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Interpreting the
13C NMR spectrum of
2-methylbutane
[Author
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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-methylbutane
[spectra
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13C NMR spectrum of
CH3CH2CH(CH3)2
Links associated
with 2-methylbutane
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C-13
NMR spectroscopy - spectra index
See also
comparing the infrared, mass, 1H NMR and
13C NMR
spectra of the 3 alkane isomers of C5H12
Introductory note on the 13C NMR spectrum of 2-methylbutane
Students and teachers please note that my explanation of the
carbon-13 NMR spectrum of 2-methylbutane is designed for advanced, but
pre-university, chemistry courses.
The description does not involve
the chemical shift δ
spin-spin coupling effects for 2-methylbutane 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-methylbutane molecule.
The most common solvent used for investigating the C13 NMR
spectrum of compounds like 2-methylbutane, 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-methylbutane here.
2-methylbutane C5H12
 ,
,
For more
see The molecular structure and
naming of alkanes
Interpreting the C-13 NMR spectrum of 2-methylbutane
As you can see from the diagram above there are
4 different 13C chemical shift lines in the C-13 NMR spectrum of
2-methylbutane
indicating 4 different chemical environments of the carbon atoms.
(CH3)2CHCH2CH3
Note the 4 colours indicating the
4 different chemical environments of the carbon atoms in
2-methylbutane - 13C chemical shifts
(a) to (d)
on the diagram.
Two of the carbon atoms
have the same
chemical shift a, so are equivalent to each other
in the 2-methylbutane molecule, i.e. they are present in identical
chemical environments.
The carbon-13 NMR spectra a provides direct evidence of
4 different
13C carbon atom environments in the 2-methylbutane molecule from 4 different chemical
shifts (ppm).
Note the left-hand pair of
carbon atoms
of the methyl groups are in identical 13C chemical environments - plane of
symmetry about the CH group.
Summary of the 13C NMR spectrum of
2-methylbutane
the ¹³C NMR spectrum of
2-methylbutane is a textbook example for teaching carbon
environments, symmetry, and chemical shift trends in saturated
hydrocarbons.
Key Features of the
¹³C NMR Spectrum of 2-Methylbutane
2-methylbutane (C5H12)
is a branched alkane with four distinct carbon
environments, despite having five carbon atoms.
This is due to symmetry:
two methyl groups are equivalent.
- All signals appear in the 0–50 ppm
range, typical of sp³ hybridized alkyl carbons.
- No deshielded carbons (e.g. carbonyl,
aromatic, alkene) are present.
- No splitting is observed in standard
proton-decoupled spectra.
Table
of ¹³C Chemical Shifts and Assignments
of the ¹³C NMR Spectrum of 2-Methylbutane
| Chemical Shift (δ, ppm) |
Carbon Type |
Environment |
Notes |
|
~15-17?, 22.3 ppm |
CH3 |
Terminal methyl groups |
Two equivalent methyls |
|
~14–15?, 11.8 ppm |
CH3 |
End primary methyl. |
Isolated methyl group |
|
~27–30?, 31.9 ppm |
CH2 |
Methylene (–CH2– between CH2
and CH) |
Slightly deshielded by branching |
|
~35–38?, 30.0 ppm |
CH |
Methine (central CH bonded to CH3
and CH2) |
Most deshielded due to branching |
Common
Misconceptions in ¹³C NMR spectrum interpretation
- Assuming one signal per carbon
atom: Equivalent carbons give
one signal — not five for C₅H₁₂.
- Expecting splitting patterns:
Standard ¹³C spectra are proton-decoupled, so no splitting
is seen.
- Misreading chemical shifts:
Students may expect high ppm values (e.g. 170–200 ppm), which only occur in
carbonyls.
- Ignoring symmetry:
Equivalent methyl groups (e.g. -CH2–CH2–) produce a
single peak.
Exam Revision Tips
for ¹³C NMR Spectroscopy questions
What to Focus On
- Count unique environments:
Use symmetry to identify equivalent carbons.
- Know typical shift ranges:
- Alkyl (sp³): 0–50 ppm
- Alkene/aromatic: 100–150 ppm
- Carbonyl: 160–220 ppm
- Link shifts to structure:
More substituted carbons are more deshielded (higher ppm).
Memory Aids
- “More branches = more ppm”
— tertiary carbons shift downfield.
- “Symmetry simplifies spectra”
— fewer signals than atoms.
- “Alkanes live below 50 ppm”
— helps eliminate wrong assignments.
Exam Strategy
- Sketch molecule
and label each carbon.
- Predict number of signals
before looking at spectrum.
- Use shift ranges
to eliminate impossible assignments.
