3.6.3Physical Chemistry
Infrared Spectroscopy — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 89 of 102
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.
Part 1 of 6How Infrared Spectroscopy Works
1
Covalent bonds behave like springs and can stretch and bend at specific natural frequencies.
2
An infrared beam is passed through the sample, and bonds absorb radiation that matches their vibration frequency.
3
The spectrometer records the amount of radiation transmitted at each wavenumber, in cm⁻¹, where wavenumber is the reciprocal of wavelength.
4
Each absorption appears as a downward peak, and its position, width and strength show which bonds are present.
5
Matching these peaks against data for known bonds allows the functional groups in the molecule to be identified.
Part 2 of 6Characteristic Absorptions
C=O
- C=O A strong, sharp peak at 1680–1750 cm⁻¹ shows a carbonyl group in an aldehyde, ketone, acid or ester.
O–H in alcohols
- O–H in alcohols A broad peak at 3230–3550 cm⁻¹, widened by hydrogen bonding, indicates an alcohol.
O–H in acids
- O–H in acids A very broad peak at 2500–3000 cm⁻¹ indicates a carboxylic acid, usually with a C=O peak as well.
C–H
- C–H Absorptions at 2850–3300 cm⁻¹ appear in almost every organic compound.
C≡N and C=C
- C≡N and C=C A nitrile absorbs at 2220–2260 cm⁻¹, and a C=C bond at 1620–1680 cm⁻¹.
| Bond | Wavenumber / cm⁻¹ |
|---|---|
| N–H | 3300–3500 |
| O–H (alcohols) | 3230–3550 |
| O–H (carboxylic acids) | 2500–3000 |
| C–H | 2850–3300 |
| C≡N | 2220–2260 |
| C=O | 1680–1750 |
| C=C | 1620–1680 |
| C–O | 1000–1300 |
Part 3 of 6Using Spectra in Practice
Using Spectra in Practice
Fingerprint region
- Fingerprint region Below about 1500 cm⁻¹ the pattern is unique to each compound, so it can be matched to a database.
Telling groups apart
- Telling groups apart An alcohol has O–H but no C=O, a ketone has C=O but no O–H, and a carboxylic acid has both.
Checking purity
- Checking purity Unexpected peaks, such as an O–H peak in a pure carbonyl product, reveal impurities or unreacted starting material.
Combined methods
- Combined methods Overlapping bands mean infrared is often used with mass spectrometry to confirm a structure.
Part 4 of 6Infrared and Global Warming
Infrared and Global Warming
Earth's radiation
- Earth's radiation The Earth absorbs sunlight and re-emits the energy as long-wavelength infrared radiation.
Greenhouse gases
- Greenhouse gases CO₂, CH₄ and H₂O contain bonds that absorb infrared radiation and vibrate more strongly.
Trapped energy
- Trapped energy The absorbed energy is re-emitted in all directions, so some of it warms the lower atmosphere.
Human impact
- Human impact Higher concentrations of greenhouse gases from human activity increase this absorption and contribute to global warming.
Part 5 of 6Worked Example
Question
- An unknown liquid with formula C₃H₆O shows a strong sharp absorption at 1715 cm⁻¹ and no broad peak above 3000 cm⁻¹.
- Deduce which functional group is present and name a possible compound.
- 1715 cm⁻¹ lies within the C=O range 1680–1750 cm⁻¹, so a carbonyl group is present.
- There is no broad O–H absorption at 3230–3550 cm⁻¹ or 2500–3000 cm⁻¹, so it is not an alcohol or a carboxylic acid.
- C₃H₆O with a carbonyl group and no O–H fits propanone, CH₃COCH₃ (propanal would also fit).
Answer The compound contains C=O and no O–H, and propanone is a possible identity.
Part 6 of 6Exam Checklist & Tips
3.6.3 – Key Points for the Exam
- Wavenumber The reciprocal of wavelength, measured in cm⁻¹, used to show infrared absorptions.
- Fingerprint region The region below about 1500 cm⁻¹ that is unique to each compound.
- Bond vibration Stretching or bending of a covalent bond that absorbs infrared radiation of a matching frequency.
- Greenhouse gas A gas whose bonds absorb infrared radiation emitted by the Earth.
Common Mistakes
- Quoting a wavenumber range without naming the bond, or the bond without the range.
- Saying greenhouse gases absorb ultraviolet radiation, which belongs to ozone depletion.
- Confusing the broad O–H peak of an acid, at 2500–3000 cm⁻¹, with that of an alcohol.
- Using the fingerprint region to identify a single functional group rather than to match a whole compound.
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