Advanced Organic Chemistry: Infrared spectrum of 2-iodobutane CH3CHICH2CH3

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Interpreting and explaining the infrared spectrum of 2-iodobutane

[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-iodobutane (IR spectra) [spectra page updated April 1st 2026 *]

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 Infrared spectroscopy - spectra index

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 2-iodobutane

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

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

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

infrared spectrum of 2-iodobutane C4H9I CH3CHICH2CH3 wavenumbers cm-1 functional group detection fingerprint pattern identification of sec-butyl iodide doc brown's advanced organic chemistry revision notes 

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

2-iodobutane (sec-butyl iodide), C4H9I, CH3-CHI-CH2-CH3

The molecular structure and naming of haloalkanes

Interpretation of the infrared spectrum of 2-iodobutane

The most prominent infrared absorption lines of 2-iodobutane

For 2-iodobutane, 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 that you see in compounds like 2-iodobutane.

C-C-C skeletal vibrations from the CH3-CH-CH2-CH2 grouping occur at wavenumbers ~1140 to 1255 cm-1 and 750 to 720 cm-1.

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

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

Key words & phrases: isomer of molecular formula C4H9I CH3CHICH2CH3 CH3CH2CHICH3 image and diagram explaining the infrared spectrum of 2-iodobutane, complete infrared absorption spectrum of 2-iodobutane, comparative spectra of 2-iodobutane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of 2-iodobutane, important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum of 2-iodobutane, revision of infrared spectroscopy of 2-iodobutane, fingerprint region analysis of 2-iodobutane, how to identify 2-iodobutane from its infrared spectrum, identifying organic compounds like 2-iodobutane from their infrared spectrum, how to analyse the absorption bands in the infrared spectrum of 2-iodobutane detection of functional groups in the 2-iodobutane molecule example of the infrared spectrum of a molecule like 2-iodobutane with a functional group  interpreting interpretation of the infrared spectrum of 2-iodobutane shows presence of functional group haloalkane halogenoalkane alkyl iodide sec-butyl iodide Diagram of absorption of wavenumber peaks in the infrared spectrum of 2-iodobutane. Characteristic peak wavenumbers in the infrared spectrum of 2-iodobutane. Finger print identification pattern using the infrared spectrum of 2-iodobutane. Revision notes on the infrared spectrum of 2-iodobutane. Matching and deducing the structure of the 2-iodobutane molecule from  its infrared spectrum. Infrared spectroscopy of aliphatic halogenoalkanes iodoalkanes, infrared spectra of 2-iodobutane, an isomer of molecular formula C4H9I How do you interpret the infrared absorption spectrum of 2-iodobutane How to interpret the infrared spectrum of 2-iodobutane Explanatory diagram of the infrared spectrum of the 2-iodobutane molecule in terms of its molecular structure. Listing data of the prominent main wavenumber peaks troughs in the infrared spectrum of 2-iodobutane. How to explain the infrared spectrum of 2-iodobutane. Use of the infrared spectrum of 2-iodobutane, identification of 2-iodobutane from its infrared spectrum - fingerprint wavenumber pattern to identify the 2-iodobutane molecule. The uses of the infrared spectrum of the 2-iodobutane molecule. The distinctive features of the infrared spectrum of the 2-iodobutane molecule explained interpretation diagram explaining the peaks-trough of the transmittance of the infrared spectrum of 2-iodobutane what does the infrared spectrum tell you about the structure and properties of the 2-iodobutane molecule? How is infrared spectrum of 2-iodobutane used to identify 2-iodobutane?

Links associated with 2-iodobutane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The mass spectrum of 2-iodobutane (sec-butyl iodide)

The H-1 NMR spectrum of 2-iodobutane (sec-butyl iodide)

The C-13 NMR spectrum of 2-iodobutane (sec-butyl iodide)

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-iodobutane) 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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