Functional groups: S3.2.9 Infrared spectra IB DP Chemistry Study Notes - New Syllabus 2025
Functional groups: Classification of organic compounds – IB DP Chemistry- Study Notes
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Structure 3.2.9 – Infrared (IR) Spectra
Structure 3.2.9 – Infrared (IR) Spectra
Infrared (IR) spectroscopy is a technique used to identify the types of chemical bonds present in a molecule based on their vibrational transitions. Each type of covalent bond absorbs infrared radiation at characteristic wavenumbers (measured in cm–1), producing an IR spectrum. This allows us to deduce certain structural features of a molecule.
The absorbed frequencies are recorded as peaks in an IR spectrum, measured in wavenumbers (cm–1). These peaks are characteristic of particular types of bonds, and can therefore be used to identify functional groups and bond types in a molecule.
Key Concepts
- IR radiation causes bonds to vibrate — either stretching or bending.
- Different bonds absorb IR radiation at different characteristic wavenumbers depending on bond type and strength.
- The functional group region (1500–4000 cm–1) is used to identify the presence of particular bond types.
- The fingerprint region (below 1500 cm–1) is unique for every molecule but is not used for general bond identification.
Interpreting the Functional Group Region of an IR Spectrum
What is the Functional Group Region?
- The functional group region of an IR spectrum lies between 1500 and 4000 cm–1.
- It contains absorption peaks that correspond to specific functional groups and bond types. This region is most useful for identifying the types of bonds present in a molecule.
- Different bonds absorb infrared radiation at characteristic frequencies, and their absorption patterns can be matched to known reference values to determine the presence of functional groups such as alcohols, carboxylic acids, alkanes, alkenes, alkynes, amines, and more.
How to Interpret It
- Locate strong and distinctive peaks in the 1500–4000 cm–1 range.
- Compare the peak wavenumbers to known bond ranges (see table below).
- Look at the shape and intensity to distinguish overlapping peaks.
Table of Characteristic Absorption Frequencies
| Bond Type | Wavenumber Range (cm−1) | Appearance / Notes |
|---|---|---|
| O–H (alcohol, free) | 3600–3650 | Strong, sharp |
| O–H (hydrogen bonded) | 3200–3550 | Strong, broad |
| N–H (primary or secondary amine/amide) | 3300–3500 | Medium, may have 1 or 2 peaks |
| C–H (alkane) | 2850–2960 | Sharp, medium |
| C–H (alkene/aromatic) | 3000–3100 | Sharp |
| C–H (alkyne) | 3300 | Strong, sharp |
| C≡C (alkyne) | 2100–2260 | Weak |
| C≡N (nitrile) | 2210–2260 | Medium to strong |
| C=C (alkene) | 1620–1680 | Medium |
| C=O (carbonyl groups) | 1670–1750 | Strong, sharp |
| C–O (ether, ester, alcohol) | 1000–1300 | Strong, sharp (just below 1500) |
Example
IR Spectrum shows peaks at 1715 cm–1, 2950 cm–1, and 3300 cm–1. Identify the functional groups.
▶️ Answer/Explanation
- 1715 cm–1: Indicates the presence of a carbonyl group (C=O).
- 2950 cm–1: Suggests C–H stretching from alkanes.
- 3300 cm–1: May be due to an N–H bond (amine) or ≡C–H bond (terminal alkyne).
Conclusion: The compound likely contains a carbonyl group, alkane chains, and possibly an amine or a terminal alkyne group.
IR Radiation and Greenhouse Gases
Certain gases in the atmosphere—known as greenhouse gases—can absorb infrared (IR) radiation. These gases include:
- Carbon dioxide (CO2)
- Water vapour (H2O)
- Methane (CH4)
- Nitrous oxide (N2O)
- Ozone (O3)
These molecules can absorb IR radiation because they undergo changes in their vibrational energy states when exposed to radiation in the infrared region (typically 600–1500 cm–1).
Why Some Gases Absorb IR
- Molecules absorb IR if the vibration causes a change in the dipole moment.
- Symmetrical diatomic molecules (like O2 or N2) do not absorb IR because they do not have a dipole moment.
Impact on the Atmosphere
When greenhouse gases absorb IR radiation from the Earth’s surface, they re-emit it in all directions, including back toward Earth. This leads to the greenhouse effect, which contributes to global warming and climate change.
Example
Explain why methane (CH4) and water vapor (H2O) are considered greenhouse gases, but nitrogen (N2) and oxygen (O2) are not.
▶️Answer/Explanation
- Methane (CH4) and water vapor (H2O) can absorb infrared radiation due to their polar bonds and asymmetric vibrational modes that change the dipole moment of the molecule.
- This allows them to trap heat by absorbing and re-emitting IR radiation, contributing to the greenhouse effect.
- Nitrogen (N2) and oxygen (O2) are symmetrical, nonpolar molecules. Their vibrations do not change the dipole moment, so they do not absorb IR radiation and are not greenhouse gases.