Organic molecules have various functional groups such as alcohols, amines, alkanes, and alkenes. But how do we know which functional groups are in a molecule? We use infrared spectroscopy (IR) to identify them. This technique works because each bond has a unique vibration frequency that we can differentiate. In this blog, we will discuss IR and how it works, show you the IR table, explore IR of organic compounds, and highlight its pros and cons.

Description of infrared spectroscopy

Infrared spectroscopy is a tool used to identify functional groups in organic molecules. There are two types of spectrometers used: dispersive and Fourier transform. The process of IR spectroscopy involves passing a beam of radiation through a sample in a spectrometer. The sample absorbs the radiation, and the absorptions are then analyzed and printed or displayed on a computer as an absorption spectrum. This spectrum shows how much radiation is absorbed by the chemical at different frequencies.

Infrared spectroscopy table

As we can see in the image below, the table has two columns. 'Bond' represents the functional groups of different organic compounds. 'Wavenumber' represents the number of waves in a given wavelength or distance. From the table, we also know that bonds in different functional groups absorb different frequencies of infrared radiation. This is the basis to distinguish functional groups with infrared spectroscopy.

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Infrared spectroscopy of organic compounds. All organic compounds absorb infrared radiation. This infrared radiation is absorbed by bonds between the molecules at different wavelengths.

Infrared Spectroscopy: Vibration of organic compounds

Think of a pair of atoms as constantly vibrating. When organic molecules absorb infrared radiation, the bonds between the atoms vibrate even more. This causes the covalent bonds in the molecule to stretch, bend, or twist. Every molecule vibrates at a specific frequency, and each bond within the molecule has its own unique natural vibration frequency. The amount of vibration caused depends on three primary factors: bond strength (stronger bonds vibrate at a higher frequency), mass of the atom (heavier atoms vibrate at a lower frequency), and bond length.

Infrared Spectroscopy: Identifying organic molecules

An infrared spectrum of a molecule is a graph that is produced once the process of infrared spectroscopy has been performed. We can see an example below.

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In the infrared spectrum, transmittance is plotted along the y-axis, while the wavenumber is plotted on the x-axis. The spectrum consists of dips in transmittance at certain wavelengths, which are referred to as peaks. These peaks represent the vibrations caused when the molecule absorbs infrared radiation. Transmittance measures the percentage of radiation that passes through a sample, while the wavenumber is a measure of frequency that is inversely proportional to the wavelength. The peaks in the IR spectra point downwards.

By analyzing the peaks in the IR spectrum, we can identify the functional groups in the molecule. The infrared spectroscopy data table is used to match the peaks in the spectrum with the functional groups that could have caused them to occur. The functional groups of the molecule can be found in the region between 4000 cm-1 and 1500 cm-1 of the infrared spectra.

Infrared Spectroscopy: Fingerprint region

The fingerprint region is the area of the spectrum that is below 1500 cm-1. This region contains absorptions for some complicated vibrations which are usually caused by the bending or stretching of single bonds. Due to this, the pattern in this region is very complicated and is unique to the molecule. There is a database available in which the infrared spectra of known organic molecules have been recorded. Therefore, the infrared spectra produced for a complex unknown compound can be compared with the database.

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Infrared spectroscopy has several advantages such as not requiring special preparation of samples, high scan speed, and high resolution. It also has a wide range of applications in both qualitative and quantitative analysis of organic compounds. However, there are also disadvantages to this technique. Infrared spectroscopy may not be applicable to samples containing water as the solvent absorbs infrared radiation. The structure of a compound cannot be fully clarified based on a single infrared radiation spectrum. It may also be limited to certain conditions for quantitative analysis.

In summary, infrared spectroscopy is a useful analytical technique for identifying functional groups within organic molecules. Each bond within a molecule has a unique vibration frequency, and the peaks in the spectra represent the vibrations caused by the absorption of infrared radiation. While it has its limitations, the advantages of infrared spectroscopy make it a valuable tool in organic chemistry.

Infrared Spectroscopy

What is infrared spectroscopy?

Infrared spectroscopy is an analytical technique used to identify the functional groups within organic molecules.  

What is infrared spectroscopy used for?

Infrared spectroscopy can be used to identify which functional groups are present in different organic molecules. 

What is the basic principle of infrared spectroscopy?

All molecules vibrate at a specific frequency, so they absorb frequencies according to their own unique characteristics. 

Can infrared spectroscopy detect impurities?

Yes, it can.

Why is potassium bromide used in infrared spectroscopy?

Potassium bromide is used as a carrier for the sample in infrared spectroscopy, since it does not absorb any IR radiation. So, there is no intereference in absorbance.

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