Question

Which one of the following five compounds produced the IR spectrum shown below?

Short answer

The compound is:

Benzyl alcohol

Step-by-step solution

IR spectroscopy

Covalent bonds can be compared to strings and vibrate along their bond axis. These vibrations can be stretching or bending. The frequencies of vibrations of an organic molecule are quantized, and therefore, a molecule will have vibrational energy levels.

Some of the bonds in a molecule can absorb infrared radiation and undergo a vibrational excitation. This means that the molecule can move to a higher vibrational energy level and vibrate with higher amplitude without change in frequency. The frequency of radiation absorbed will equal the natural vibrational frequency of the molecule.

IR spectrum

Different functional groups of a molecule will have different frequencies of vibrations. The frequency of vibration of a particular functional group is always a constant. Therefore, the IR spectrum can be used to identify the functional groups.

A molecule may contain more than one functional group. The functional groups in a molecule can be identified from the peaks in the IR spectrum using the standard value for the wavenumber of the absorption of each functional group.

Identifying the compound from the given spectrum

The IR spectrum of the given compound contains a peak at approximately 1500-1430 $c{m}^{-1}$ and weak peaks between 1600 and 2000 . This indicates the presence of a benzene ring in the compound.

The strong peak at 3300 indicates the presence of an –OH group in the compound.

The absence of a strong peak at 1600 shows the absence of an amine group in the compound. Also, the compound is not a carboxylic acid because the absorption peak for the –C=O bond at 1700 is not present in the spectrum.

Therefore, the compound can be either benzyl alcohol or phenol. The peaks near 3000 are used to determine the hydrogen atoms in the molecule. There are peaks above 3000 and below 3000 . Therefore, the compound contains both and hybridized carbon-hydrogen bonds.

Phenol does not have any hydrogen attached to an $s{p}^3$ carbon. Therefore, the given compound is benzyl alcohol.

Benzyl alcohol