Interpreting the infrared
spectrum of hexane
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 hexane
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Infrared spectroscopy - spectra index
comparing infrared, mass, 1H NMR & 13C NMR
spectra of the structural alkane isomers of C6H14
Spectra obtained from a liquid film of hexane. The right-hand part of the of the
infrared spectrum of hexane, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of hexane and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of hexane.
see The molecular structure,
naming of alkanes
the infrared spectrum of hexane
The most prominent infrared absorption lines of hexane
Strong C-H stretching vibration absorption bands at
wavenumbers 2940 to 2880 cm-1 for the CH2 and
CH3 groups in hexane.
Strong C-H deformation vibration absorptions
at wavenumbers 1480 to 1365 cm-1 for the CH2
and CH3 groups in hexane.
Moderate C-C skeletal vibration absorption due to
-(CH2)4- grouping at 750 to 720 cm-1,
the shortest linear alkane in which this can occur.
All of these infrared absorption vibrations are
characteristic of saturated alkyl structures in molecules,
exemplified by linear alkanes themselves e.g. hexane.
The absence of other specific functional group bands
will show that particular functional group is absent from the hexane
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
exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower
aliphatic alkanes (non-cyclic alkanes).
Infrared spectra below.
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
Infrared spectra above, mass spectra below.
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
2,3-dimethylbutane: m/z 43, 41, 42 and 71
Mass spectra above, 1H NMR spectra below.
1H NMR SPECTRA: They can
all be distinguished by their different integrated proton ratios -
need very high resolution.
δ shifts, H ratio 3:2:2 (6:4:4 in formula)
δ shifts, H ratio 6:3:2:2:1
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)
1H NMR spectra above, 13C NMR spectra below.
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
(ii) 2-methylpentane: 5 13C
(iii) 3-methylpentane: 4 13C
(iv) 2,2-dimethylbutane: 4 13C
(v) 2,3-dimethylbutane: 2 13C
13C NMR spectra above.
Key words & phrases: image and diagram explaining the infrared spectrum
of hexane, complete infrared absorption spectrum of hexane, comparative spectra of
hexane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of
important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum
of hexane, revision of infrared spectroscopy of hexane, fingerprint region analysis of
hexane, how to identify hexane from its infrared spectrum, identifying organic
compounds like hexane from their infrared spectrum,
how to analyse the absorption bands in the infrared spectrum of hexane detection of functional groups in the hexane molecule example of the infrared spectrum of a
molecule like hexane with a functional group interpreting interpretation of the infrared spectrum of hexane
Diagram of absorption of wavenumber
peaks in the infrared spectrum of hexane. Characteristic peak wavenumbers in the infrared
spectrum of hexane. Finger print identification pattern using the infrared
spectrum of hexane. Revision notes on the infrared spectrum of hexane. Matching
and deducing the structure of the hexane molecule from its infrared
spectrum. Infrared spectroscopy of aliphatic
alkanes, infrared spectra of
hexane, an isomer of molecular formula C6H14
Links associated with hexane
The chemistry of ALKANES
revision notes INDEX
The mass spectrum of hexane
The H-1 NMR spectrum of hexane
The C-13 NMR spectrum of hexane
Infrared spectroscopy index
ALL SPECTROSCOPY INDEXES
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