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Interpreting and
explaining the infrared
spectrum of 2-methylpentane
[Author
©
Dr
Phil Brown GRIC, 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-methylpentane
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CH3CH2CH2CH(CH3)2
Links associated
with 2-methylpentane
The chemistry of ALKANES and the petrochemical
industry
This is a BIG
website, PLEASE take time to explore it
Infrared spectroscopy - spectra index
See also
comparing infrared, mass, 1H NMR & 13C NMR
spectra of the structural alkane isomers of C6H14
Isomers of molecular formula C6H14
(Mr = 86)
Introductory note on the infrared spectrum of 2-methylpentane
Students and teachers please note
my explanation of the infrared
spectrum of 2-methylpentane is designed
for advanced, but pre-university, chemistry courses.
Based in
the infrared spectrum diagram for 2-methylpentane, only some of the most
prominent peaks for particular bond vibrations are discussed,
particularly if 2-methylpentane has a functional group with a particular
characteristic wavenumber peak.
The infrared spectrum of
2-methylpentane is
unique and the whole, or selected wavenumbers, can be used to
fingerprint its identity, sometimes analysing a mixture
containing 2-methylpentane or following its change of concentration in a
reaction.
Spectra obtained from a liquid film of 2-methylpentane. The right-hand part of the of the
infrared spectrum of 2-methylpentane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of 2-methylpentane and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of
2-methylpentane.
2-methylpentane C6H14,
 ,
,
For more
see The molecular structure,
classification and
naming of alkanes
Interpretation of the infrared spectrum of 2-methylpentane
The most prominent infrared absorption lines of
2-methylpentane
Strong C-H stretching vibration absorption bands at
wavenumbers 2940 to 2880 cm-1 for the CH2 and
CH3 groups in 2-methylpentane.
Strong C-H deformation vibration absorptions
at wavenumbers 1480 to 1365 cm-1 for the CH2
and CH3 groups in 2-methylpentane.
All of these infrared absorption vibrations are
characteristic of saturated alkyl structures in molecules, exemplified
by branched alkanes themselves e.g. 2-methylpentane.
2-methylpentane does
not have a specific organic functional group, so there is no
particular characteristic absorption band (wavenumber band) in its
infrared spectrum
The absence of other specific functional group bands
will show that particular functional group is absent from the
2-methylpentane
molecular
structure.
Summary of 2-methylpentane's infrared spectrum
2-Methylpentane shows characteristic IR absorptions for saturated
hydrocarbons, with key peaks around 2950 cm⁻¹ (C–H stretch) and 1450–1375
cm⁻¹ (C–H bends).
It lacks
functional group peaks like C=O or O–H, making its spectrum relatively
simple.
Key Infrared Absorptions of 2-Methylpentane
2-Methylpentane (C6H14)
is a branched alkane. Its IR spectrum is dominated by vibrations from C–H
bonds in methyl and methylene groups.
Here's a breakdown of the most prominent
peaks:
| Wavenumber
(cm⁻¹) |
Assignment |
Description |
| ~2955 |
Asymmetric C–H stretch (CH3) |
Strong, sharp peak from methyl groups |
| ~2925 |
Asymmetric C–H stretch (CH2) |
Strong, slightly broader than CH₃
stretch |
| ~2870 |
Symmetric C–H stretch (CH3/CH2) |
Weaker than asymmetric stretches |
| ~1465 |
CH2
bending (scissoring) |
Medium intensity, often split |
| ~1375 |
CH3
bending (umbrella mode) |
Medium intensity, diagnostic for
methyl groups |
| ~720 |
CH2
rocking (long chain alkanes) |
Weak, may be absent in short chains |
Sources: NIST Chemistry WebBook, Master Organic
Chemistry
Common Misconceptions in IR Interpretation
- Mistaking alkane C–H stretches for
functional groups: Students
may confuse the 2950–2850 cm⁻¹ region with O–H or N–H stretches. Emphasize
that alkane C–H stretches are sharp and less broad.
- Expecting strong peaks for alkanes:
Alkanes lack polar bonds, so their IR peaks are often weaker than those of
functional groups like C=O or O–H.
