Interpreting the infrared spectrum of 2-chlorobutane

See also Comparing infrared, mass, 1H NMR & 13C NMR spectra of the 4 structural isomers of C₄H₉Cl

Introductory note on the infrared spectrum of 2-chlorobutane

Students and teachers please note my explanation of the infrared spectrum of 2-chlorobutane is designed for advanced, but pre-university, chemistry courses.

Based in the infrared spectrum diagram for 2-chlorobutane, only some of the most prominent peaks for particular bond vibrations are discussed, particularly if 2-chlorobutane has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of 2-chlorobutane is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing 2-chlorobutane or following its change of concentration in a reaction.

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

Interpretation of the infrared spectrum of 2-chlorobutane

The most prominent infrared absorption lines of 2-chlorobutane

For 2-chlorobutane you observe infrared C-H stretching vibrations at wavenumbers ~2880 to 3080 cm^-1,

and C-H bending vibrations at wavenumbers ~1300 to 1500 cm^-1.

These are typical infrared absorptions of molecules like 2-chlorobutane with alkyl structures in them.

Characteristic groups of C-Cl vibration absorptions at wavenumbers ~580 to 780 cm^-1 that you expect in halogenoalkane molecules like 2-chlorobutane.

The absence of other specific functional group bands will show that a particular functional group is absent from the 2-chlorobutane molecular structure.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C4H9Cl 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 1-chlorobutane, 2-chlorobutane, 1-chloro-2-methylpropane and 2-chloro-2-methylpropane image sizes.  These four molecules are structural isomers of molecular formula C4H9Cl and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, chloroalkanes, alkyl chlorides).
INFRARED SPECTRA (above): 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. The infrared spectrum of 2-chloro-2-methylpropane is noticeably simpler in the fingerprint region, perhaps due to the greater symmetry of the molecule.
MASS SPECTRA (above): Theoretically, all four can give the parent molecular ions of m/z 92 and 94, but they are all relatively tiny peaks. 2-chlorobutane and 2-chloro-2-methylpropane give a base ion peak of m/z 57. The base ion peak for 1-chlorobutane is m/z 56 and that of 1-chloro-2-methylpropane is m/z 43. Each gives different patterns of pairs of m/z values two mass units apart, in the peak height ratio of 3:1, if the positive fragment contains a chlorine atom (35Cl or 37Cl) e.g look for m/z pairs 49/51, 63/65 and 77/79 in their mass spectra.
1H NMR SPECTRA (above): The 1H NMR spectra of all four molecules give different integrated proton ratios i.e.1-chlorobutane four peaks of ratio 3:2:2:2; 2-chlorobutane four peaks of ratio 3:3:2:1, 1-chloro-2-methylpropane three peaks of ratio 6:2:1 and 2-chloro-2-methylpropane gives just one peak '1' (effectively no ratio involved), so all four molecular structures can be distinguished from each other by their 1H NMR spectra proton ratios, numbers of peaks and (n+1) rule splitting patterns.
13C NMR SPECTRA (above): The 13C NMR spectra of the four molecules show various numbers of carbon-13 chemical environments i.e 1-chlorobutane and 2-chlorobutane show four 13C NMR resonances, 1-chloro-2-methylpropane three 13C NMR resonances and 2-chloro-2-methylpropane only two 13C resonances (3 and 2 chemical environments respectively. Therefore 1-chloro-2-methylpropane and 2-chloro-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-chlorobutane and 2-chlorobutane cannot be distinguished from each other from their number of 13C NMR resonance lines - other data would be required.

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