- Compare with isomers:
e.g., pentane versus 2-methylbutane.
|
Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the 3 alkane isomers of C5H12
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 pentane,
2-methylbutane and 2,2-dimethylpropane image sizes. |
 |
 |
 |
Comparing the
infrared
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane
exemplify infrared spectra of the alkane homologous series CnH2n+2
hydrocarbon
molecules, where n = 5 |
|
INFRARED SPECTRA
(above): There are, as expected, differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, but there is no
specific infrared absorption band for a functional group. The
infrared spectra of pentane and 2-methylbutane seem very
similar, but that of 2,2-dimethylpropane seems much simpler. |
 |
 |
 |
Comparing the
mass
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane
exemplify the mass spectra of the alkane series CnH2n+2
hydrocarbon
molecules, where n = 5 |
|
MASS SPECTRA (above):
All three hydrocarbons show some similarities in their mass
spectra e.g. m/z ions 27 to 29 for [C2Hx]+
(x = 2 and 4). The molecular ion peaks will
be the same for all three isomers (m/z 72),
but it is very tiny for 2,2-dimethypropane. The pattern ratios
for m/z 39 to 43 are similar for pentane and 2-methylbutane, but
m/z 42 and 43 ions are almost absent from the
2,2-dimethylpropane spectrum. The base peak ion for pentane is
m/z 43, but for 2-methylbutane and 2,2-dimethylpropane it is m/z
57. |
 |
 |
 |
Comparing the
1H proton NMR
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane exemplify the 1H proton NMR spectra of the alkane
homologous series CnH2n+2
hydrocarbon
molecules where, n = 5 |
|
1H NMR SPECTRA (above): The 1H NMR spectra of
all three molecules give different proton ratios for the
different 1H chemical environments i.e. pentane's
proton ratio is 3:2:1 (from 6:4:2 H's in the molecule).
2-methylbutane's proton ratio is 6:1:2:3 and
2,2-dimethylpropane's doesn't have a proton ratio, all hydrogen
atoms are equivalent. This means all three isomeric C5H12
hydrocarbons can be distinguished from their 1H NMR spectra. |
 |
 |
 |
Comparing the
carbon-13 NMR
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane exemplify the carbon-13 NMR spectra of
members of the alkane homologous series CnH2n+2
hydrocarbon
molecules, where n = 5 |
|
13C NMR SPECTRA
(above): The
13C NMR spectra of the three molecules show different numbers of
carbon-13 chemical environments i.e different numbers of 13C NMR
resonance lines. So, pentane gives three 13C chemical
shifts,
2-methylbutane four and 2,2-dimethylpropane two. This means all
three isomeric C5H12 hydrocarbons can be
distinguished from their 13C NMR spectra. |
Key words & phrases: isopentane methylbutane Interpreting the C-13 NMR spectra of 2-methylbutane, C-13 nmr spectrum of
2-methylbutane, understanding the
carbon-13 nmr spectrum of 2-methylbutane, explaining the line pattern in the high
resolution C-13 nmr spectra of 2-methylbutane, revising the C-13 nmr spectrum of
2-methylbutane, ppm
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carbon atoms in 2-methylbutane examining the c13 nmr spectrum of
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13-c nmr spectrum of 2-methylbutane how do you sketch and interpret the C-13 NMR
spectrum of 2-methylbutane isopentane
methylbutane Molecular structure diagram of the
carbon-13 NMR diagram for the 13C NMR spectrum of 2-methylbutane. Deducing the number
of different chemical environments of the carbon atoms in the 2-methylbutane molecule
from the 13C chemical shifts in the carbon-13 NMR spectrum of 2-methylbutane. Revision
notes on the carbon-13 NMR spectrum of 2-methylbutane. Matching and deducing the
structure of the 2-methylbutane molecule from its 13C NMR spectrum.
Carbon-13 NMR spectroscopy of type,
13C NMR spectra of 2-methylbutane, an isomer of molecular formula C5H12
How do you interpret the chemical shifts of the C-13 NMR spectrum
of 2-methylbutane How to interpret the C-13 NMR spectrum of
2-methylbutane Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the number of different carbon atom
environments in the 2-methylbutane molecule from its carbon-13 NMR spectrum to
help work out the molecular structure of the 2-methylbutane molecule? The uses
and distinctive features of the carbon-13 NMR spectrum of the
2-methylbutane
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-methylbutane
molecule? explaining the decoupled carbon-13 NMR spectrum of
2-methylbutane
with a detailed diagram of all the C-13 chemical shifts and
intensities
Links associated
with
2-methylbutane
The chemistry of ALKANES and the petrochemical
industry
The infrared spectrum for
2-methylbutane
The mass spectrum for 2-methylbutane
The H-1 NMR spectrum for
2-methylbutane
C-13
NMR spectroscopy index
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