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Interpreting infrared
spectrum of 2,3-dimethylpentane
[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 & AP honors chemistry courses: Molecular
spectroscopy - analysing
infrared spectrum of
2,3-dimethyl pentane
[updated
31st 2025]
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brown
Re-edit infrared spectrum of CH3CH(CH3)CH(CH3)CH2CH3
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Links associated with 2,3-dimethylpentane
Infrared spectroscopy - spectra index
See also
comparing the
1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16
Introductory note on the infrared spectrum of 2,3-dimethylpentane
Students and teachers please note
my explanation of the infrared
spectrum of 2,3-dimethylpentane is designed
for advanced, but pre-university, chemistry courses.
Based in
the infrared spectrum diagram for 2,3-dimethylpentane, only some of the most
prominent peaks for particular bond vibrations are discussed,
particularly if 2,3-dimethylpentane has a functional group with a particular
characteristic wavenumber peak.
The infrared spectrum of
2,3-dimethylpentane is
unique and the whole, or selected wavenumbers, can be used to
fingerprint its identity, sometimes analysing a mixture
containing 2,3-dimethylpentane or following its change of concentration in a
reaction.
Spectra obtained from a liquid film of 2,3-dimethylpentane. The right-hand part of the of the
infrared spectrum of 2,3-dimethylpentane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of 2,3-dimethylpentane and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of
2,3-dimethylpentane.
2,3-dimethylpentane
C7H16
For more
see
The molecular structure,
classification and
naming of alkanes
Interpretation of
the infrared spectrum of 2,3-dimethylpentane
The most prominent infrared absorption lines of
2,3-dimethylpentane
Strong C-H stretching vibration absorptions at
wavenumbers 2940 to 2880 cm-1 for the CH2 and
CH3 groups in 2,3-dimethypentane.
Several strong C-H deformation vibration absorptions
at wavenumbers 1480 to 1365 cm-1 for the CH2
and CH3 groups in 2,3-dimethypentane.
There is a weak -CH- stretching vibration
absorption at ~1340 cm-1.
Several strong C-C skeletal vibration
absorptions associated with a C-(CH3)2
group occur at wavenumbers 840 to 790 and 1175 to 1140 cm-1.
There are no specific characteristic lines for
alkanes like heptane because C-C and C-H vibrations are common to so
many organic molecules and alkanes like 2,3-dimethypentane have no functional
group that gives a characteristic vibration.
The absence of other specific functional group bands
will show that a particular functional group is absent from the
2,3-dimethylpentane
molecular
structure.
Key points
about the infrared spectrum of 2,3-dimethylpentane
2,3-dimethylpentane shows no functional group peaks in its IR spectrum; key
absorptions arise from C–H stretching and bending typical of saturated
alkanes.
Overview: IR
Spectrum of 2,3-dimethylpentane
2,3-dimethylpentane is a branched alkane (C7H16)
with no polar functional groups.
Its IR spectrum is dominated by C–H
stretching and bending vibrations, and a complex fingerprint
region.
This makes it an ideal example for
understanding alkane IR profiles and distinguishing saturated hydrocarbons
from functionalized compounds.
Prominent IR
Absorptions of 2,3-dimethylpentane
| Wavenumber
(cm⁻¹) |
Assignment |
Notes |
| ~2950–2850 |
C–H stretch (sp³) |
Multiple overlapping
peaks due to methyl and methylene groups |
| ~1465 |
CH2
bending (scissoring) |
Common in alkanes;
moderate intensity |
| ~1375 |
CH3
bending (umbrella mode) |
Characteristic of methyl
groups |
| ~720–730 |
CH2
rocking (long-chain alkanes) |
Weak; more prominent in
linear chains |
| 1500–600 |
Fingerprint region |
Complex; useful for
compound identification |
Common
Misconceptions in Exams
- Mistaking alkane IR spectra as
“featureless”: While lacking
strong functional group peaks for C=O, O-H or N-H, alkanes still show
diagnostic C–H stretches and bends as with 2,3-dimethylpentane.
- Assuming absence of peaks means no
compound: Even simple
hydrocarbons have identifiable IR features.
