Advanced Organic Chemistry: Infrared spectrum of hexane

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Interpreting the infrared spectrum of hexane

Doc 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

See also comparing infrared, mass, 1H NMR & 13C NMR spectra of the structural alkane isomers of C6H14

infrared spectrum of hexane wavenumbers cm-1 functional group detection fingerprint pattern identification of hexane doc brown's advanced organic chemistry revision notes 

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.

Hexane C6H14, alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b

For more see The molecular structure, classification and naming of alkanes

Interpretation of 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 molecular structure.
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).
Infrared spectra below.

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.
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 prominent

2,3-dimethylbutane: m/z 43, 41, 42 and 71 prominent
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.

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)
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 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
13C NMR spectra above.

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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

All Advanced Organic Chemistry Notes

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