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Advanced Organic Chemistry: Infrared spectrum of 2,3-dimethylbutane

Interpreting the infrared spectrum of 2,3-dimethylbutane

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 2,3-dimethylbutane

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

infrared spectrum of 2,3-dimethylbutane wavenumbers cm-1 functional group detection fingerprint pattern identification of 2,3-dimethylbutane doc brown's advanced organic chemistry revision notes 

Spectra obtained from a liquid film of 2,3-dimethylbutane. The right-hand part of the of the infrared spectrum of 2,3-dimethylbutane, wavenumbers ~1500 to 400 cm-1 is considered the fingerprint region for the identification of 2,3-dimethylbutane and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of 2,3-dimethylbutane.

2,3-dimethylbutane 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 2,3-dimethylbutane

The most prominent infrared absorption lines of 2,3-dimethylbutane

For 2,3-dimethylbutane, there are strong C-H stretching vibration absorption bands at wavenumbers 2940 to 2880 cm-1 for the CH2 and CH3 groups.

Several strong C-H deformation vibration absorptions at wavenumbers 1480 to 1365 cm-1 for the CH2 and CH3 groups in 2,3-dmethybutane (a double peak).

Several strong C-C skeletal vibration absorptions associated with a C-(CH3)2 group occur at wavenumbers 1175 to 1140 cm-1 and 840 to 790 cm-1.

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 2,3-dimethylbutane 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.

Key words & phrases: image and diagram explaining the infrared spectrum of 2,3-dimethylbutane, complete infrared absorption spectrum of 2,3-dimethylbutane, comparative spectra of 2,3-dimethylbutane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of 2,3-dimethylbutane, important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum of 2,3-dimethylbutane, revision of infrared spectroscopy of 2,3-dimethylbutane, fingerprint region analysis of 2,3-dimethylbutane, how to identify 2,3-dimethylbutane from its infrared spectrum, identifying organic compounds like 2,3-dimethylbutane from their infrared spectrum, how to analyse the absorption bands in the infrared spectrum of 2,3-dimethylbutane detection of functional groups in the 2,3-dimethylbutane molecule example of the infrared spectrum of a molecule like 2,3-dimethylbutane with a functional group  interpreting interpretation of the infrared spectrum of 2,3-dimethylbutane

Diagram of absorption of wavenumber peaks in the infrared spectrum of 2,3-dimethylbutane. Characteristic peak wavenumbers in the infrared spectrum of 2,3-dimethylbutane. Finger print identification pattern using the infrared spectrum of 2,3-dimethylbutane. Revision notes on the infrared spectrum of 2,3-dimethylbutane. Matching and deducing the structure of the 2,3-dimethylbutane molecule from  its infrared spectrum. Infrared spectroscopy of aliphatic alkanes, infrared spectra of 2,3-dimethylbutane, a structural isomer of molecular formula C6H14


Links associated with 2,3-dimethylbutane

The chemistry of ALKANES revision notes INDEX

The mass spectrum of 2,3-dimethylbutane

The H-1 NMR spectrum of 2,3-dimethylbutane

The C-13 NMR spectrum of 2,3-dimethylbutane

Infrared spectroscopy index

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