What is Infrared Spectroscopy?
Infrared (IR) spectroscopy uses infrared light to identify functional groups in molecules by detecting how they absorb specific IR wavelengths. Different bonds vibrate at different frequencies, creating a unique 'fingerprint' for each compound.
IR spectroscopy measures the absorption of infrared light (wavelength 2.5–25 μm, wavenumber 400–4000 cm⁻¹) by molecular bonds, revealing functional groups present.
Step-by-step worked examples
A compound shows a strong absorption peak at 1700 cm⁻¹. What functional group does this suggest?
1700 cm⁻¹ is in the C=O stretch region (carbonyl). This indicates an aldehyde, ketone, carboxylic acid, ester, or amide. Further analysis of other peaks (O-H at ~3300 cm⁻¹ for acid, or N-H) refines the identification.
Ethanol (CH₃CH₂OH) shows peaks at 2900 (C-H), 3300 (O-H), and 1050 (C-O) cm⁻¹. Explain each peak.
2900 cm⁻¹: C-H stretching (alkyl groups) 3300 cm⁻¹: O-H stretching (hydroxyl group, broad peak) 1050 cm⁻¹: C-O stretching (alcohol C-O bond) These peaks confirm ethanol's functional groups.
Phenol (C₆H₅OH) shows a O-H peak at 3200 cm⁻¹, not 3300. Why might it differ from aliphatic alcohols?
Phenol's O-H is directly bonded to an aromatic ring. The aromatic structure shifts the O-H stretching frequency slightly. Hydrogen bonding in phenol can also broaden the peak and shift it to lower frequency.
Flashcards
Quick quiz
Q1.A spectrum shows a strong, sharp peak at 2150 cm⁻¹. Which bond is this likely?
Q2.IR peaks for O-H in carboxylic acids are typically…
Q3.What does a C=O peak at ~1650 cm⁻¹ suggest, rather than 1700 cm⁻¹?
Q4.Why is the fingerprint region (below 1500 cm⁻¹) useful?
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Common mistakes
IR spectroscopy identifies the actual structure of a compound. — Correct: IR identifies functional groups present, not full structure — NMR and MS are needed for complete structure.
All C=O bonds absorb at exactly 1700 cm⁻¹. — Correct: C=O frequency varies (1650 cm⁻¹ for amides, 1750 cm⁻¹ for acid chlorides) due to conjugation and hybridization.
Wavenumber is the same as wavelength. — Correct: Wavenumber (cm⁻¹) = 1/wavelength (cm); they are inversely related.
Broad peaks are always due to hydrogen bonding. — Correct: Broad peaks usually indicate hydrogen bonding, but can also result from multiple vibrational modes.
FAQ
What does 'IR-active' mean?
A bond is IR-active if its vibration changes the molecular dipole moment, allowing it to absorb IR light.
Why does solvent choice matter in IR spectroscopy?
Solvents like KBr and CCl₄ are used for solid/liquid samples. Some solvents absorb IR and mask peak regions.
Can IR spectroscopy detect all functional groups?
No — symmetrical groups (like C=C in nonpolar alkenes) may not show IR absorption; NMR is better for these.
What is an 'IR-inactive' group?
A group whose vibration does not change the dipole moment (e.g. symmetrical diatomic molecules like O₂).