- Confusing CH3
and CH2
bending modes: CH3
appears near 1375 cm⁻¹, CH2 near 1465 cm⁻¹—important for
structural analysis.
- Overinterpreting fingerprint
region: It’s useful for
matching known spectra but not for assigning specific groups without
reference.
Exam Revision Tips
(AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, AP)
- Know your baseline:
Alkanes like 2,3-dimethylpentane provide a reference for spectra without
functional groups.
- Focus on C–H regions:
Learn the typical sp³ C–H stretch (~2950–2850 cm⁻¹) and methyl/methylene
bends.
- Compare spectra:
Practice distinguishing alkanes from alcohols, ketones, and carboxylic acids
by presence/absence of O–H, C=O (carboxylic acids have both), etc.
- Use elimination logic:
In multiple-choice questions, rule out compounds with strong IR peaks (e.g.,
O–H ~3300 cm⁻¹, C=O ~1700 cm⁻¹) if absent.
- Fingerprint region strategy:
Use it for matching known compounds, not for deducing structure from
scratch.
- Interpretation: Be ready
to interpret spectra without labels—practice assigning peaks based
on wavenumber and intensity.
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Comparing the
1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16
You can distinguish all 9 isomers from a data combination of their number of
1H NMR
chemical shifts,
and their resulting integrated 1H proton ratios, plus, their number of
13C
chemical shifts. |
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Name of the alkane structural isomer of molecular
formula C7H16 |
Abbreviated structural formulae
of the nine isomers of molecular formula C7H16 (interpretation complications with 3-methylhexane and
2,3-dimethylpentane because they exhibit R/S isomerism due to a
chiral carbon) |
Skeletal formula of the
nine
alkane isomers of
molecular formula C7H16 |
Number of 1H NMR chemical shifts (δ) and
proton ratio (links
to spectrum) |
Number of 13C chemical shifts (δ)
(links
to spectrum) |
|
heptane |
 |
 |
4 δ: proton ratio: 3:2:2:1 (6:4:4:2 in the molecule) |
4 δ shifts |
|
2-methylhexane |
 |
 |
6 δ: proton ratio :
6:3:2:2:2:1 |
6 δ shifts |
|
3-methylhexane |
 |
 |
7 δ: proton ratio:
3:3:3:2:2:2:1 (simplification) ! |
7
δ shifts |
|
3-ethylpentane |
 |
 |
3 δ: proton ratio:
9:6:1 |
3 δ
shifts |
|
2,2-dimethylpentane |
 |
 |
4 δ: proton ratio:
9:3:2:2 |
5 δ shifts |
|
2,3-dimethylpentane |
|
|
6 δ: proton ratio:
6:3:3:2:1:1 (simplification) ! |
6 δ
shifts (simplification) !!! |
|
2,4-dimethylpentane |
 |
 |
3 δ: proton ratio:
12:2:2 |
3 δ
shifts |
|
3,3-dimethylpentane |
 |
 |
3 δ: proton ratio:
3:3:2 (6:4:4 in the molecule) |
4 δ
shifts |
|
2,2,3-trimethylbutane |
 |
 |
3 δ: proton ratio:
9:6:1 |
4 δ shifts |
Key words & phrases: C7H16 image and diagram explaining the infrared spectrum
of 2,3-dimethylpentane, complete infrared absorption spectrum of
2,3-dimethylpentane, comparative spectra of
2,3-dimethylpentane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
2,3-dimethylpentane,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of 2,3-dimethylpentane, revision of infrared spectroscopy of 2,3-dimethylpentane, fingerprint region analysis of
2,3-dimethylpentane, how to identify 2,3-dimethylpentane from its infrared spectrum, identifying organic
compounds like 2,3-dimethylpentane from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of
2,3-dimethylpentane detection of no functional groups in the
2,3-dimethylpentane molecule example of the infrared spectrum of a
molecule like 2,3-dimethylpentane with no functional group
interpreting interpretation of the infrared spectrum of
2,3-dimethylpentane shows presence
of no functional groups
(CH3)2CHCH(CH3)CH2CH3
Diagram of absorption of wavenumber
peaks in the infrared spectrum of 2,3-dimethylpentane. Characteristic peak wavenumbers in the infrared
