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Interpreting the infrared
spectrum of 3-ethylpentane
[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-ethylpentane
[updated
October 24th 2025]
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HC(CH2CH3)3
The chemistry of ALKANES and the petrochemical
industry
Links associated with 3-ethylpentane
This is a BIG
website, PLEASE 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 3-ethylpentane
Students and teachers please note
my explanation of the infrared
spectrum of 3-ethylpentane is designed
for advanced, but pre-university, chemistry courses.
Based in
the infrared spectrum diagram for 3-ethylpentane, only some of the most
prominent peaks for particular bond vibrations are discussed,
particularly if 3-ethylpentane has a functional group with a particular
characteristic wavenumber peak.
The infrared spectrum of
3-ethylpentane is
unique and the whole, or selected wavenumbers, can be used to
fingerprint its identity, sometimes analysing a mixture
containing 3-ethylpentane or following its change of concentration in a
reaction.
Spectra obtained from a liquid film of 3-ethylpentane. The right-hand part of the of the
infrared spectrum of 3-ethylpentane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of 3-ethylpentane and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of
3-ethylpentane.
3-ethylpentane
CH(CH2CH3)3
For more
see
The molecular structure,
classification and
naming of alkanes
Interpretation of
the infrared spectrum of 3-ethylpentane
The most prominent infrared absorption lines of
3-ethylpentane
Most of the strong absorptions are due to the vibrations of the C-H bonds
e.g. stretching and bending deformations at ~3000 to 2900 cm-1 and ~1500
to 1400 cm-1.
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.
Skeletal C-C vibrations show an absorption ~750 to
720 cm-1 for -(CH2)n-, where n is
>3, which is obviously the case with the carbon chain of
3-ethylpentane.
There is no characteristic absorption band for a
functional group, as alkanes don't have one.
There are no specific characteristic lines for
alkanes like 3-ethylpentane because C-C and C-H vibrations are common to so
many organic molecules and alkanes like octane 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
3-ethylpentane
molecular
structure.
Key points about the infrared
spectrum of 2-ethylpentane
The IR spectrum of 2-ethylpentane shows
characteristic alkane C–H stretching and bending vibrations, with no peaks
for functional groups like O–H, C=O, or C≡C.
Key IR Spectrum
Features of 2-Ethylpentane
2-Ethylpentane is a branched alkane (C7H16) with no
polar functional groups. Its IR spectrum is dominated by C–H stretching and
bending vibrations typical of saturated hydrocarbons:
| Wavenumber (cm⁻¹) |
Bond / Vibration |
Description |
| 2950–2850 |
C–H stretch (sp³ alkyl) |
Multiple sharp peaks; symmetric
and asymmetric stretching of methyl/methylene |
| 1470–1450 |
CH2 bending
(scissoring) |
Medium intensity; confirms
presence of methylene groups |
| 1380–1365 |
CH3
bending (umbrella mode) |
Medium intensity; diagnostic for
methyl groups |
| <1300 |
Fingerprint region |
Complex pattern; useful for
compound comparison but not functional group ID |
Sources: Master Organic Chemistry, Specac IR Frequency Lookup
Common
Misconceptions in Exams
- Expecting functional group peaks: Students may
mistakenly look for O–H or C=O stretches—these are absent in alkanes.
- Overinterpreting the fingerprint region: Peaks below
1300 cm⁻¹ are often misassigned; they’re best used for matching known
spectra.
- Confusing alkane C–H stretches with aromatic or alkene C–H:
Alkane C–H stretches are lower in frequency (~2950 cm⁻¹) and lack adjacent
unsaturation.
- Assuming IR is uninformative for alkanes: While simple,
IR can confirm absence of functional groups and presence of alkyl chains.
Exam Revision Tips
- Use IR to eliminate possibilities: Absence of O–H
(~3300 cm⁻¹), C=O (~1700 cm⁻¹), or C≡C (~2100–2260 cm⁻¹) helps narrow down
structure.
- Learn alkane C–H regions: Memorize 2950–2850 cm⁻¹ for
sp³ C–H stretches and 1470/1380 cm⁻¹ for bending modes.
- Compare with branched versus straight alkanes:
Branching affects fingerprint region subtly—use for advanced interpretation.
- Practice with spectra overlays: Annotate known spectra
to reinforce peak assignments and structural implications.
- Use IR with other techniques: Combine with mass or NMR
data to confirm hydrocarbon identity and branching.
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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 3-ethylpentane, complete infrared absorption spectrum of 3-ethylpentane, comparative spectra of
3-ethylpentane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
3-ethylpentane,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of 3-ethylpentane, revision of infrared spectroscopy of 3-ethylpentane, fingerprint region analysis of
3-ethylpentane, how to identify 3-ethylpentane from its infrared spectrum, identifying organic
compounds like 3-ethylpentane from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of 3-ethylpentane detection of
no functional groups in the 3-ethylpentane molecule example of the infrared spectrum of a
molecule like 3-ethylpentane with a no functional group absorptions
interpreting interpretation of the infrared spectrum of 3-ethylpentane shows presence
of no functional groups
(CH3CH2)3CH CH3CH2CH(CH2CH3)CH2CH3
Diagram of absorption of wavenumber
peaks in the infrared spectrum of 3-ethylpentane. Characteristic peak wavenumbers in the infrared
spectrum of 3-ethylpentane. Finger print identification pattern using the infrared
spectrum of 3-ethylpentane. Revision notes on the infrared spectrum of
3-ethylpentane. Matching
and deducing the structure of the 3-ethylpentane molecule from its infrared
spectrum. Infrared spectroscopy of aliphatic alkanes, infrared spectra of
3-ethylpentane, a structural isomer of molecular formula C7H16 How do you
interpret the infrared absorption spectrum of 3-ethylpentane How to interpret
the infrared spectrum of 3-ethylpentane Explanatory diagram of the infrared
spectrum of the 3-ethylpentane molecule in terms of its molecular structure.
Listing data of the prominent main wavenumber peaks troughs in the infrared
spectrum of 3-ethylpentane. How to explain the infrared spectrum of
3-ethylpentane. Use of the infrared spectrum of 3-ethylpentane, identification
of 3-ethylpentane from its infrared spectrum - fingerprint wavenumber pattern to
identify the 3-ethylpentane molecule. The uses of the infrared spectrum of the
3-ethylpentane molecule. The distinctive features of the infrared spectrum of
the 3-ethylpentane molecule explained. explaining the peaks-trough of the
transmittance of the infrared spectrum of 3-ethylpentane what does the infrared
spectrum tell you about the structure and properties of the 3-ethylpentane
molecule? How is infrared spectrum of 3-ethylpentane used to identify
3-ethylpentane?
Links associated with 3-ethylpentane
The mass spectrum of
3-ethylpentane
The
H-1 NMR spectrum of 3-ethylpentane
The
C-13 NMR spectrum of 3-ethylpentane
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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