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Interpreting the infrared
spectrum of 2,3-dimethylbutane
[Author
©
Dr WP 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
2,3-dimethylbutane
[updated
October
31st 2025]
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Links associated with 2,3-dimethylbutane
Infrared spectroscopy - spectra index
See also
comparing infrared, mass, 1H NMR & 13C NMR
spectra of the structural alkane isomers of C6H14
Introductory note on the infrared spectrum of 2,3-dimethylbutane
Students and teachers please note
my explanation of the infrared
spectrum of 2,3-dimethylbutane is designed
for advanced, but pre-university, chemistry courses.
Based in
the infrared spectrum diagram for 2,3-dimethylbutane, only some of the most
prominent peaks for particular bond vibrations are discussed,
particularly if 2,3-dimethylbutane has a functional group with a particular
characteristic wavenumber peak.
The infrared spectrum of
2,3-dimethylbutane is
unique and the whole, or selected wavenumbers, can be used to
fingerprint its identity, sometimes analysing a mixture
containing 2,3-dimethylbutane or following its change of concentration in a
reaction.
Spectra obtained from a liquid film of 2,3-dimethylbutane. The right-hand part of the of the
infrared spectrum of 2,3-dimethylbutane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of 2,3-dimethylbutane and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of
2,3-dimethylbutane.
2,3-dimethylbutane C6H14
 ,
,
For more
see The molecular structure,
classification and
naming of alkanes
Interpretation of
the infrared spectrum of 2,3-dimethylbutane
The most prominent infrared absorption lines of
2,3-dimethylbutane
For 2,3-dimethylbutane, there are strong C-H stretching vibration absorption bands at
wavenumbers 2940 to 2880 cm-1 for the CH2 and
CH3 groups.
Several strong C-H deformation vibration absorptions
at wavenumbers 1480 to 1365 cm-1 for the CH2
and CH3 groups in 2,3-dmethybutane (a double peak).
Several strong C-C skeletal vibration
absorptions associated with a C-(CH3)2
group occur at wavenumbers 1175 to 1140 cm-1 and 840
to 790 cm-1.
All of these infrared absorption vibrations are
characteristic of saturated alkyl structures in molecules,
exemplified by branched alkanes themselves e.g.
2,3-dimethylbutane.
The absence of other specific functional group bands
will show that particular functional group is absent from the
2,3-dimethylbutane
molecular
structure.
Key points
about the infrared spectrum of 2,3-dimethylbutane
2,3-dimethylbutane shows a simple IR
spectrum typical of branched alkanes, dominated by C–H stretching and
bending vibrations, with no peaks above 3000 cm⁻¹ due to the absence of
polar functional groups.
Key IR Spectrum
Features of 2,3-Dimethylbutane
2,3-Dimethylbutane (C6H14)
is a highly branched alkane with no IR-active functional groups like O–H,
N–H, or C=O.
Its spectrum is characterized by:
- No strong absorptions above 3000 cm⁻¹
- Multiple C–H stretching peaks due to methyl and
methylene groups
- Distinct CH3 bending modes in the
fingerprint region
- Weak skeletal C–C vibrations below 1200 cm⁻¹
Prominent
Wavenumbers Table
| Bond Type |
Vibration
Mode |
Wavenumber (cm⁻¹) |
Notes |
| C–H (sp³) stretch |
Symmetric & asymmetric |
2850–2960 |
Multiple peaks from
methyl/methylene |
| CH3 bend (umbrella) |
Deformation |
~1375 |
Strong, diagnostic for methyl
groups |
| CH2 bend |
Scissoring |
~1450 |
Often overlaps with CH₃ bending |
| C–C stretch |
Skeletal vibration |
~800–1200 |
Weak, in fingerprint region |
Sources:
NIST IR spectrum database
Common
Misconceptions in Exams
- Assuming absence of peaks means no useful data: Even
simple spectra confirm absence of functional groups.
- Overinterpreting fingerprint region: It’s useful for
comparison but not diagnostic alone.
- Confusing CH3 and CH2 bending modes:
Their peaks are close but arise from different group types.
Exam Revision Tips
For AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, and US AP Chemistry:
- Memorize key functional group regions: Even if absent
here, they’re crucial for other compounds.
- Use IR to eliminate possibilities: No O–H or C=O means
it’s not an alcohol, carboxylic acid, or ketone.
- Practice comparative analysis: Match unknown spectra to
known references, especially in fingerprint region.
- Link symmetry to IR activity: Highly symmetrical
molecules like 2,3-dimethylbutane may show fewer IR-active modes.
- Combine with other techniques: IR confirms bond types,
but mass and NMR give structural detail.
|
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). |
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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. |
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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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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 |
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13C NMR spectra above. |
Key words & phrases: image and diagram explaining the infrared spectrum
of 2,3-dimethylbutane, complete infrared absorption spectrum of
2,3-dimethylbutane, comparative spectra of
2,3-dimethylbutane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
2,3-dimethylbutane,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of 2,3-dimethylbutane, revision of infrared spectroscopy of 2,3-dimethylbutane, fingerprint region analysis of
2,3-dimethylbutane, how to identify 2,3-dimethylbutane from its infrared spectrum, identifying organic
compounds like 2,3-dimethylbutane from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of
2,3-dimethylbutane detection of functional groups in the 2,3-dimethylbutane molecule example of the infrared spectrum of a
molecule like 2,3-dimethylbutane with a functional group
interpreting interpretation of the infrared spectrum of 2,3-dimethylbutane
Diagram of absorption of wavenumber
peaks in the infrared spectrum of 2,3-dimethylbutane. Characteristic peak wavenumbers in the infrared
spectrum of 2,3-dimethylbutane. Finger print identification pattern using the infrared
spectrum of 2,3-dimethylbutane. Revision notes on the infrared spectrum of
2,3-dimethylbutane. Matching
and deducing the structure of the 2,3-dimethylbutane molecule from its infrared
spectrum. Infrared spectroscopy of aliphatic
alkanes, infrared spectra of
2,3-dimethylbutane, a structural isomer of molecular formula C6H14 How do you interpret the infrared absorption spectrum of
2,3-dimethylbutane How
to interpret the infrared spectrum of 2,3-dimethylbutane Explanatory diagram of the
infrared spectrum of the 2,3-dimethylbutane molecule in terms of its molecular
structure. Listing data of the prominent main wavenumber peaks
troughs in the infrared spectrum of 2,3-dimethylbutane. How to explain the infrared
spectrum of 2,3-dimethylbutane. Use of the infrared spectrum of
2,3-dimethylbutane, identification
of 2,3-dimethylbutane from its infrared spectrum - fingerprint wavenumber pattern
to identify the 2,3-dimethylbutane molecule. The uses of the infrared spectrum of
the 2,3-dimethylbutane molecule. The distinctive features of the infrared spectrum
of the 2,3-dimethylbutane molecule explained interpretation diagram explaining the
peaks-trough of the transmittance of the infrared spectrum of
2,3-dimethylbutane
what does the infrared spectrum tell you about the structure and
properties of the 2,3-dimethylbutane molecule? How is infrared spectrum of
2,3-dimethylbutane used
to identify 2,3-dimethylbutane?
Links associated with 2,3-dimethylbutane
The chemistry of ALKANES
revision notes INDEX
The mass spectrum of
2,3-dimethylbutane
The H-1 NMR spectrum of
2,3-dimethylbutane
The C-13 NMR spectrum of
2,3-dimethylbutane
Infrared spectroscopy index
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