Interpreting and explaining the infrared spectrum of 1-iodo-2-methylpropane (isobutyl iodide)

[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-iodo-2-methylpropane (IR spectra) [updated Nov 10th 2025]

See also comparison of the infrared, mass, 1H NMR and 13C NMR spectra of the four isomers of C₄H₉I

Introductory note on the infrared spectrum of 1-iodo-2-methylpropane

Students and teachers please note my explanation of the infrared spectrum of 1-iodo-2-methylpropane is designed for advanced, but pre-university, chemistry courses.

Based in the infrared spectrum diagram for 1-iodo-2-methylpropane, only the most prominent peaks for particular bond vibrations are discussed, particularly if 1-iodo-2-methylpropane has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of 1-iodo-2-methylpropane is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing 1-iodo-2-methylpropane or following its change of concentration in a reaction.

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

Interpretation of the infrared spectrum of 1-iodo-2-methylpropane

The most prominent infrared absorption lines of 1-iodo-2-methylpropane

For 1-iodo-2-methylpropane, the C-H stretching vibration absorptions peak at wavenumbers ~2845 to 2975 cm^-1.

C-H bending/deformation vibrations absorption bands peaking at wavenumbers ~1365 to 1470 cm^-1.

These two sets of absorption bands are typical of most alkyl structures you find in aliphatic compounds like halogenoalkanes.

C-C-C skeletal vibrations from the CH₃-C-CH₃ grouping occur at wavenumbers ~1170 to 1140 cm^-1 and 840 to 790 cm^-1.

Characteristic groups of C-I stretching vibration absorptions occur at wavenumbers ~600 to 500 cm^-1 that you expect in 1-iodo-2-methylpropane.

The absence of other specific functional group bands will show that a particular functional group is absent from the 1-iodo-2-methylpropane molecular structure.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C4H9I 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-iodobutane, 2-iodobutane, 1-iodo-2-methylpropane and 2-iodo-2-methylpropane image sizes.  These four molecules are structural isomers of molecular formula C4H9I and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, iodoalkanes, alkyl iodides).
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 ion of m/z 184, but it is only a relatively tiny peak for 2-iodo-2-methylpropane. All four give the base ion peak of m/z 57. All four give prominent peaks for m/z ions 29 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.
1H NMR SPECTRA (above): The 1H NMR spectra of all three molecules give different proton ratios i.e.1-iodobutane four peaks 3:2:2:2, 2-iodobutane four peaks 3:3:2:1, 1-iodo-2-methylpropane three peaks 6:2:1 and 2-iodo-2-methylpropane 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-iodobutane and 2-iodobutane show four 13C NMR resonances, 1-iodo-2-methylpropane three 13C NMR resonances and 2-iodo-2-methylpropane only two 13C resonances. Therefore 1-iodo-2-methylpropane and 2-iodo-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-iodobutane and 2-iodobutane 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-iodo-2-methylpropane) 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.