spectrum of 2,3-dimethylpentane. Finger print identification pattern using the infrared
spectrum of 2,3-dimethylpentane. Revision notes on the infrared spectrum of
2,3-dimethylpentane. Matching
and deducing the structure of the 2,3-dimethylpentane molecule from its infrared
spectrum. Infrared spectroscopy of aliphatic alkanes, infrared spectra of
2,3-dimethylpentane, a structural isomer of molecular formula C7H16 How do you interpret the infrared absorption spectrum of
2,3-dimethylpentane How
to interpret the infrared spectrum of 2,3-dimethylpentane Explanatory diagram of the
infrared spectrum of the 2,3-dimethylpentane molecule in terms of its molecular
structure. Listing data of the prominent main wavenumber peaks
troughs in the infrared spectrum of 2,3-dimethylpentane. How to explain the infrared
spectrum of 2,3-dimethylpentane. Use of the infrared spectrum of
2,3-dimethylpentane, identification
of 2,3-dimethylpentane from its infrared spectrum - fingerprint wavenumber pattern
to identify the 2,3-dimethylpentane molecule. The uses of the infrared spectrum of
the 2,3-dimethylpentane molecule. The distinctive features of the infrared spectrum
of the 2,3-dimethylpentane molecule explained interpretation diagram explaining the
peaks-trough of the transmittance of the infrared spectrum of
2,3-dimethylpentane
what does the infrared spectrum tell you about the structure and
properties of the 2,3-dimethylpentane molecule? How is infrared spectrum of
2,3-dimethylpentane used
to identify 2,3-dimethylpentane?
Links associated with 2,3-dimethylpentane
The mass spectrum of
2,3-dimethylpentane
The H-1 NMR spectrum of
2,3-dimethylpentane
The C-13 NMR spectrum of
2,3-dimethylpentane
The chemistry of ALKANES
revision notes INDEX
Infrared spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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Infrared spectra of the isomers of C7H16
The infrared
spectrum of heptane
The
infrared spectrum of 2-methylhexane
The
infrared spectrum of 3-methylhexane
The
infrared spectrum of 3-ethylpentane
The infrared spectrum of
2,2-dimethylpentane
The infrared spectrum of
2,3-dimethylpentane
The infrared spectrum of
2,4-dimethylpentane
The infrared spectrum of
3,3-dimethylpentane
The infrared spectrum
of 2,2,3-trimethylbutane
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Mass spectra of the isomers of C7H16
The mass
spectrum of heptane
The mass
spectrum of 2-methylhexane
The mass
spectrum of 3-methylhexane
The
mass spectrum of 3-ethylpentane
The mass spectrum of
2,2-dimethylpentane
The mass spectrum of
2,3-dimethylpentane
The mass spectrum of
2,4-dimethylpentane
The mass spectrum of
3,3-dimethylpentane
The mass
spectrum of 2,2,3-trimethylbutane
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1H NMR spectra of the isomers of C7H16
The H-1 NMR
spectrum of heptane
The
H-1 NMR spectrum of 2-methylhexane
The
H-1 NMR spectrum of 3-methylhexane
The
H-1 NMR spectrum of 3-ethylpentane
The H-1 NMR spectrum of
2,2-dimethylpentane
The H-1 NMR spectrum of
2,3-dimethylpentane
The H-1 NMR spectrum of
2,4-dimethylpentane
The H-1 NMR spectrum of
3,3-dimethylpentane
The H-1
NMR spectrum of 2,2,3-trimethylbutane
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13C NMR spectra of the isomers of C7H16
The C-13 NMR
spectrum of heptane
The
C-13 NMR spectrum of 2-methylhexane
The
C-13 NMR spectrum of 3-methylhexane
The
C-13 NMR spectrum of 3-ethylpentane
The C-13 NMR spectrum of
2,2-dimethylpentane
The C-13 NMR spectrum of
2,3-dimethylpentane
The C-13 NMR spectrum of
2,4-dimethylpentane
The C-13 NMR spectrum of
3,3-dimethylpentane
The
C-13 NMR spectrum of 2,2,3-trimethylbutane
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