Advanced Organic Chemistry: Infrared spectrum (1-methylethyl)benzene (cumene)

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Interpreting and explaining  the infrared spectrum of (1-methylethyl)benzene, 2-phenylpropane (cumene, isopropylbenzene)

[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 cumene [spectra page updated Mar 22nd 2026 *]

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 Infrared spectroscopy - spectra index

Introductory note on the infrared spectrum of cumene

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

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

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

C9H12 infrared spectrum of (1-methylethyl)benzene (cumene) wavenumbers cm-1 functional group detection fingerprint pattern identification of cumene 2-phenylpropane doc brown's advanced organic chemistry revision notes 

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

 (1-methylethyl)benzene, C6H5CH(CH3)2 , 2-phenylpropane, C9H12, , (cumene)

The molecular structure and naming of aromatic compounds

Interpretation of the infrared spectrum of (1-methylethyl)benzene (cumene)

The most prominent infrared absorption lines of (1-methylethyl)benzene (cumene)

Strong absorption at wavenumbers 3080 to 3030 cm-1 due to =C-H aryl C-H stretching vibrations.

Strong C-H stretching vibrations from the alkyl groups (CH2 and CH3) at wavenumbers 2975 to 2845 cm-1.

benzene aromatic ring stretching vibrations with peaks close to wavenumbers 1600 and 1500 cm-1.

Technically, the carbon-carbon bond order is 1.5 (shown as ) and not 2.0 for a full C=C bond.

C-H side-chain alkyl vibrations are observed at wavenumbers 1470 to 1370 cm-1.

Other aryl C-H vibration absorptions occur at wavenumbers 770 to 690 cm-1 characteristic of a monosubstituted benzene ring.

The absence of other specific functional group bands will show that particular functional group is absent from the (1-methylethyl)benzene (cumene) molecular structure.

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Links associated with (1-methylethyl)benzene (cumene)

The mass spectrum of (1-methylethyl)benzene or  2-phenylpropane (Cumene)

The H-1 NMR spectrum of (1-methylethyl)benzene  or  2-phenylpropane (Cumene)

The C-13 NMR spectrum of (1-methylethyl)benzene  or  2-phenylpropane (Cumene)

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