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Interpreting and
explaining the infrared
spectrum of 2,2-dimethylpropane
[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
2,2-dimethylpropane
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See also
comparing the infrared, mass,
1H NMR and 13C NMR
spectra of the 3 alkane isomers of C5H12
Isomers of molecular formula C5H12
(Mr = 72)
Introductory note on the infrared spectrum of 2,2-dimethylpropane
Students and teachers please note
my explanation of the infrared
spectrum of 2,2-dimethylpropane is designed
for advanced, but pre-university, chemistry courses.
Based in
the infrared spectrum diagram for 2,2-dimethylpropane, only some of the most
prominent peaks for particular bond vibrations are discussed,
particularly if 2,2-dimethylpropane has a functional group with a particular
characteristic wavenumber peak.
The infrared spectrum of
2,2-dimethylpropane is
unique and the whole, or selected wavenumbers, can be used to
fingerprint its identity, sometimes analysing a mixture
containing 2,2-dimethylpropane or following its change of concentration in a
reaction.
Spectra obtained from 2,2-dimethylpropane in its gaseous
state (boiling point oC). The right-hand part of the of the
infrared spectrum of 2,2-dimethylpropane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of 2,2-dimethylpropane and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of
2,2-dimethylpropane.
2,2-dimethylpropane C5H12
 ,
,
For more
see The molecular structure and
naming of alkanes
Interpretation of
the infrared spectrum of 2,2-dimethylpropane
The most prominent infrared absorption lines of
2,2-dimethylpropane
Most of these are due to the vibrations of the
C-H bonds e.g. stretching and bending at ~2975 to 2845 cm-1 and ~1470
to 1370 cm-1.
There is no characteristic absorption band for a
functional group, as alkanes don't have one.
There are C-C skeletal vibration absorptions at 1175
to 1140 cm-1, which include C-(CH3)2
and C-(CH3)3 vibrations
Note:
(i) The infrared spectrum of vapours tends to be
simpler than those for liquid or solid films.
(ii) The absorption peaks are usually much sharper
than in liquid/solid film spectra.
(iii) In the condensed state of liquid or solid,
molecules interact with each other (vibrate against each other) and
affect the individual vibrational energy level modes of the
molecules, often producing a broadening effect on the absorption
bands.
(iv) In the gaseous state, the molecules are much
further apart and are less likely to interfere with each other.
The absence of other specific functional group bands
will show that particular functional group is absent from the
2,2-dimethylpropane
molecular
structure.
Key
points about the infrared spectrum of 2,2-dimethylpropane
2,2-Dimethylpropane shows a simple IR spectrum dominated by alkane C–H
stretching and bending vibrations, with no functional group peaks above 3000
cm⁻¹.
Its
fingerprint region is key for identification.
Key IR Features of
2,2-Dimethylpropane
2,2-Dimethylpropane (neopentane) is a
highly symmetrical, branched alkane with no polar functional groups.
Its IR spectrum reflects this simplicity:
- No strong absorptions above 3000
cm⁻¹: No O–H, N–H, or C=O
groups.
- C–H stretching (sp³ hybridized):
Moderate peaks just below 3000 cm⁻¹.
- C–H bending (methyl groups):
Strong absorptions in the fingerprint region (1350–1470 cm⁻¹).
- Fingerprint region:
Complex but characteristic pattern due to methyl rocking and wagging.
Prominent IR
Absorptions Table
| Wavenumber
(cm⁻¹) |
Assignment |
Intensity |
Notes |
| 2950–2850 |
C–H stretch (alkane, sp³) |
Medium |
Symmetric and asymmetric
methyl stretches |
| 1470–1450 |
CH3 asymmetric
bend (scissoring) |
Strong |
Diagnostic for methyl
groups |
| 1390–1370 |
CH3
symmetric bend (rocking) |
Strong |
Often split due to
multiple CH3
groups |
| <1300 |
Fingerprint region (C–C,
CH3
wag) |
Variable |
Complex, used for
compound ID |
Common
Misconceptions
- Mistaking absence of peaks for
purity: A “clean” spectrum
with few peaks doesn’t mean the sample is pure—it reflects the molecule’s
simplicity.
- Overinterpreting the fingerprint
region: While useful for
comparison, it’s not reliable for functional group identification.
- Assuming all hydrocarbons show
strong IR signals: Nonpolar
bonds like C–C and symmetric C–H stretches have weak IR activity.
Exam Revision Tips
For A-levels (AQA, Edexcel, OCR, WJEC,
CCEA), CIE, IB, and US AP Chemistry:
- Focus on functional group regions:
Know where O–H, N–H, C=O, and C≡C appear. For hydrocarbons like
2,2-dimethylpropane, emphasize C–H stretches and bends.
- Use IR to confirm absence:
In questions, IR can rule out alcohols, ketones, or acids if no peaks appear
above 3000 cm⁻¹.
- Practice with spectra overlays:
Compare IR spectra of straight-chain versus branched alkanes to spot subtle
differences in methyl bending.
- Link to molecular symmetry:
Highly symmetrical molecules like neopentane show fewer IR-active modes—use
this to explain sparse spectra.
