Americas
Europe
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?
Explore our app and discover over 50 million learning materials for free.
Lerne mit deinen Freunden und bleibe auf dem richtigen Kurs mit deinen persönlichen Lernstatistiken
Jetzt kostenlos anmeldenThere 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.
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:
An absorption spectrum is a graph that shows a chemical absorbing radiation over a range of frequencies.
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
All organic compounds absorb infrared radiation. This infrared radiation is absorbed by bonds between the molecules at different wavelengths.
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:
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.
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
Like all techniques, infrared spectroscopy can come in very handy, but it also has its pitfalls. Let's see what they are.
Infrared spectroscopy is an analytical technique used to identify the functional groups within organic molecules.
Infrared spectroscopy can be used to identify which functional groups are present in different organic molecules.
All molecules vibrate at a specific frequency, so they absorb frequencies according to their own unique characteristics.
Yes, it can.
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.
Flashcards in Infrared Spectroscopy15
Start learningWhat is infrared spectroscopy?
An analytical technique used to identify the functional groups within organic molecules.
What are two types of spectrometers used in infrared spectroscopy?
- A dispersive infrared radiation spectrometer
- A Fourier transform infrared radiation spectrometer.
What is an absorption spectrum?
A plot that shows chemicals absorbing radiation over a range of frequencies.
What steps take place in the process of infrared spectroscopy?
- 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.
Which two columns do an infrared spectroscopy table have?
- Bond
- Wavenumber
Which three factors affect how much vibration is caused between the bonds?
- Bond strength
- Mass of the atom
- Bond length
Already have an account? Log in
How would you like to learn this content?
How would you like to learn this content?
Free chemistry cheat sheet!
Everything you need to know on . A perfect summary so you can easily remember everything.
The first learning app that truly has everything you need to ace your exams in one place
Sign up to highlight and take notes. It’s 100% free.
Save explanations to your personalised space and access them anytime, anywhere!
Sign up with Email Sign up with AppleBy signing up, you agree to the Terms and Conditions and the Privacy Policy of StudySmarter.
Already have an account? Log in
Explanations 
Exams 
Magazine 
