Identifying Compounds and Functional Groups by Infrared Spectroscopy

Identifying Compounds and Functional Groups by Infrared Spectroscopy


To prepare both solid and liquid IR samples in order to obtain and interpret the IR spectra of the organic compounds and identify the functional groups of each compound.


An IR spectrum is a plot that demonstrates the relationship between the percent of IR radiation transmittance versus the frequency of the radiation. This plot is produced using the principles of IR spectroscopy by which chemists often utilize to identify and analyze various compound and their functional groups. These functional groups give rise to specific absorption bands, or peaks, which are identifiable and persistent regardless of the structure of the said compound. Thus, chemists can also use IR spectroscopy to identify unknown compounds by observing reference compounds. In IR spectroscopy, vibration mechanics is an extremely important concept.  In a molecule each atom vibrates around its equilibrium position and there are two types of vibrations that produce that most important peaks for identifying organic compounds – stretching and bending. Bending vibrations require less energy than the stretching vibrations therefore the bending motions absorb at a lower frequency and have smaller wavenumbers.

Like it was mentioned earlier, the IR spectra can be used to identify the various functional groups of a compound. The major functional groups that can be identified by IR spectroscopy are OH, NH, CH (sp, sp2, sp3), CC, CN triple bond, carbonyl, and CO single bond. The typical frequency ranges for OH groups are 3590 – 3650 cm-1 or they could be broad from 2500 – 3000 cm-1. For NH the frequency ranges from 3300 – 3500 cm-1. For CH groups that are sp hybridized the range is 3300 cm-1, for sp2 the range is 3000 – 3100 cm-1, and for sp3 the range is 2800 – 3000 cm-1. For CC double bond groups, the range 1620 – 1680 cm-1 and for triple bonds it is 2100 – 2260 cm-1. The CN triple bond group has a range of 2220 – 2260 cm-1 and the carbonyl group has a range of 1630 – 1780 cm-1. The CO single bond group has a range of 1000 – 1300 cm-1.

In this experiment particularly, the type of spectrophotometer used was a Fourier-transform infrared spectroscopy (FTIR). This technique is used to obtain and detect solid, liquid or gas emission. It collects high-spectral-resolution data over a wide range.

Table of Physical Properties and Hazards:


Physical Properties




Molar mass: 136.15 g/mol

Melting point: 0°C

Boiling point: 248°C

Physical state: Liquid

p-anisaldehyde is an irritant. Prevent eye, skin, and clothing contact. Avoid inhaling fumes and ingesting the compound. It should be used in a fume hood.

Benzoic acid


Molar mass: 122.12 g/mol

Melting point: 122°C

Boiling point: 249.2°C

Physical state: Liquid

Benzoic acid causes irritation. Prevent eye, skin, and clothing contact. Avoid inhaling and ingesting this compound.

Potassium bromide (KBr)

Molar mass: 119.002 g/mol

Melting point: 730°C

Boiling point: 1435°C

Physical state: Solid

Potassium bromide causes irritation. Prevent eye, skin, and clothing contact. Avoid inhaling and ingesting this compound.


In order to produce an IR spectrum, sample must be prepared. In this experiment, both a solid and a liquid IR sample was prepared. To prepare the liquid IR sample, KBr plates were used. These plates were important because the compound, KBr, is not detected by the IR spectra and would therefore not influence the final reading. One drop of p-anisaldehyde was placed in the middle of a clean KBr plate. A second KBr plate was then used to spread the compound around. It was placed on top of the first and pressed gently down on the drop to ensure that air bubbles were not present. The salt plates were then placed into a holder which was then covered and placed gently into the IR spectrophotometer. To prepare the IR spectrophotometer, the background was collected and finally the IR spectrum reading was obtained.

To compose the solid sample, the plates were prepared from scratch. This was done by combining benzoic acid and KBr into a grinded, pasted mix. This mixture was set aside for moment, while the bolt system was assembled by screwing two large bolts together. The mixture was then transferred into the open region of the screw and bolt system. Another screw was used to press down into the paste and pressure was applied to prepare the paste into an evenly transparent sample. This sample was then placed into the IR spectrophotometer and the IR spectrum was acquired.           


Liquid Sample

Wave numbers (cm-1)

Functional Group

2937.1, 2966.7

CH, sp3

2739.6, 2840.4

CHO, aldehyde




C=C, aromatic

Solid Sample

Wave numbers (cm-1)

Functional Group

2561.56 – 3071.88



C=C, aromatic



Discussion and Conclusion:

In the final analysis the IR spectra obtained helped us to determine the various functional groups for both p-anisaldehyde and benzoic acid. Both compounds were recognizable based upon their functional groups. For example, the carboxylic acid demonstrated that this IR spectrum belonged to the solid sample or the benzoic acid compound, while the presence of aldehyde p-anisaldehyde demonstrated functional groups of aldehydes and plenty of C-H bonds. Thus, it is evident that this method can be used to identify unknown compounds by using what you know about the compound’s structure and reference IR’s. As mentioned earlier, the different methods of sample preparation were used in this lab. An advantage of using the liquid method is that it is short and requires less time and energy. The disadvantages of using a solid sample for an IR spectrum is that it is more tedious to prepare. Determining the amount of KBr and benzoic acid was an approximation as the spatula used to obtain these compounds was not a reliable measuring tool.


National Center for Biotechnology Information. PubChem Database. Potassium bromide,

CID=253877,  (accessed on Mar. 31, 2019)

National Center for Biotechnology Information. PubChem Database. Benzoic acid, CID=243,  (accessed on Mar. 31, 2019)

National Center for Biotechnology Information. PubChem Database. 4-Methoxybenzaldehyde,

CID=31244,  (accessed on Mar. 31, 2019)

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