Advanced Organic pre-university Chemistry: The infrared spectrum of urea (NH2)2C=O

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Interpreting and explaining the infrared spectrum of urea (plus brief comment on mass & NMR spectra)

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses, IB chemistry & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy - analysing the infrared spectra of urea [spectra page updated April 4th 2026 *]

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 Infrared spectroscopy - spectra index

Introductory note on the infrared spectrum of urea

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

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

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

infrared spectrum of urea CH4N2O CO(NH2)2 O=C(NH2)2 wavenumbers cm-1 functional group detection NH2 C=O fingerprint pattern identification of urea doc brown's advanced organic chemistry revision notes  

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

Urea, CH4N2O, CO(NH2)2, O=C(NH2)2

Interpretation of the infrared spectrum of urea

The most prominent infrared absorption lines of urea (wavenumber bands in cm-1)

Twin peaks of a broad band from N-H stretching vibrations at 3200 to 3600 cm-1.

At ~1700 cm-1 is the stretching vibration absorption band for the C=O carbonyl group.

At ~1600-1650 cm-1 is a double peak absorption band for other N-H vibrations.

Both the peak absorptions at ~1450 and 1150 cm-1 maybe both due to C-N vibrations?

The absence of other specific functional group bands will show that a particular functional group is absent from the urea molecular structure.

 

Note on the NMR spectra for urea, molecular structure  O=C(NH2)2

Urea is a symmetrical molecule.

You would expect one 1H NMR chemical shift because all 4 protons are equivalent to each other (~5.7 ppm).

Since the molecule only has one carbon atom, you would only expect to see one 13C NMR chemical shift in the spectrum (~160 ppm).

 

Note on the mass spectrum of urea, CH4N2O, CO(NH2)2, O=C(NH2)2

The base peak ion is m/z 17 and the molecular ion M m/z 60.

From NIST spectra data, the most abundant ions are

m/z value 16 17 43 (M-NH3) ? 44 (M-NH2) ? 60 (M)
ion identity [NH2]+ [NH3]+ [CHNO]+ ? [CH2NO]+ ? [CH4N2O]+

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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 (mass spectrum, 1H NMR spectrum, 13C NMR spectrum and infrared spectra of urea) 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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