Lab 8 Identifying an Unknown Compound by Infrared Spectroscopy

Lab 8 – Identifying an Unknown Compound by Infrared Spectroscopy


The aim was to prepare IR samples. Identify and analyze functional groups of liquid and solid organic compounds using IR spectroscopy.


Spectroscopy is an analytical technique that helps determine structure by destroying little to no sample. One common type of spectroscopy is known as infrared (IR) spectroscopy which measures the bond vibration frequencies in a molecule and is used to determine certain functional groups. The IR spectrum plot shows the percentage of IR radiation that passes through a sample (measured as percent transmission) on the y-axis and the frequency of the radiation (measured in wavenumbers) on the x-axis. The functional groups can be determined from the spectrum because they give rise to particular absorption bands, or peaks that occur at or close to the same frequency, despite the structure of the rest of the molecule. When working with an unknown molecule, the IR spectra indicates what type of vibrational modes the molecule responds with after absorbing light, and when you figure out the peaks that correspond to each motion, you can essentially figure out the functional groups present in the molecule as well as the molecule you might be dealing with. Generally, it is helpful to divide the IR spectrum into two parts; 4000 – 1500 cm-1 portion (helpful for identifying functional groups) and the 1500 – 600 cm-1 portion (useful for comparing the spectrum of unknown compound with the spectra of known compounds for identification purposes). Some factors that can affect IR spectra include conjugation and resonance. Some functional groups may be shifted because they can experience single or double bond character from resonance forms in conjugated systems. For instance, conjugation of a carbonyl group would shift the IR peak to lower wavenumber because the CO bond is weakened when conjugating the carbonyl, leading to the IR peak shifting down. If there is little to no conjugation, the electrons are firmly based in the CO double bond, but if resonance structures or conjugation are introduced then those electrons become delocalized, weakening the bond and lowering the wavenumbers. In this experiment, we prepare IR samples to identify and analyze functional groups of p-anisaldehyde (liquid) and benzoic acid (solid) using IR spectroscopy.


Table 1 Table of physical properties of compounds that were used in this experiment



Benzoic Acid




Melting Point (ºC)



Boiling Point (ºC)



Physical Properties

Colorless to pale yellow liquid

Benzoic acid appears as a white crystalline solid. It is slightly soluble in water.

Known Hazards

Irritant to the skin and harmful if swallowed

Causes skin irritation and serious eye damage. May cause damage to organs through prolonged or repeated exposure.


Liquid Sample

The liquid IR sample of p-anisaldehype was prepared by placing one drop of the sample between two salt plates, which were wiped using acetone and kimwipes to clean any residue off the plates. Salt plates made out of KBr or NaCl are used instead of glass because they are transparent to the IR radiation and they aren’t aromatic. Once a drop was placed on the center of the plates, they were squished together to form a neat sample, which means it’s in a pure, liquid form. The liquid sample was then carefully placed on a holder and the holder was placed on the FT-IR machine to generate the spectrum. The IR spectrum data was printed out and used to identify specific functional groups.

Solid Sample   

The solid sample of benzoic acid was prepared by placing a 2:1 ratio of benzoic acid to potassium bromide and grinding them together using a mortar and pestle to create a fine film of crystals. After this, transfer the mixture into a bolt with two screws to make a transparent film. The solid samples take a lot of time to prepare. The crystals must be grinded if they are not in fine crystal form and the fine crystals must be compressed to form very thin film, which requires a lot of skill because you can easily mess up. The same process used in the liquid sample to obtain the IR spectrum was conducted.


The functional groups and wavenumber values for the different absorptions in the IR spectra of p-anisaldehyde and benzoic acid are attached.


“Benzoic Acid.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine,

“4-Methoxybenzaldehyde.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine,

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