Welcome to Topic 7B: Infrared Spectroscopy!
Ever wondered how chemists can tell exactly which functional groups are in a mysterious clear liquid? They don't just guess; they use Infrared (IR) Spectroscopy. Think of it like a molecule’s "vibrational fingerprint." Every bond in a molecule vibrates, and by shining infrared light through a sample, we can see which specific "notes" the molecule is playing.
In this chapter, we focus on identifying specific bonds and functional groups. Don't worry if it seems like a lot of numbers at first—your Edexcel Data Booklet will be your best friend during the exam!
1. How Does it Work? (The Simple Version)
Molecules aren't rigid sticks; the bonds between atoms act like tiny springs. These springs can stretch and bend. When we hit them with infrared radiation, the bonds absorb energy that matches their natural vibration frequency.
- Absorption: If the frequency of the IR light matches the frequency of the bond's vibration, the bond absorbs that energy.
- The Result: We see a "dip" or a "peak" on the IR spectrum graph.
Did you know? Heavier atoms and stronger bonds vibrate at different frequencies. It’s just like a guitar string: a thick string (heavy atom) vibrates more slowly than a thin one!
2. Reading the IR Spectrum
When you look at an IR spectrum, it looks like a series of upside-down mountains. Here is what you need to know about the axes:
- Vertical Axis (y-axis): This is transmittance (%). High transmittance (at the top) means light passed straight through. Low transmittance (the dips/peaks) means the light was absorbed by a bond.
- Horizontal Axis (x-axis): This is the wavenumber, measured in \(cm^{-1}\). It’s a bit like frequency. The scale usually goes from \(4000\ cm^{-1}\) on the left to about \(400\ cm^{-1}\) on the right.
Quick Tip: Edexcel exams focus on the area above \(1500\ cm^{-1}\). The area below \(1500\ cm^{-1}\) is called the fingerprint region. While unique to every molecule, it is usually too complex to "read" by eye, so we use it for direct comparison with a database.
3. Identifying the Key Functional Groups
According to the Pearson Edexcel syllabus, you need to be able to recognize and predict the following specific absorptions. You will find these values in your Data Booklet, so focus on recognizing the shape of the peaks.
A. The Hydroxyl Group (\(O-H\)) in Alcohols
In alcohols, the \(O-H\) bond creates a strong, broad, and smooth peak. It usually appears between \(3230–3550\ cm^{-1}\). It looks like a deep, rounded "U" shape.
B. The Carbonyl Group (\(C=O\)) in Aldehydes and Ketones
This is one of the easiest peaks to spot! It is strong and sharp, like a long sword pointing down. It typically appears between \(1630–1750\ cm^{-1}\). If you see a very deep, narrow spike in the middle of the spectrum, it's likely a \(C=O\) bond.
C. The Carboxylic Acid Group (\(C=O\) and broad \(O-H\))
Carboxylic acids are "double trouble." They have both a \(C=O\) peak AND an \(O-H\) peak. However, the \(O-H\) in a carboxylic acid is much broader than the one in an alcohol (typically \(2500–3300\ cm^{-1}\)). It often overlaps with the \(C-H\) peaks, making the left side of the spectrum look like a messy, "hairy" beard.
D. The Amine Group (\(N-H\))
Found in amines and amides, this absorption is usually between \(3300–3500\ cm^{-1}\). It is typically less intense and sharper than an alcohol \(O-H\) peak. Sometimes it has two small "points" if it's a primary amine (\(NH_{2}\)), looking a bit like a vampire fang!
E. The Alkene Group (\(C=C\))
This is a weak to medium peak found between \(1620–1680\ cm^{-1}\). It is often quite small and can be easy to miss, especially compared to the giant \(C=O\) peak that appears in the same general neighborhood.
F. The Alkane/General Group (\(C-H\))
Almost all organic molecules have \(C-H\) bonds. These appear as sharp spikes just below the \(3000\ cm^{-1}\) mark (usually \(2850–2950\ cm^{-1}\)). Since almost every organic compound has them, they aren't very useful for identifying a specific functional group, but they are good for orientation.
4. Summary Table for Quick Revision
Note: Always check your Data Booklet for the exact ranges during the exam!
| Bond | Functional Group | Typical Shape |
|---|---|---|
| \(O-H\) | Alcohol | Broad and smooth "U" |
| \(O-H\) | Carboxylic Acid | Very broad, "hairy" beard |
| \(C=O\) | Aldehyde / Ketone / Acid | Strong, sharp "sword" |
| \(N-H\) | Amine | Small, sharp spikes |
| \(C=C\) | Alkene | Weak/small spike |
5. Step-by-Step: Deducing a Structure
If you are given an unknown spectrum in Paper 2 or Paper 3, follow these steps:
- Check the \(1630–1750\ cm^{-1}\) region: Is there a big sharp spike? Yes? You have a carbonyl (\(C=O\)).
- Check the \(3000–3500\ cm^{-1}\) region: Is there a broad dip?
- If it's smooth and separate from the \(C-H\) peaks, it’s an alcohol (\(O-H\)).
- If it's huge, messy, and swallows the \(C-H\) peaks, it's a carboxylic acid (\(O-H\)). (Check for the \(C=O\) peak to confirm!)
- Look for \(N-H\): If there are small spikes around \(3300\ cm^{-1}\) but the molecule doesn't act like an alcohol, suspect an amine.
- Cross-reference: Use mass spectrometry data (Topic 7A/19A) if provided to confirm the molar mass and fragments.
Common Mistake to Avoid: Confusing the \(O-H\) of an alcohol with the \(O-H\) of a carboxylic acid. Remember: the acid \(O-H\) is much broader and shifted to the right, often looking like it's centered around \(3000\ cm^{-1}\), whereas the alcohol \(O-H\) is centered higher up around \(3300\ cm^{-1}\).
Key Takeaway
Infrared spectroscopy is all about identifying bonds by where they absorb energy. Use the sharp \(C=O\) and the broad \(O-H\) as your primary landmarks on the "map" of the spectrum. Always justify your answer by quoting the specific wavenumber ranges from the Data Booklet!