There are many different functional groups in organic molecules, including alcohols, amines, alkanes, and alkenes, to name a few. But how do we identify which functional groups are present in different organic molecules?
For this, we use a technique called infrared spectroscopy (IR). This works because different bonds will have vibrations of different frequencies, allowing us to differentiate between them.
- We will describe infrared spectroscopy and how it works.
- We'll look at the infrared spectroscopy table.
- Then we will explore infrared spectroscopy of organic compounds.
- Finally, we will outline some of the key advantages and disadvantages of infrared spectroscopy.
Description of infrared spectroscopy
Infrared spectroscopy is an analytical technique used to identify the functional groups within organic molecules.
There are two types of spectrometers used in infrared spectroscopy, a dispersive infrared radiation spectrometer, and a Fourier transform infrared radiation spectrometer.
There are a few steps that take place in the process of infrared spectroscopy. These are as follows:
- A beam of radiation is passed through a sample.
- The sample, which is in a spectrometer, absorbs the infrared radiation.
- Once the absorptions are detected and analysed, the absorption spectrum is printed or displayed on a computer.
An absorption spectrum is a graph that shows a chemical absorbing radiation over a range of 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.
Infrared spectroscopy table
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
A pair of atoms constantly vibrates. When the organic molecules absorb infrared radiation, the bonds between the different atoms vibrate even more. Due to this, the covalent bonds in the molecule also vibrate and are forced to either stretch, bend, or twist. All the molecules vibrate at a specific frequency. Each bond within a molecule has a unique natural vibration frequency. How much vibration is caused depends on three main factors:
- Bond strength – stronger bonds vibrate at a higher frequency.
- Mass of the atom – heavier atoms vibrate at a lower frequency.
- 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.
Example of an infrared spectrum of a molecule
In the infrared spectrum, the transmittance is plotted along the y-axis, whilst the wavenumber is plotted on the x-axis. As we can see, the spectrum consists of a series of dips in transmittance at certain wavelengths which are (confusingly) called ‘peaks’. These peaks represent the vibrations caused when infrared radiation is absorbed.
Transmittance measures the percentage of radiation that passes through a sample.
The wavenumber is the number of waves at a given distance. This distance is known as 'wavelength'. Wavenumber is 1/wavelength, so they are inversely proportional. It is a measure used for frequency.
The peaks point downwards in the IR spectra.
This information can enable us to identify functional groups in the molecule. The infrared spectroscopy data table as shown above is used to match the different peaks of 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.
Spectrum highlighting the position of the fingerprint region and the functional group region
Advantages and disadvantages of infrared spectroscopy
Like all techniques, infrared spectroscopy can come in very handy, but it also has its pitfalls. Let's see what they are.
Advantages of infrared spectroscopy
- The samples being viewed don’t require any special preparation such as samples having radioactive dye put into them.
- It has a high scan speed. All the information needed for the whole frequency range can be obtained within seconds.
- The infrared spectrometer has a really high resolution.
- It has a wide range of applications in both qualitative analysis and quantitative analysis, so it can be used to analyse almost all organic compounds.
Disadvantages of infrared spectoscopy
- If a sample contains water, then infrared spectroscopy is not applicable. A solvent like water would absorb infrared radiation.
- There may be complications sometimes. We cannot fully clarify the structure of a compound based on a single infrared radiation spectrum.
- Infrared spectroscopy may be limited to certain conditions for quantitative analysis (where numbers and not only categories are required, such as 'N-H bond').
Infrared Spectroscopy - Key takeaways
- Infrared spectroscopy is an analytical technique used to identify the functional groups within organic molecules.
- Bonds in different organic compounds absorb different frequencies of infrared radiation.
- Each bond within a molecule has a unique natural vibration frequency.
- The amount of vibration caused depends on bond strength, bond length, and mass of the atom.
- Infrared spectra consist of peaks that represent the vibrations caused when infrared radiation is absorbed.
- Data from the spectra are used to identify organic molecules by matching the different peaks to the functional groups that could have caused them to occur.
- The fingerprint region is the area of the spectrum that is below 1500 cm-1.
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