Advanced Organic Chemistry: Infrared spectrum of 2-bromobutane CH3CH2CHBrCH3

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Interpreting the infrared spectrum of 2-bromobutane (sec-butyl bromide)

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of 2-bromobutane (infrared spectra) [spectra page updated April 1st 2026 *]

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 Infrared spectroscopy - spectra index

See also comparing infrared, mass, 1H NMR & 13C NMR spectra of 4 halogenoalkane isomers of C4H9Br

Introductory note on the infrared spectrum of 2-bromobutane

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

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

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

infrared spectrum of 2-bromobutane  C4H9Br CH3CHBrCH2CH3 wavenumbers cm-1 functional group detection fingerprint pattern identification of sec-butyl bromide doc brown's advanced organic chemistry revision notes 

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

2-bromobutane, C4H9Br, CH3CHBrCH2CH3, CH3-CH(Br)-CH2-CH3

The molecular structure and naming of haloalkanes

 Secondary haloalkane/halogenoalkane and old names: sec-butyl bromide, sec-butyl bromide (secondary alkyl halide)

Interpretation of the infrared spectrum of 2-bromobutane

The most prominent infrared absorption lines of 2-bromobutane

C-H stretching vibration absorptions peak at wavenumbers ~2845 to 2975 cm-1.

C-H bending/deformation vibrations absorption bands peaking at wavenumbers ~1270 to 1470 cm-1.

C-C-C skeletal vibrations from the CH2-CH2-CH2 grouping occur at wavenumbers ~1370 to 1250 cm-1 and from CH2-CH2-CH2-CH2 750 to 720 cm-1.

These absorption bands are typical of most alkyl structures like those in 2-bromobutane.

Characteristic groups of C-Br stretching vibration absorptions at wavenumbers ~550 to 650 cm-1 that you expect in 2-bromobutane.

The absence of other specific functional group bands will show that a particular functional group is absent from the 2-bromobutane molecular structure.
Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C4H9Br

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-bromobutane, 2-bromobutane, 1-bromo-2-methylpropane and 2-bromo-2-methylpropane image sizes.  These four molecules are structural isomers of molecular formula C4H9Br and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, bromoalkanes, alkyl bromides).

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.

MASS SPECTRA (above): All four give the parent molecular ions of m/z 136 and 138, but it is only a relatively tiny peak for 2-bromobutane and 2-bromo-2-methylpropane. All four give the base ion peak of m/z 57. All four give prominent peaks for m/z ions 27, 29, 39 and 41 and all give a tiny peak from an ionised iodine atom at m/z 127. They look quite similar to me and lack a clear fingerprint fragmentation pattern. There are small differences in the relative abundances (peak heights) for pairs of ions involving 79Br/81Br isotopes e.g. m/z 93/95, 107/109 and 121/123. 1-bromo-2-methylpropane is the only one of the four to have a prominent peak for the m/z 43 ion.

1H NMR SPECTRA (above): The 1H NMR spectra of all four molecules give different integrated proton ratios i.e.1-bromobutane four peaks of ratio 3:2:2:2; 2-bromobutane four peaks of ratio 3:3:2:1, 1-bromo-2-methylpropane three peaks of ratio 6:2:1 and 2-bromo-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-bromobutane and 2-bromobutane show four 13C NMR resonances, 1-bromo-2-methylpropane three 13C NMR resonances and 2-bromo-2-methylpropane only two 13C resonances. Therefore 1-bromo-2-methylpropane and 2-bromo-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-bromobutane and 2-bromobutane cannot be distinguished from each other from their number of 13C NMR resonance lines - other data would be required.

Key words & phrases: C4H9Br CH3CHBrCH2CH3 CH3CH2CHBrCH3 image and diagram explaining the infrared spectrum of 2-bromobutane, complete infrared absorption spectrum of 2-bromobutane, comparative spectra of 2-bromobutane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of 2-bromobutane, important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum of 2-bromobutane, revision of infrared spectroscopy of 2-bromobutane, fingerprint region analysis of 2-bromobutane, how to identify 2-bromobutane from its infrared spectrum, identifying organic compounds like 2-bromobutane from their infrared spectrum, how to analyse the absorption bands in the infrared spectrum of 2-bromobutane detection of functional groups in the 2-bromobutane molecule example of the infrared spectrum of a molecule like 2-bromobutane with a functional group  interpreting interpretation of the infrared spectrum of 2-bromobutane shows presence of  functional group sec-butyl bromide sec-butyl bromide alkyl bromide alkyl halide haloalkane halogenoalkane Diagram of absorption of wavenumber peaks in the infrared spectrum of 2-bromobutane (sec-butyl bromide). Characteristic peak wavenumbers in the infrared spectrum of 2-bromobutane (sec-butyl bromide). Finger print identification pattern using the infrared spectrum of 2-bromobutane (sec-butyl bromide). Revision notes on the infrared spectrum of 2-bromobutane (sec-butyl bromide). Matching and deducing the structure of the 2-bromobutane (sec-butyl bromide) molecule from  its infrared spectrum. How do you interpret the infrared absorption spectrum of 2-bromobutane How to interpret the infrared spectrum of 2-bromobutane Explanatory diagram of the infrared spectrum of the 2-bromobutane molecule in terms of its molecular structure. Listing data of the prominent main wavenumber peaks troughs in the infrared spectrum of 2-bromobutane. How to explain the infrared spectrum of 2-bromobutane. Use of the infrared spectrum of 2-bromobutane, identification of 2-bromobutane from its infrared spectrum - fingerprint wavenumber pattern to identify the 2-bromobutane molecule. The uses of the infrared spectrum of the 2-bromobutane molecule. The distinctive features of the infrared spectrum of the 2-bromobutane molecule explained interpretation diagram explaining the peaks-trough of the transmittance of the infrared spectrum of 2-bromobutane what does the infrared spectrum tell you about the structure and properties of the 2-bromobutane molecule? How is infrared spectrum of 2-bromobutane used to identify 2-bromobutane?

Links associated with 2-bromobutane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The mass spectrum of 2-bromobutane (sec-butyl bromide)

The H-1 NMR spectrum of 2-bromobutane (sec-butyl bromide)

The C-13 NMR spectrum of 2-bromobutane (sec-butyl bromide)

Infrared spectroscopy index

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Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university advanced level chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These organic chemistry revision notes on spectroscopy (on the infrared spectrum of 2-bromobutane) are suitable for use of pre-university students studying AQA advanced level chemistry, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE advanced level chemistry, CCEA advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry.

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