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Interpreting the infrared
spectrum of 2-methylpentane
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Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US
K12 grade 11 grade 12 organic chemistry courses involving molecular
spectroscopy analysing infrared spectra of 2-methylpentane
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Infrared spectroscopy - spectra index
See also
comparing infrared, mass, 1H NMR & 13C NMR
spectra of the structural alkane isomers of C6H14
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. 2,3-dimethylbutane.
The absence of other specific functional group bands
will show that particular functional group is absent from the
2-methylpentane
molecular
structure.
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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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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
Links associated with 2-methylpentane
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
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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