Infrared spectroscopy (IR spectroscopy) is a powerful analytical technique taught in IB Chemistry Topic 11 (Measurement & Data Processing) and Topic 21 (HL Organic Chemistry). It allows chemists to identify functional groups in organic molecules based on how they absorb infrared radiation. IR spectra give unique “fingerprints” of molecules, making this technique essential for structure determination and confirming whether a reaction has succeeded.
What Is Infrared Spectroscopy?
Infrared spectroscopy is an analytical technique that measures how molecules absorb infrared radiation to identify functional groups based on characteristic bond vibrations.
When IR radiation hits a molecule:
- Bonds vibrate (stretch or bend)
- Different bonds absorb different frequencies
- The instrument records these absorptions as peaks
These peaks reveal which functional groups are present.
Why Molecules Absorb Infrared Radiation
Covalent bonds behave like vibrating springs.
When IR radiation matches the natural vibration frequency of a bond, the bond absorbs energy and vibrates more strongly.
Bond vibrations occur due to:
- Stretching (lengthening and shortening)
- Bending (changing angles)
Different types of bonds absorb IR at different frequencies depending on:
- Bond strength
- Bond polarity
- Mass of atoms involved
For example:
- O–H bonds vibrate at higher frequencies
- C–H bonds vibrate at moderate frequencies
- C–O, C–N, and C–C bonds vibrate at lower frequencies
What IR Spectroscopy Is Used For
IR spectroscopy is mainly used for identifying functional groups in organic compounds.
Key uses include:
1. Determining functional groups
Each functional group has characteristic absorption peaks:
- Alcohols → broad O–H peak around 3200–3600 cm⁻¹
- Carboxylic acids → very broad O–H + strong C=O at ~1700 cm⁻¹
- Alkenes → C=C stretch around 1640–1680 cm⁻¹
- Aldehydes/ketones → sharp C=O around 1710–1740 cm⁻¹
- Amines → N–H stretches around 3300–3500 cm⁻¹
These key signals allow quick identification.
2. Confirming whether a reaction occurred
IR can show changes in functional groups:
- Disappearance of an O–H peak → alcohol oxidized to aldehyde/ketone
- Appearance of a C=O peak → new carbonyl formed
- Loss of a C=C peak → alkene underwent addition reaction
Chemists often use IR to confirm product formation after synthesis.
3. Comparing unknown samples
IR spectra act like fingerprints.
No two organic molecules have identical IR spectra in the fingerprint region (below 1500 cm⁻¹).
This allows comparison to reference spectra.
4. Detecting impurities
Extra peaks in the IR spectrum may indicate the presence of unwanted compounds, solvents, or side products.
Understanding an IR Spectrum
An IR spectrum has:
- Y-axis: % transmittance (how much light passes through)
- X-axis: Wavenumber (cm⁻¹), representing frequency
A dip (peak downward) means absorption of IR radiation.
Chemists focus on:
- Functional group region: 1500–4000 cm⁻¹
- Fingerprint region: below 1500 cm⁻¹
The fingerprint region is complex but unique to each molecule.
Common Peaks You Must Know for IB Chemistry
Broad O–H peak (alcohols):
3200–3600 cm⁻¹
Very broad O–H peak (carboxylic acids):
2500–3300 cm⁻¹ + strong C=O at ~1700 cm⁻¹
C=O stretch:
Around 1700 cm⁻¹
C=C stretch:
Around 1640–1680 cm⁻¹
Sharp C–H stretches:
Around 2850–3100 cm⁻¹
These appear frequently in exam interpretation questions.
Why IR Spectroscopy Is Important
IR is essential in:
- Organic synthesis
- Pharmaceutical quality control
- Environmental testing
- Food and cosmetic analysis
- Forensic science
It is fast, non-destructive, and provides clear structural information.
Common IB Misunderstandings
“IR spectroscopy gives molecular formulas.”
Incorrect. IR only shows functional groups, not exact structure.
“A strong peak means a high concentration.”
Peak strength depends mainly on bond polarity, not quantity.
“Fingerprint region is not useful.”
It is extremely useful for verifying identity against reference spectra.
FAQs
Can IR distinguish between aldehydes and ketones?
Not easily; both show similar C=O peaks. Further tests or techniques are needed.
Can IR show isomers?
Functional group isomers often have different IR peaks, but stereoisomers usually do not.
Does IR work on all compounds?
It works best on covalent compounds; ionic salts generally do not show useful spectra.
Conclusion
Infrared spectroscopy identifies functional groups by measuring how molecular bonds absorb infrared radiation. It is essential for confirming reaction outcomes, analyzing organic structures, and comparing unknown samples. In the IB Chemistry curriculum, understanding IR interpretation is vital for Paper 2 and HL organic questions.
