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Interpreting the infrared
spectrum of 2-methylhexane
[Author
©
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-methylhexane
[spectra
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CH3CH2CH2CH2CH(CH3)2
Links associated
with 2-methylhexane
The chemistry of ALKANES and the petrochemical
industry
This is a BIG
website, you need to take time to explore it
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-methylhexane
Students and teachers please note
my explanation of the infrared
spectrum of 2-methylhexane is designed
for advanced, but pre-university, chemistry courses.
Based in
the infrared spectrum diagram for 2-methylhexane, only some of the most
prominent peaks for particular bond vibrations are discussed,
particularly if 2-methylhexane has a functional group with a particular
characteristic wavenumber peak.
The infrared spectrum of
2-methylhexane is
unique and the whole, or selected wavenumbers, can be used to
fingerprint its identity, sometimes analysing a mixture
containing 2-methylhexane or following its change of concentration in a
reaction.
Spectra obtained from a liquid film of 2-methylhexane. The right-hand part of the of the
infrared spectrum of 2-methylhexane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of 2-methylhexane and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of
2-methylhexane.
2-methylhexane C7H16
The molecular structure and
naming of alkanes
Interpretation of
the infrared spectrum of 2-methylhexane
The most prominent infrared absorption lines of
2-methylhexane
From 2975 to 2845 cm-1 there are multiple
peaks amounting to strong absorption due to C-H stretching
vibrations in the CH, -CH2- or -CH3 groups in
2-methylhexane.
The are other C-H absorptions due to C-H deformation
vibrations of the CH2 groups at 1480 to 1440 cm-1.
These overlap with C-H vibrations of the CH3
groups at wavenumbers 1385 to 1370 cm-1 and 1470 to 1435
cm-1.
There are weak C-H vibrations of the -CH- group at
~1340 cm-1, close to the C-H vibrations of the -CH3
groups.
There are strong C-C skeletal vibration absorptions
at 1175 to 1140 cm-1 from the C(CH3)2
grouping, and similarly, but, weaker, an absorption at 840 to 790 cm-1.
There are no specific characteristic lines for
alkanes like 2-methylhexane because C-C and C-H vibrations are common to so
many organic molecules and alkanes like 2-methylhexane 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-methylhexane
molecular
structure.
Key IR
Spectrum Features of the infrared spectrum of 2-Methylhexane
2-Methylhexane (C7H16) is a branched
alkane, so its IR spectrum lacks peaks for functional groups
like –OH, C=O, or C≡C. Instead, it displays characteristic
alkane vibrations:
|
Wavenumber (cm⁻¹) |
Assignment |
Intensity |
Notes |
| 2975–2845 |
C–H stretching (CH3,
CH2,
CH) |
Strong |
Multiple overlapping peaks |
| 1480–1440 |
CH2
bending (scissoring) |
Medium |
Often overlaps with CH3
bending |
| 1470–1435 |
CH3
bending (asymmetric deformation) |
Medium |
May appear as shoulder or
merged peak |
| 1385–1370 |
CH3
symmetric bending |
Medium |
Distinctive for methyl
groups |
| ~1340 |
CH bending (methine group) |
Weak |
From central –CH– in
branched structure |
| 1175–1140 |
C–C skeletal vibrations
(C(CH3)2
group) |
Strong |
Indicates branching |
| 840–790 |
C–C skeletal bending |
Weak |
Less diagnostic, part of
fingerprint region |
Common
Misconceptions in IR Interpretation
- Mistaking alkane peaks for functional groups:
Students may incorrectly assign CH stretches to –OH or C=O due
to overlapping regions.
- Ignoring fingerprint region: The 1500–400
cm⁻¹ range is often skipped, yet it contains unique skeletal
vibrations useful for compound identification.
- Assuming absence of peaks means no information:
Even simple alkanes like 2-methylhexane have diagnostic CH and
C–C vibrations.
