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Interpreting the infrared
spectrum of 3,3-dimethylpentane
[Author
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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 & AP honors chemistry courses: Molecular
spectroscopy - analysing
infrared spectrum of
3,3-dimethylpentane
[spectra updated
Mar 19th 2026 *]
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Infrared spectroscopy - spectra index
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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 3,3-dimethylpentane
Students and teachers please note
my explanation of the infrared
spectrum of 3,3-dimethylpentane is designed
for advanced, but pre-university, chemistry courses.
Based in
the infrared spectrum diagram for 3,3-dimethylpentane, only some of the most
prominent peaks for particular bond vibrations are discussed,
particularly if 3,3-dimethylpentane has a functional group with a particular
characteristic wavenumber peak.
The infrared spectrum of
3,3-dimethylpentane is
unique and the whole, or selected wavenumbers, can be used to
fingerprint its identity, sometimes analysing a mixture
containing 3,3-dimethylpentane or following its change of concentration in a
reaction.
Spectra obtained from a liquid film of 3,3-dimethylpentane. The right-hand part of the of the
infrared spectrum of 3,3-dimethylpentane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of 3,3-dimethylpentane and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of
3,3-dimethylpentane.
3,3-dimethylpentane C7H16
The molecular structure and
naming of alkanes
Interpretation of
the infrared spectrum of 3,3-dimethylpentane
The most prominent infrared absorption lines of
3,3-dimethylpentane
Strong C-H stretching vibration absorptions at
wavenumbers 2940 to 2880 cm-1 for the CH2 and
CH3 groups in 3,3-dimethypentane.
Several strong C-H deformation vibration absorptions
at wavenumbers 1480 to 1365 cm-1 for the CH2
and CH3 groups in 3,3-dimethypentane.
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 3,3-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
3,3-dimethylpentane
molecular
structure.
Key
points about the infrared spectrum of 3,3-dimethylpentane
Structural
Overview: 3,3-Dimethylpentane
Molecular formula: C7H16
Structure: Branched saturated hydrocarbon (alkane), but
quite symmetrical
Functional groups: None beyond alkyl C–H bonds
Key IR Absorptions
for 3,3-Dimethylpentane
| Wavenumber (cm⁻¹) |
Bond Type |
Vibration Mode |
Description |
| 2950–2850 |
C–H (sp³) stretch |
Stretching |
Strong, broad band for alkyl C–H |
| 1470–1450 |
C–H bend (CH2/CH3) |
Bending (scissoring) |
Medium intensity, CH2/CH3
deformation |
| 1380–1365 |
CH3
symmetric bend |
Bending (umbrella) |
Medium–strong, diagnostic for CH3 |
| 800–700 |
C–C skeletal |
Rocking/wagging |
Weak, fingerprint region |
Note: No peaks for C=C, C≡C, C=O, O–H, or N–H are
expected.
Common
Misconceptions
| Misconception |
Clarification |
| All organic compounds show
strong IR peaks |
Not true—alkanes show only C–H
and C–C vibrations, which are less distinctive. |
| IR always identifies the
compound |
IR is best for functional
group identification, not full structure determination. |
| No peak = no bond |
Some bonds (e.g. C–C)
give weak or overlapping peaks, especially in the
fingerprint region. |
| IR shows molecular weight |
No—mass spectrometry
is used for molecular mass, not IR. |
Exam Revision Tips
- Focus on functional groups: IR is most useful for
identifying O–H, N–H, C=O, C=C, C≡C, etc.
- Alkanes show only C–H stretches and bends—look for
2950–2850 cm⁻¹ and ~1375 cm⁻¹.
- No strong peaks above 3000 cm⁻¹ in alkanes—this helps
rule out alcohols, acids, amines.
- Use IR with other spectra (e.g. NMR, MS) for full
identification.
- Fingerprint region (<1500 cm⁻¹) is complex—don’t
overinterpret unless comparing known spectra.
Practice Question
(Exam Style)
Question:
The IR spectrum of a hydrocarbon shows strong absorptions at 2960
cm⁻¹ and 2870 cm⁻¹, and medium peaks at 1465 cm⁻¹ and 1375 cm⁻¹.
(a) What type of compound is likely responsible for this spectrum?
(b) Explain the origin of the peaks at 2960 cm⁻¹ and 1375 cm⁻¹.
(c) Why are there no peaks above 3100 cm⁻¹ or around 1700 cm⁻¹?
Model Answer
(a) The compound is likely a saturated alkane,
such as 3,3-dimethylpentane.
(b)
- 2960 cm⁻¹: C–H stretching in alkyl (sp³) groups
- 1375 cm⁻¹: CH3 symmetric bending (umbrella
mode)
(c)
- No peaks above 3100 cm⁻¹: No O–H, N–H,
or sp² C–H bonds
- No peak at 1700 cm⁻¹: No C=O (carbonyl) group present
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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 3,3-dimethylpentane, complete infrared absorption spectrum of
3,3-dimethylpentane, comparative spectra of
3,3-dimethylpentane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
3,3-dimethylpentane,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of 3,3-dimethylpentane, revision of infrared spectroscopy of 3,3-dimethylpentane, fingerprint region analysis of
3,3-dimethylpentane, how to identify 3,3-dimethylpentane from its infrared spectrum, identifying organic
compounds like 3,3-dimethylpentane from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of
3,3-dimethylpentane detection of no functional groups in the
3,3-dimethylpentane molecule example of the infrared spectrum of a
molecule like 3,3-dimethylpentane with no functional group
interpreting interpretation of the infrared spectrum of 3,3-dimethylpentane shows presence
of no functional groups
CH3CH2C(CH3)2CH2CH3 Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
3,3-dimethylpentane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 3,3-dimethylpentane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 3,3-dimethylpentane. The m/e m/z value of the base ion peak in the
mass spectrum of 3,3-dimethylpentane. Possible examples of equations showing the formation
of the ionised fragments in 3,3-dimethylpentane. Revision notes on the mass spectrum of
3,3-dimethylpentane.
Matching and deducing the structure of the 3,3-dimethylpentane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic alkanes,
mass spectra of 3,3-dimethylpentane, a structural isomer of molecular formula
C7H16
How do you interpret the infrared absorption spectrum of
3,3-dimethylpentane How
to interpret the infrared spectrum of 3,3-dimethylpentane Explanatory diagram of the
infrared spectrum of the 3,3-dimethylpentane molecule in terms of its molecular
structure. Listing data of the prominent main wavenumber peaks
troughs in the infrared spectrum of 3,3-dimethylpentane. How to explain the infrared
spectrum of 3,3-dimethylpentane. Use of the infrared spectrum of
3,3-dimethylpentane, identification
of 3,3-dimethylpentane from its infrared spectrum - fingerprint wavenumber pattern
to identify the 3,3-dimethylpentane molecule. The uses of the infrared spectrum of
the 3,3-dimethylpentane molecule. The distinctive features of the infrared spectrum
of the 3,3-dimethylpentane molecule explained interpretation diagram explaining the
peaks-trough of the transmittance of the infrared spectrum of
3,3-dimethylpentane
what does the infrared spectrum tell you about the structure and
properties of the 3,3-dimethylpentane molecule? How is infrared spectrum of
3,3-dimethylpentane used
to identify 3,3-dimethylpentane?
Links associated with 3,3-dimethylpentane
The mass spectrum of
3,3-dimethylpentane
The H-1 NMR spectrum of
3,3-dimethylpentane
The C-13 NMR spectrum of
3,3-dimethylpentane
The chemistry of ALKANES
revision notes INDEX
Infrared spectroscopy index
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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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