Organic Analysis - Infrared (IR) Spectroscopy (A-Level Chemistry)
Infrared (IR) Spectroscopy
Infrared Spectroscopy
Infrared waves are part of the electromagnetic spectrum.
When infrared radiation is passed through a compound, some of the radiation is absorbed by the bonds within the molecule, and some is transmitted.
When chemical bonds absorb infrared radiation their kinetic energy increases and they vibrate faster.
Different chemical bonds absorb different frequencies of infrared radiation. These frequencies are measured in wavenumbers, where:
Wavelength = 1 / wavenumber (cm-1)
The table shows the infrared wavenumbers for different bond types: This table will be given to you on your exam data sheet.
Interpreting an IR Spectrum
The IR spectrum is a measure of the % transmittance of IR against the wavenumber.
It shows you what radiation frequencies the organic molecule under investigation absorbs and hence the type of bonds present. This information can be used to identify the functional groups present in the molecule.
Infrared spectroscopy can also be used to identify impurities in a sample.
To interpret the IR spectrum of a given molecule, you need to look carefully at the wavenumber of the troughs and their shapes. For example:
- O-H in alcohols – produce a strong, broad trough at a wavenumber 3230-3550 cm-1
- O-H in acids – produce a strong, broad trough as a wavenumber 2500-3000 cm-1
- C=O bond – this is a strong, narrow trough at 1680-1750 cm-1 in aldehydes, ketones, carboxylic acids and esters.
Worked example: Which bond does this trough represent?
Answer: This transmittance is produced by an O-H bond in a carboxylic acid. The trough is between 2500 and 3000 cm-1
Worked example: Identify the functional groups in the molecule C6H12O2, using the infrared spectrum given below. (2 marks)
Answer: Signal at 3230-3350 shows an O-H (alcohol) (1)
and C=O at 1680-1750 (1)
There are separate -OH and C=O bonds in the molecule, so it is not a carboxylic acid.
Worked example: Identify whether this is the spectrum of a primary amine, tertiary amine or a nitrile. Explain your choice.
Answer: There is no trough at 2220-2260 cm-1, so it has no C≡N bond, so it is not a nitrile.
There is no clear trough at 3300 to 3500 cm-1 so there is no N-H bond.
It must be a tertiary amine.
Fingerprint Region
The “fingerprint region” is unique for all molecules.
It is in the region of 500 and 1500 cm-1. It represents the wavenumbers absorbed by the C-C and C-O bonds as well as the vibrations of the whole molecule.
The fingerprint region is unique to each molecule, and can be used to identify a molecule by comparing it with known samples in databases.
Infrared Radiation and Global warming
Greenhouse Effect
The Sun emits different forms of radiation. Out of this radiation, short-wavelength ultraviolet, visible, and the shorter wavelengths of infrared radiation get transmitted to the Earth.
The Earth absorbs some of the energy from these waves warming it up.
The Earth then emits them back to the atmosphere as longer wavelength infrared waves.
This radiation gets absorbed and reflected back to earth by atmospheric green-house gases such as carbon dioxide, water vapor and methane, keeping the Earth warm.
This is called the the green-house effect.
Global Warming
Greenhouse gases include carbon dioxide, water vapour, methane, and all other atmospheric gases containing atmospheric gases containing C=O, O–H and C–H bonds.
The bonds within molecules of these greenhouse gases absorb infrared radiation. This increases their kinetic energy causing the gases to heat up. As a result the temperature of the atmosphere increases, which in turn warms the earth.
The temperature of the earth is maintained in this way so it is high enough to support life.
Without the greenhouse gases in the atmosphere, the Earth would be too cold to maintain life.
However, the rapid increase in carbon dioxide levels over the last hundred or so years, due to increased combustion of fossil fuels, has led to the largest increase in temperature the planet has known in the shortest time.
This is referred to as global warming.
FAQs
Infrared (IR) spectroscopy is a technique used in Organic Chemistry to identify the functional groups in a molecule. IR spectroscopy works by measuring the absorption of infrared radiation by the molecule and using this information to determine the functional groups present in the molecule.
IR spectroscopy works by shining infrared radiation at a sample of the molecule and measuring the amount of radiation absorbed by the molecule. Different functional groups in a molecule will absorb different wavelengths of infrared radiation, and the resulting IR spectrum can be used to identify the functional groups present in the molecule.
IR spectroscopy is a quick and reliable way to identify the functional groups in a molecule, and it is widely used in the field of Organic Chemistry. The technique can be used to identify the functional groups in a pure substance, as well as in mixtures of substances. IR spectroscopy is also non-destructive, so the sample can be used for other analysis after the IR analysis has been performed.
IR spectroscopy is used in Organic Chemistry to identify the functional groups in a molecule and to confirm the structure of the molecule. The IR spectrum of the molecule can be compared to reference spectra to determine the functional groups present, and this information can be used to confirm the structure of the molecule. IR spectroscopy is also used to determine the purity of a substance and to study the interactions between molecules.
The limitations of IR spectroscopy in Organic Chemistry include its sensitivity to impurities in the sample and its inability to provide structural information about the molecule. IR spectroscopy can also be affected by the presence of other molecules in the sample, and it may not be possible to obtain a clear IR spectrum for highly complex molecules.
IR spectroscopy is often used in conjunction with other analytical techniques in Organic Chemistry, such as nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). These techniques can be used together to provide a more complete picture of the structure of a molecule, and IR spectroscopy is often used to confirm the results obtained from other techniques.
IR spectroscopy is widely used in industry and research to identify the functional groups in a molecule and to confirm the structure of the molecule. The technique is used in the pharmaceutical industry to identify active ingredients in drugs and to study the interactions between drugs and other molecules. IR spectroscopy is also used in the food and beverage industry to identify contaminants and to study the composition of food and drink products.
The study of IR spectroscopy is important for A-Level Chemistry students as it provides a fundamental understanding of the technique and its applications in Organic Chemistry. Understanding IR spectroscopy will help students to identify the functional groups in a molecule and to confirm the structure of the molecule, which is essential knowledge for many areas of chemistry and related fields. The study of IR spectroscopy will also provide students with a valuable tool for solving problems in Organic Chemistry and for conducting further research in the field.
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