Exam Revision Tips
for IR Spectroscopy
These tips apply across AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB,
and US AP Chemistry syllabi:
What to Focus On
- Functional group regions: Know key
wavenumbers for –OH (~3200–3600), C=O (~1700), C≡C (~2100–2260),
and C=C (~1600).
- Alkane signatures: Recognize CH stretches
(2975–2845) and CH3/CH2 bends (1480–1370).
- Fingerprint region: Use it to distinguish
isomers or confirm compound identity.
Strategy Tips
- Annotate spectra: Label peaks with bond
types and wavenumbers.
- Compare spectra: Practice with isomers
(e.g., hexane vs. 2-methylhexane) to see branching effects.
- Link to structure: Use IR to support or
refute proposed molecular structures in multi-technique
questions (e.g., IR + NMR).
Practical Exam Advice
- Don’t overinterpret: Absence of functional
group peaks is still informative.
- Use elimination: Rule out compounds with
incompatible IR features.
- Watch for distractors: Some spectra may
include solvent or impurity peaks—focus on major absorptions.
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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. |
|
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-methylhexane, complete infrared absorption spectrum of 2-methylhexane, comparative spectra of
2-methylhexane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
2-methylhexane,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of 2-methylhexane, revision of infrared spectroscopy of 2-methylhexane, fingerprint region analysis of
2-methylhexane, how to identify 2-methylhexane from its infrared spectrum, identifying organic
compounds like 2-methylhexane from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of 2-methylhexane detection of
no functional groups in the 2-methylhexane molecule example of the infrared spectrum of a
molecule like 2-methylhexane with a functional group ?
interpreting interpretation of the infrared spectrum of 2-methylhexane shows presence
of no functional group
(CH3)2CHCH2CH2CH2CH3
Diagram of absorption of wavenumber
peaks in the infrared spectrum of 2-methylhexane. Characteristic peak wavenumbers in the infrared
spectrum of 2-methylhexane. Finger print identification pattern using the infrared
spectrum of 2-methylhexane. Revision notes on the infrared spectrum of
2-methylhexane. Matching
and deducing the structure of the 2-methylhexane molecule from its infrared
spectrum. Infrared spectroscopy of aliphatic alkanes, infrared spectra of
2-methylhexane, a structural isomer of molecular formula C7H16 How do you interpret the infrared absorption spectrum of
2-methylhexane How
to interpret the infrared spectrum of 2-methylhexane Explanatory diagram of the infrared spectrum of the
2-methylhexane molecule
in terms of its molecular structure.
Listing data of the prominent main wavenumber peaks troughs in the infrared
spectrum of 2-methylhexane. How to explain the infrared spectrum of
2-methylhexane. Use of
the infrared spectrum of 2-methylhexane, identification of
2-methylhexane from its
infrared spectrum - fingerprint wavenumber pattern to identify the
2-methylhexane
molecule. The uses of the infrared spectrum of the 2-methylhexane molecule. The
distinctive features of the infrared spectrum of the 2-methylhexane molecule
explained. explaining the peaks-trough of the transmittance of the infrared
spectrum of 2-methylhexane what does the infrared spectrum tell you about the
structure and properties of the 2-methylhexane molecule? How is infrared
spectrum of 2-methylhexane used to identify 2-methylhexane?
Links associated with 2-methylhexane
The mass
spectrum of 2-methyhexane
The
H-1 NMR spectrum of 2-methylhexane
The
C-13 NMR spectrum of 2-methylhexane
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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H-1 proton NMR spectra of ALKANES
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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C-13 carbon-13 NMR spectra
of ALKANES
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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spectroscopy are
suitable for use of pre-university students studying AQA advanced level
chemistry, Edexcel advanced level chemistry, OCR advanced level
chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level
chemistry, CIE advanced level chemistry, US grade 11-12 AP honors
chemistry courses and they will also prove useful to
1st year undergraduate students of chemistry.
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