Interpreting and explaining the infrared spectrum of 1-bromobutane

[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 1-bromobutane (IR spectra) [spectra page updated Mar 30th 2026 *]

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

and the Isomers of molecular formula C₄H₉X  (where X = F, Cl, Br or I)

Introductory note on the infrared spectrum of 1-bromobutane

Students and teachers please note my explanation of the infrared spectrum of 1-bromobutane is designed for advanced, but pre-university, chemistry courses.

Based in the infrared spectrum diagram for 1-bromobutane, only the most prominent peaks for particular bond vibrations are discussed, particularly if 1-bromobutane has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of 1-bromobutane is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing 1-bromobutane or following its change of concentration in a reaction.

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

Interpretation of the infrared spectrum of 1-bromobutane

The most prominent infrared absorption lines of 1-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 840 to 790 cm^-1.

These absorption bands are typical of most alkyl structures you see in molecules like 1-bromobutane.

Characteristic groups of C-Br stretching vibration absorptions at wavenumbers ~550 to 750 cm^-1 that you expect in 1-bromobutane.

The absence of other specific functional group bands will show that a particular functional group is absent from the 1-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.

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 1-bromobutane) 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.