- Overinterpreting the fingerprint
region: Below 1300 cm⁻¹, peaks
are complex and not easily assigned without reference spectra. Focus on the
3000–1300 cm⁻¹ region for exam purposes.
Exam Revision Tips for IR Spectroscopy
These tips align with AQA, Edexcel, OCR, WJEC,
CCEA, CIE, IB, and US AP Chemistry syllabi:
- Know the key regions:
- O–H stretch:
~3200–3600 cm⁻¹ (broad)
- C=O stretch:
~1700 cm⁻¹ (strong, sharp)
- C–H stretch (alkanes):
~2850–2960 cm⁻¹
- C=C stretch:
~1640–1680 cm⁻¹
- Use IR to confirm absence of
functional groups: For
hydrocarbons like 2-methylpentane, the lack of O–H or C=O peaks is just as
important as what is present.
- Compare spectra:
Practice comparing IR spectra of alkanes, alcohols, ketones, and carboxylic
acids to spot differences quickly.
- Link structure to spectrum:
Understand how branching (like in 2-methylpentane) affects peak intensity
and position slightly, but doesn’t introduce new functional group peaks.
- Use IR with other techniques:
Exams may combine IR with mass spectrometry or NMR. Be ready to triangulate
evidence.
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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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 |
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13C NMR spectra above. |
Key words & phrases:: isohexane
image and diagram explaining the infrared spectrum of 2-methylpentane, complete infrared absorption spectrum of 2-methylpentane, comparative spectra of
2-methylpentane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
2-methylpentane,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of 2-methylpentane, revision of infrared spectroscopy of 2-methylpentane, fingerprint region analysis of
2-methylpentane, how to identify 2-methylpentane from its infrared spectrum, identifying organic
compounds like 2-methylpentane from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of 2-methylpentane detection of functional groups in the
2-methylpentane molecule example of the infrared spectrum of a
molecule like 2-methylpentane with a functional group interpreting
interpretation of the infrared spectrum of 2-methylpentane:
isohexane Diagram of absorption of wavenumber
peaks in the infrared spectrum of 2-methylpentane. Characteristic peak wavenumbers in the infrared
spectrum of 2-methylpentane. Finger print identification pattern using the infrared
spectrum of 2-methylpentane. Revision notes on the infrared spectrum of
2-methylpentane. Matching
and deducing the structure of the 2-methylpentane molecule from its infrared
spectrum. Infrared spectroscopy of aliphatic
alkanes, infrared spectra of
2-methylpentane, an isomer of molecular formula C6H14 How do you interpret
the infrared absorption spectrum of 2-methylpentane How to interpret the
infrared spectrum of 2-methylpentane Explanatory diagram of the infrared
spectrum of the 2-methylpentane molecule in terms of its molecular structure.
Listing data of the prominent main wavenumber peaks troughs in the infrared
spectrum of 2-methylpentane. How to explain the infrared spectrum of
2-methylpentane. Use of the infrared spectrum of 2-methylpentane, identification
of 2-methylpentane from its infrared spectrum - fingerprint wavenumber pattern
to identify the 2-methylpentane molecule. The uses of the infrared spectrum of
the 2-methylpentane molecule. The distinctive features of the infrared spectrum
of the 2-methylpentane molecule explained. explaining the peaks-trough of the
transmittance of the infrared spectrum of 2-methylpentane what does the infrared
spectrum tell you about the structure and properties of the 2-methylpentane
molecule? How is infrared spectrum of 2-methylpentane used to identify
2-methylpentane?
Links associated with 2-methylpentane
Isomers of molecular formula C5H12
(Mr = 72)
The chemistry of ALKANES
revision notes INDEX
The mass spectrum of
2-methylpentane
The H-1 NMR spectrum of
2-methylpentane
The C-13 NMR spectrum of
2-methylpentane
Infrared spectroscopy index
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spectroscopy on the infrared spectrum of 2-methylpentane, diagnostic
features, data analysed, useful infrared spectra exam revision comment notes 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. |
