🔹 1. Infrared (IR) Spectroscopy
🔸 Principle:
- Infrared radiation is absorbed by covalent bonds, causing them to vibrate (stretch or bend).
- Different functional groups absorb IR radiation at specific frequencies, producing peaks in an IR spectrum.
🔸 Analysis of IR Spectrum:
- The x-axis shows wavenumber (cm⁻¹), inverse of wavelength.
- The y-axis shows transmittance – lower peaks indicate higher absorption.
- Use the Data Booklet to match absorption ranges with functional groups.
| Functional Group | Approx. Absorption Range (cm⁻¹) | Description |
|---|---|---|
| O–H (alcohol) | 3200–3550 (broad) | Strong, broad |
| O–H (carboxylic acid) | 2500–3300 (very broad) | Overlaps with C–H |
| C=O (carbonyl) | 1680–1750 | Sharp, strong |
| N–H (amine/amide) | 3300–3500 | Sharp, medium |
| C–H (alkane) | ~2850–2960 | Medium |
| C=C (alkene) | ~1620–1680 | Weak to medium |
| C≡C or C≡N | ~2100–2260 | Weak |
🔸 Uses:
- To identify functional groups.
- To monitor purity (e.g., absence of extra peaks).
🔹 2. Mass Spectrometry (MS)
🔸 Principle:
- A compound is ionized to form a molecular ion (M⁺).
- The instrument detects ions based on mass-to-charge ratio (m/e).
- Most common ions have +1 charge, so m/e ≈ mass.
🔸 2.1 Interpreting Mass Spectra:
1️⃣ m/e Values and Isotopic Abundances
- Peaks represent ions with specific m/e.
- Relative abundances shown by peak height or percentage.
- Isotopes show multiple peaks for elements like Cl, Br, etc.
🔸 2.2 Calculating Relative Atomic Mass (Aᵣ)
Given isotopic masses and their abundances: Aᵣ=[ ∑(isotopic mass×abundance)] / ∑abundances
🔸 Example:
| Isotope | Mass | Abundance (%) |
|---|---|---|
| 35Cl | 35 | 75 |
| 37Cl | 37 | 25 |
Ar=(35×75+37×25)/100=35.5
🔸 2.3 Determining Molecular Mass
- The molecular ion peak (M⁺) gives the molecular mass of the compound.
- It’s the highest m/e value (excluding smaller fragment peaks).
- Peaks lower than M⁺ represent fragmentation.
🔸 2.4 Fragmentation Patterns
- Molecules break into smaller fragments → characteristic peaks.
- Use molecular formula, likely fragment groups, and masses to identify.
- Example: C₂H₅⁺ → m/e = 29, CH₃⁺ → m/e = 15
🔸 2.5 [M+1]⁺ Peak and Number of Carbon Atoms
- Due to ¹³C isotope (1.1% natural abundance).
- Formula:
n= [100×abundance of [M+1]⁺ peak ] / [1.1×abundance of M⁺ peak ]
- n = number of carbon atoms
🔸 2.6 [M+2]⁺ Peak – Identifying Halogens
- Chlorine: 35Cl (75%), 37Cl (25%) → 3:1 peak ratio (M⁺ : [M+2]⁺)
- Bromine: 79Br (50%), 81Br (50%) → 1:1 peak ratio
✅ A strong [M+2]⁺ peak signals Cl or Br presence.
✍️ Summary of Skills
| Task | Technique |
|---|---|
| Identify functional groups | IR spectrum |
| Find molecular mass | Molecular ion peak (MS) |
| Calculate Aᵣ | Isotopic data (MS) |
| Identify fragments | Fragment peaks (MS) |
| Count carbon atoms | [M+1]⁺ peak (MS) |
| Detect Cl/Br | [M+2]⁺ peak (MS) |
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