Back to: Organic Chemistry 200 Level
Welcome to class!
Hello superstar! Imagine trying to understand a song you’ve never heard before — you pay attention to the beats, the melody, and the lyrics to figure out what it’s about. In the same way, chemists “listen” to molecules using spectroscopy to discover their structure. Today, we will focus on two powerful tools: Infrared (IR) Spectroscopy and Ultraviolet–Visible (UV–Vis) Spectroscopy.
Spectroscopy II
Infrared (IR) Spectroscopy
Infrared spectroscopy works by shining infrared light on a molecule and measuring how it is absorbed. This causes the bonds in the molecule to vibrate — stretching or bending — in specific ways depending on the atoms involved. Each type of bond absorbs at a particular frequency, producing a unique pattern known as an IR spectrum. This spectrum is like the molecule’s fingerprint; no two compounds have exactly the same one.
The IR spectrum has two main parts. The functional group region (around 4000–1500 cm⁻¹) helps identify the types of bonds and functional groups present. For example, an O–H stretch appears between 3200–3600 cm⁻¹, while a C=O stretch is usually found near 1700 cm⁻¹. The fingerprint region (1500–400 cm⁻¹) is more complex but very useful for confirming the identity of a molecule.
In real life, IR spectroscopy can be used to check if a product has been altered — for instance, to detect if pure honey has been mixed with sugar syrup.
Ultraviolet–Visible (UV–Vis) Spectroscopy
UV–Vis spectroscopy looks at how molecules absorb light in the ultraviolet and visible parts of the spectrum. When this light is absorbed, electrons in the molecule are excited from a lower energy level to a higher one.
Molecules with conjugated systems — alternating single and double bonds — often absorb in this range. The more conjugated a molecule is, the longer the wavelength of light it will absorb. A practical example is beta-carotene, found in carrots, which absorbs light in the visible range and gives carrots their bright orange colour.
In everyday applications, UV–Vis spectroscopy is used to test the presence of compounds in solutions and to measure the concentration of coloured solutions. For instance, the deep red of palm oil can be linked to its carotene content using this method.
Summary
- IR spectroscopy detects bond vibrations and identifies functional groups in molecules.
- UV–Vis spectroscopy examines electronic transitions, especially in conjugated systems.
- Both methods help in identifying, studying, and measuring substances in the laboratory and industry.
Evaluation
- What does the functional group region in an IR spectrum show?
- Why do conjugated systems absorb light in the visible range?
- Give one real-life use of UV–Vis spectroscopy.
- State one difference between IR spectroscopy and UV–Vis spectroscopy.
Well done! You’re learning to understand the unique “signals” molecules send out. With Afrilearn, every lesson strengthens your ability to see and interpret the invisible details of the chemical world.