Infrared Spectroscopy: Analyzing Organic Compound Structures

Expert reviewed • 23 November 2024 • 5 minute read

Introduction

Infrared (IR) spectroscopy serves as a powerful analytical tool for identifying functional groups and molecular structures in organic compounds. This technique is fundamental to modern chemical analysis and forms a crucial part of HSC Chemistry Module 8, Section 2.

Principles of IR Spectroscopy

Infrared spectroscopy works by exposing molecules to infrared radiation, which causes their covalent bonds to vibrate in two primary ways:

  • Bond Stretching: The bond length between atoms increases and decreases
  • Bond Bending: The angle between bonds changes

Different functional groups absorb IR radiation at characteristic frequencies (measured in wavenumbers, cm⁻¹), creating a unique spectral fingerprint for each molecule.

Characteristic IR Absorptions

Key Functional Groups and Their IR Signals

Functional GroupBond TypeWavenumber (cm⁻¹)Peak Characteristics
AlkaneC–H2850–3300Sharp
AlkeneC=C1620–1680Medium
AlkyneC≡C2100–2260Sharp
AlcoholO–H3230–3550Broad
AlcoholC–O1000–1300Medium
CarbonylC=O1680–1750Strong
Carboxylic AcidO–H2500–3000Very broad
Amine/AmideN–H3300–3500Broad
Amine/AmideC–N1030–1230Medium
NitrileC≡N2220–2260Sharp

Analysis of Common Functional Groups

Alkanes

  • Characteristic sharp C–H absorption at ν=3100 cm−1\nu = 3100 \text{ cm}^{-1} [Figure 2: Alkane IR spectrum]

Alcohols

Ethanol shows two key features:

  • Broad O–H stretch at ν=3391 cm−1\nu = 3391 \text{ cm}^{-1}
  • Sharp C–H stretch at ν=2981 cm−1\nu = 2981 \text{ cm}^{-1}

Carbonyls (Ketones and Aldehydes)

  • Strong C=O absorption at ν=1700–1800 cm−1\nu = 1700\text{--}1800 \text{ cm}^{-1}
  • C–H stretch near ν=3000 cm−1\nu = 3000 \text{ cm}^{-1}

Carboxylic Acids

Distinctive features include:

  • Broad O–H stretch at ν=3500 cm−1\nu = 3500 \text{ cm}^{-1}
  • Strong C=O absorption at ν=1800 cm−1\nu = 1800 \text{ cm}^{-1}

Esters

Key identifiers:

  • Sharp C=O stretch at ν=1752 cm−1\nu = 1752 \text{ cm}^{-1}
  • C–O stretch at ν=1055 cm−1\nu = 1055 \text{ cm}^{-1}
  • Absence of O–H stretch

Amines and Amides

Primary and secondary amines/amides show:

  • N–H stretch at ν=3300–3500 cm−1\nu = 3300\text{--}3500 \text{ cm}^{-1}
  • Amides show additional C=O stretch at ν=1680–1750 cm−1\nu = 1680\text{--}1750 \text{ cm}^{-1}

Nitriles

  • Sharp C≡N stretch at ν=2250 cm−1\nu = 2250 \text{ cm}^{-1}

Practice Questions

  • What spectral features would you expect to see in the IR spectrum of ethanoic acid?
  • How can you distinguish between an ester and a ketone using IR spectroscopy?
  • Why do tertiary amines show no absorption in the 3300-3500 cm⁻¹ region?

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Identifying Organic Compounds: Tests for Aldehydes, Ketones, and Carboxylic Acids

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Understanding Mass Spectrometry for Organic Compound Analysis

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