2.10.3Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Using Infrared Spectrometry — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 16 of 53 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6How Infrared Spectroscopy Works
Infrared spectra give a quick way to identify functional groups in an unknown sample.
Vibrations
- Vibrations Covalent bonds stretch and bend at set frequencies, and a bond absorbs infrared radiation that matches its vibration frequency.
Spectrum
- Spectrum The spectrum plots percentage transmittance against wavenumber in cm⁻¹, and downward peaks are absorptions.
Dependence
- Dependence The wavenumber depends on bond strength and on the masses of the atoms, so different functional groups have characteristic values.
Fingerprint region
- Fingerprint region Below about 1500 cm⁻¹ there is a complex pattern unique to each molecule, which can be matched to a database.
Section 2 of 6Characteristic Absorptions
Characteristic Absorptions
O–H alcohol
- O–H alcohol Broad absorption between 3200 and 3600 cm⁻¹.
O–H carboxylic acid
- O–H carboxylic acid Very broad absorption between 2500 and 3300 cm⁻¹, often overlapping the C–H peaks.
C–H
- C–H Absorption between 2850 and 3100 cm⁻¹ in most organic compounds.
C=O
- C=O Strong sharp absorption between 1680 and 1750 cm⁻¹ in aldehydes, ketones, acids and esters.
C=C
- C=C Absorption between 1620 and 1680 cm⁻¹, usually weaker than the carbonyl peak.
C–O
- C–O Strong absorption between 1000 and 1300 cm⁻¹ in alcohols, esters and acids.
| Bond | Wavenumber / cm⁻¹ |
|---|---|
| O–H (alcohol) | 3200–3600 |
| O–H (acid) | 2500–3300 |
| C–H | 2850–3100 |
| C=O | 1680–1750 |
| C=C | 1620–1680 |
Section 3 of 6Interpreting a Spectrum
1
Check first for a strong peak in the 1680 to 1750 cm⁻¹ range, which indicates a carbonyl group.
2
Check for a broad peak in the 3200 to 3600 cm⁻¹ range, which suggests an alcohol O–H.
3
A very broad peak from 2500 to 3300 cm⁻¹ together with a carbonyl peak indicates a carboxylic acid.
4
Look for absence of peaks as well, since a missing O–H peak rules out alcohols and acids.
5
State the bond and its range when naming a functional group, quoting the wavenumber seen on the spectrum.
Section 4 of 6Applications
Applications
Reaction monitoring
- Reaction monitoring During oxidation of an alcohol to an aldehyde, the broad O–H peak disappears and a C=O peak appears.
Greenhouse gases
- Greenhouse gases CO₂, H₂O and CH₄ absorb infrared radiation emitted by Earth, which is why they trap heat in the atmosphere.
Breathalysers
- Breathalysers Some devices use infrared absorption to measure ethanol in breath.
Purity
- Purity The presence of an unexpected O–H peak may show that a product is contaminated with water or alcohol.
Section 5 of 6Worked Example
Question
- A compound shows a strong sharp peak at 1715 cm⁻¹ and a very broad absorption from 2500 to 3300 cm⁻¹.
- Which functional group is present?
- A peak at 1715 cm⁻¹ lies in the 1680–1750 cm⁻¹ range for C=O.
- A very broad absorption from 2500–3300 cm⁻¹ is the characteristic O–H of a carboxylic acid.
- A carbonyl and acid O–H together indicate –COOH.
Answer A carboxylic acid group, –COOH.
Section 6 of 6Exam Checklist & Tips
2.10.3 – Key Points for the Exam
- Wavenumber The reciprocal of wavelength, measured in cm⁻¹, used to label infrared spectra.
- Absorption The taking in of radiation of a particular frequency by a bond.
- Fingerprint region The region below about 1500 cm⁻¹ with a pattern unique to each compound.
- Transmittance The percentage of radiation passing through the sample.
- Greenhouse gas A gas that absorbs infrared radiation from the Earth and warms the atmosphere.
Common Mistakes
- Saying infrared radiation breaks bonds; it only makes them vibrate more.
- Mixing up the wavenumber ranges of O–H in alcohols and carboxylic acids.
- Forgetting to state both the bond and the wavenumber in an answer.
- Reading peaks as upward on a transmittance spectrum.
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