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Interpreting the infrared
spectrum of 2-methylbutane
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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
infrared spectrum of
2-methylbutane
[spectra
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infrared spectrum of
CH3CH2CH(CH3)2
Links associated
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Infrared 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 infrared spectrum of 2-methylbutane
Students and teachers please note
my explanation of the infrared
spectrum of 2-methylbutane is designed
for advanced, but pre-university, chemistry courses.
Based in
the infrared spectrum diagram for 2-methylbutane, only some of the most
prominent peaks for particular bond vibrations are discussed,
particularly if 2-methylbutane has a functional group with a particular
characteristic wavenumber peak.
The infrared spectrum of
2-methylbutane is
unique and the whole, or selected wavenumbers, can be used to
fingerprint its identity, sometimes analysing a mixture
containing 2-methylbutane or following its change of concentration in a
reaction.
Spectra obtained from a liquid film of 2-methylbutane. The right-hand part of the of the
infrared spectrum of 2-methylbutane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of 2-methylbutane and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of
2-methylbutane.
2-methylbutane C5H12
 ,
,
For more
see The molecular structure and
naming of alkanes
Interpretation of
the infrared spectrum of 2-methylbutane
The most prominent infrared absorption lines of
2-methylbutane
Most of these are due to the vibrations of the
C-H bonds e.g. stretching and bending at ~2975 to 2845 cm-1 and ~1470
to 1370 cm-1.
There is no characteristic absorption band for a
functional group, as alkanes don't have one!
There are C(CH3)2 skeletal vibration absorptions at 1175
to 1140 cm-1.
The absence of a specific functional group band
will show that a particular functional group is absent from the
pentane molecular structure.
The absence of other specific functional group bands
will show that particular functional group is absent from the
2-methylbutane
molecular
structure.
Summary of the infrared spectrum of 2-methylbutane
2-Methylbutane shows characteristic IR absorptions for alkanes, with
prominent C–H stretching and bending vibrations.
No
functional group peaks are present.
Key IR Spectrum
Features of 2-Methylbutane
2-Methylbutane (C5H12),
also known as isopentane, is a branched alkane. Its IR spectrum is dominated
by C–H stretching and bending vibrations, typical of
saturated hydrocarbons.
There are no functional groups,
so the spectrum lacks strong polar absorptions.
| Wavenumber (cm⁻¹) |
Vibration Type |
Assignment |
Intensity |
Notes |
| 2950–2850 |
Stretching |
C–H stretch (sp³) |
Medium–strong |
Multiple overlapping peaks |
| 1470–1450 |
Bending (scissoring) |
CH2 scissoring |
Medium |
Often appears as a doublet |
| 1390–1370 |
Bending (rocking/symmetric) |
CH3 symmetric
bending (umbrella) |
Medium |
Useful for methyl identification |
| 1300–1100 |
Bending (wagging/twisting) |
CH2
wagging/twisting |
Weak |
Often broad and less diagnostic |
| Below 1000 |
Fingerprint region |
Complex skeletal vibrations |
Weak–medium |
Not useful for simple alkanes |
Sources: NIST Chemistry WebBook, LibreTexts IR
interpretation guide
Common
Misconceptions in IR Analysis of Alkanes
- Mistaking C–H stretches for
functional group peaks:
Students often expect strong peaks near 1700 cm⁻¹ (carbonyl) or 3300 cm⁻¹
(O–H/N–H), which are absent in alkanes.
- Overinterpreting the fingerprint
region: Below 1000 cm⁻¹, the
spectrum becomes complex and is not useful for simple molecules like
2-methylbutane.
- Assuming absence of peaks means no
molecule: Hydrocarbons have
subtle IR features; lack of strong peaks doesn’t mean the sample is missing.
Exam Revision Tips
for IR Spectroscopy questions on alkanes
What to Focus On
- Look for absence of strong polar
peaks (no O–H ~3300 cm⁻¹, no C=O ~1700 cm⁻¹).
- Recognize C–H stretches (2850–2950
cm⁻¹) as hallmark of alkanes.
- Identify absence of functional
group peaks as diagnostic for
saturated hydrocarbons.
- Use CH3 bending at
~1375 cm⁻¹ to confirm methyl
groups.
- Compare spectra:
Practice distinguishing alkanes from alcohols, ketones, and carboxylic
acids.
Memory Aids
- “C–H stretches live in the 2900s”
– helps recall alkane region.
- “No peaks near 1700 or 3300? It’s
probably an alkane” – helps eliminate other groups.
Exam Strategy
- Annotate spectra
with wavenumbers and assignments.
- Use elimination:
If no carbonyl or hydroxyl peaks, rule out those groups.
- Practice with known spectra:
Use NIST or textbook examples to build confidence.
|
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 image and diagram explaining the infrared spectrum
of 2-methylbutane, complete infrared absorption spectrum of 2-methylbutane, comparative spectra of
2-methylbutane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
2-methylbutane,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of 2-methylbutane, revision of infrared spectroscopy of 2-methylbutane, fingerprint region analysis of
2-methylbutane, how to identify 2-methylbutane from its infrared spectrum, identifying organic
compounds like 2-methylbutane from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of 2-methylbutane detection of ? functional groups in the 2-methylbutane molecule example of the infrared spectrum of a
molecule like 2-methylbutane with a functional group
isopentane methylbutane
Diagram of absorption of wavenumber
peaks in the infrared spectrum of 2-methylbutane. Characteristic peak wavenumbers in the infrared
spectrum of 2-methylbutane. Finger print identification pattern using the infrared
spectrum of 2-methylbutane. Revision notes on the infrared spectrum of
2-methylbutane. Matching
and deducing the structure of the 2-methylbutane molecule from its infrared
spectrum. Infrared spectroscopy of aliphatic alkanes, infrared spectra of
2-methylbutane, an isomer of molecular formula C5H12 How do you interpret the infrared absorption spectrum of
2-methylbutane How
to interpret the infrared spectrum of 2-methylbutane Explanatory diagram of the infrared spectrum of the
2-methylbutane molecule
in terms of its molecular structure.
Listing data of the prominent main wavenumber peaks troughs in the infrared
spectrum of 2-methylbutane. How to explain the infrared spectrum of
2-methylbutane. Use of
the infrared spectrum of 2-methylbutane, identification of
2-methylbutane from its
infrared spectrum - fingerprint wavenumber pattern to identify the
2-methylbutane
molecule. The uses of the infrared spectrum of the 2-methylbutane molecule. The
distinctive features of the infrared spectrum of the 2-methylbutane molecule
explained. explaining the peaks-trough of the transmittance of the infrared
spectrum of 2-methylbutane what does the infrared spectrum tell you about the
structure and properties of the 2-methylbutane molecule? How is infrared
spectrum of 2-methylbutane used to identify 2-methylbutane?
Links associated
with
2-methylbutane
The chemistry of ALKANES and the petrochemical
industry
The mass spectrum for 2-methylbutane
The H-1 NMR spectrum for
2-methylbutane
The C-13 NMR spectrum for
2-methylbutane
Infrared spectroscopy index
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