: Sketch the molecule and mark equivalent
groups to predict signal count.
- Use DEPT spectra if available: These help distinguish
CH3, CH2, and CH signals.
- Expect singlets: ¹³C spectra are usually
proton-decoupled, so no splitting.
|
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). |
|
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. |
 |
 |
|
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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 |
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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 |
 |
 |
|
13C NMR spectra above. |
Key words & phrases: isohexane Interpreting the C-13 NMR spectra of
2-methylpentane, C-13 nmr spectrum of 2-methylpentane, understanding the
carbon-13 nmr spectrum of 2-methylpentane, explaining the line pattern in the high
resolution C-13 nmr spectra of 2-methylpentane, revising the C-13 nmr spectrum of
2-methylpentane, ppm
chemical shifts of the C-13 nmr spectrum of 2-methylpentane, how to construct the diagram of
the C-13 nmr spectrum of 2-methylpentane, how to analyse the chemical shifts in the
carbon-13 NMR spectrum of 2-methylpentane deducing the chemical environment of all the
carbon atoms in 2-methylpentane examining the c13 nmr spectrum of
2-methylpentane analysing the
13-c nmr spectrum of 2-methylpentane how do you sketch and interpret the C-13 NMR spectrum
of 2-methylpentane interpreting interpretation of the C-13 NMR spectrum of
2-methylpentane isohexane Molecular structure diagram of the
carbon-13 NMR diagram for the 13C NMR spectrum of 2-methylpentane. Deducing the number
of different chemical environments of the carbon atoms in the
2-methylpentane molecule
from the 13C chemical shifts in the carbon-13 NMR spectrum of
2-methylpentane. Revision
notes on the carbon-13 NMR spectrum of 2-methylpentane. Matching and deducing the
structure of the 2-methylpentane molecule from its 13C NMR spectrum.
Carbon-13 NMR spectroscopy of aliphatic
alkanes,
13C NMR spectra of 2-methylpentane, an isomer of molecular formula C6H14 How do you interpret the chemical shifts of the C-13 NMR spectrum
of 2-methylpentane How to interpret the C-13 NMR spectrum of
2-methylpentane Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the number of different carbon atom
environments in the 2-methylpentane molecule from its carbon-13 NMR spectrum to
help work out the molecular structure of the 2-methylpentane molecule? The uses
and distinctive features of the carbon-13 NMR spectrum of the
2-methylpentane
molecule explained. What do the number and values of the chemical
shifts from the c-13 carbon-13 NMR spectrum tell us about the
2-methylpentane
molecule? explaining the decoupled carbon-13 NMR spectrum of
2-methylpentane
with a detailed diagram of all the C-13 chemical shifts and
intensities
Links associated
with
2-methylpentane
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum of
2-methylpentane
The mass spectrum of
2-methylpentane
The H-1 NMR spectrum of
2-methylpentane
C-13
NMR spectroscopy index
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