3.2.9Additional lessons
Infrared Spectra (IR) Interpretation (HL) — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 66 of 172
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 5How IR Works
Vibration
- Vibration Covalent bonds stretch and bend at specific frequencies,
- and a bond absorbs infrared radiation when the radiation frequency matches its vibration.
Dipole change
- Dipole change Only vibrations that change the molecule's dipole moment absorb, so symmetrical diatomics such as N₂ and O₂ do not absorb.
Wavenumber
- Wavenumber Absorptions are plotted against wavenumber in cm⁻¹, and the vertical axis shows transmittance, so absorptions appear as downward peaks.
Stronger bonds
- Stronger bonds Stronger bonds and lighter atoms vibrate at higher wavenumbers, so C≡C appears at a higher value than C=C or C–C.
Fingerprint region
- Fingerprint region Below about 1500 cm⁻¹ the pattern is unique to each compound and is matched against reference spectra.
Section 2 of 5Characteristic Absorptions
1
A strong, broad absorption between 3200 and 3600 cm⁻¹ indicates the O–H group of an alcohol.
2
A very broad absorption from about 2500 to 3300 cm⁻¹ shows the O–H group in a carboxylic acid.
3
A strong, sharp peak between 1700 and 1750 cm⁻¹ shows a C=O carbonyl group, as in ketones, aldehydes, acids and esters.
4
Absorptions at 2850 to 3090 cm⁻¹ come from C–H stretching, and a peak at 1620 to 1680 cm⁻¹ shows C=C.
5
A peak near 2100 to 2260 cm⁻¹ indicates a triple bond such as C≡C or C≡N.
| Bond | Wavenumber range / cm⁻¹ |
|---|---|
| O–H (alcohol) | 3200–3600 |
| O–H (acid) | 2500–3300 |
| C=O | 1700–1750 |
| C–H | 2850–3090 |
Section 3 of 5Applications
Applications
Distinguishing groups
- Distinguishing groups An alcohol shows O–H only, a ketone shows C=O only, and a carboxylic acid shows both.
Monitoring reactions
- Monitoring reactions Loss of an O–H peak and gain of a C=O peak shows oxidation of an alcohol to a carbonyl compound.
Greenhouse gases
- Greenhouse gases Molecules such as CO₂, CH₄ and H₂O absorb infrared radiation, which explains how they trap heat in the atmosphere.
Breathalysers
- Breathalysers Some breathalysers use infrared absorption by bonds in ethanol vapour to estimate blood alcohol level.
Section 4 of 5Worked Example
Question
- An unknown compound of formula C₃H₆O₂ shows a very broad absorption from 2500 to 3300 cm⁻¹ and a strong peak at 1710 cm⁻¹.
- Identify the functional group.
- The strong peak at 1710 cm⁻¹ is in the C=O range.
- The very broad absorption from 2500 to 3300 cm⁻¹ is typical of O–H in a carboxylic acid.
- The two peaks together indicate –COOH.
- A C₃ compound with –COOH is propanoic acid, CH₃CH₂COOH.
Answer Carboxylic acid; propanoic acid
Section 5 of 5Exam Checklist & Tips
3.2.9 – Key Points for the Exam
- Wavenumber The reciprocal of wavelength, measured in cm⁻¹.
- Transmittance The fraction of incident radiation passing through a sample.
- Fingerprint region The region below about 1500 cm⁻¹ with a pattern unique to each compound.
- Dipole moment change A change in charge separation during vibration, needed for IR absorption.
Common Mistakes
- Thinking that IR gives the carbon skeleton or the exact molecular formula.
- Forgetting that very broad O–H peaks in acids merge with C–H absorptions.
- Describing absorption as increasing at a peak, when percentage transmittance falls.
- Using wavelength units rather than wavenumber for the x axis.
Last updated:
\n