- Watch for distractors:
Some exam questions include IR spectra with misleading peaks—always match
peaks to known functional groups.
|
Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the 3 alkane isomers of C5H12
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 pentane,
2-methylbutane and 2,2-dimethylpropane image sizes. |
 |
 |
 |
Comparing the
infrared
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane
exemplify infrared spectra of the alkane homologous series CnH2n+2
hydrocarbon
molecules, where n = 5 |
|
INFRARED SPECTRA
(above): There are, as expected, differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, but there is no
specific infrared absorption band for a functional group. The
infrared spectra of pentane and 2-methylbutane seem very
similar, but that of 2,2-dimethylpropane seems much simpler. |
 |
 |
 |
Comparing the
mass
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane
exemplify the mass spectra of the alkane series CnH2n+2
hydrocarbon
molecules, where n = 5 |
|
MASS SPECTRA (above):
All three hydrocarbons show some similarities in their mass
spectra e.g. m/z ions 27 to 29 for [C2Hx]+
(x = 2 and 4). The molecular ion peaks will
be the same for all three isomers (m/z 72),
but it is very tiny for 2,2-dimethypropane. The pattern ratios
for m/z 39 to 43 are similar for pentane and 2-methylbutane, but
m/z 42 and 43 ions are almost absent from the
2,2-dimethylpropane spectrum. The base peak ion for pentane is
m/z 43, but for 2-methylbutane and 2,2-dimethylpropane it is m/z
57. |
 |
 |
 |
Comparing the
1H proton NMR
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane exemplify the 1H proton NMR spectra of the alkane
homologous series CnH2n+2
hydrocarbon
molecules where, n = 5 |
|
1H NMR SPECTRA (above): The 1H NMR spectra of
all three molecules give different proton ratios for the
different 1H chemical environments i.e. pentane's
proton ratio is 3:2:1 (from 6:4:2 H's in the molecule).
2-methylbutane's proton ratio is 6:1:2:3 and
2,2-dimethylpropane's doesn't have a proton ratio, all hydrogen
atoms are equivalent. This means all three isomeric C5H12
hydrocarbons can be distinguished from their 1H NMR spectra. |
 |
 |
 |
Comparing the
carbon-13 NMR
spectra of pentane, 2-methylbutane and 2,2-dimethylpropane
Pentane,
2-methylbutane and 2,2-dimethylpropane
are structural isomers of molecular formula C5H12
Pentane,
2-methylbutane and 2,2-dimethylpropane exemplify the carbon-13 NMR spectra of
members of the alkane homologous series CnH2n+2
hydrocarbon
molecules, where n = 5 |
|
13C NMR SPECTRA
(above): The
13C NMR spectra of the three molecules show different numbers of
carbon-13 chemical environments i.e. different numbers of 13C NMR
resonance lines. So, pentane gives three 13C chemical
shifts,
2-methylbutane four and 2,2-dimethylpropane two. This means all
three isomeric C5H12 hydrocarbons can be
distinguished from their 13C NMR spectra. |
Key words & phrases: neopentane
dimethylpropane
image and diagram explaining the infrared spectrum
of 2,2-dimethylpropane, complete infrared absorption spectrum of
2,2-dimethylpropane, comparative spectra of
2,2-dimethylpropane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
2,2-dimethylpropane,
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of 2,2-dimethylpropane, revision of infrared spectroscopy of 2,2-dimethylpropane, fingerprint region analysis of
2,2-dimethylpropane, how to identify 2,2-dimethylpropane from its infrared spectrum, identifying organic
compounds like 2,2-dimethylpropane from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of
2,2-dimethylpropane detection of
neopentane dimethylpropane functional groups in the
2,2-dimethylpropane molecule example of the infrared spectrum of a
molecule like 2,2-dimethylpropane with a functional group
neopentane dimethylpropane Diagram of absorption of wavenumber
peaks in the infrared spectrum of 2,2-dimethylpropane. Characteristic peak wavenumbers in the infrared
spectrum of 2,2-dimethylpropane. Finger print identification pattern using the infrared
spectrum of 2,2-dimethylpropane. Revision notes on the infrared spectrum of
2,2-dimethylpropane. Matching
and deducing the structure of the 2,2-dimethylpropane molecule from its infrared
spectrum. Infrared spectroscopy of aliphatic alkanes, infrared spectra of
2,2-dimethylpropane, an isomer of molecular formula C5H12 How do you interpret the infrared absorption spectrum of
2,2-dimethylpropane How
to interpret the infrared spectrum of 2,2-dimethylpropane Explanatory diagram of the
infrared spectrum of the 2,2-dimethylpropane molecule in terms of its molecular
structure. Listing data of the prominent main wavenumber peaks
troughs in the infrared spectrum of 2,2-dimethylpropane. How to explain the infrared
spectrum of 2,2-dimethylpropane. Use of the infrared spectrum of
2,2-dimethylpropane, identification
of 2,2-dimethylpropane from its infrared spectrum - fingerprint wavenumber pattern
to identify the 2,2-dimethylpropane molecule. The uses of the infrared spectrum of
the 2,2-dimethylpropane molecule. The distinctive features of the infrared spectrum
of the 2,2-dimethylpropane molecule explained interpretation diagram explaining the
peaks-trough of the transmittance of the infrared spectrum of
2,2-dimethylpropane
what does the infrared spectrum tell you about the structure and
properties of the 2,2-dimethylpropane molecule? How is infrared spectrum of
2,2-dimethylpropane used
to identify 2,2-dimethylpropane?
Links associated
with
2,2-dimethylpropane
Isomers of molecular formula C5H12
(Mr = 72)
The chemistry of ALKANES
revision notes INDEX
The mass spectrum for 2,2-dimethylpropane
The H-1 NMR spectrum for 2,2-dimethylpropane
The C-13 NMR spectrum for 2,2-dimethylpropane
Infrared spectroscopy index
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spectroscopy on the infrared spectrum of 2,2-dimethylpropane,
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revision comment notes
suitable for use of pre-university students studying AQA advanced level
chemistry, Edexcel advanced level chemistry, OCR advanced level
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chemistry, CIE advanced level chemistry, US grade 11-12 AP honors
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