Organic Analysis (AQA A-Level Chemistry): Revision Notes

3.6.3 Infrared Spectroscopy

How Infrared Spectroscopy Works

Absorption of Infrared Radiation

• Bonds in a molecule absorb infrared (IR) radiation at specific frequencies.
• The frequency of absorption is related to the vibrational frequency of the bond, which depends on:
  • The mass of the atoms involved.
  • The bond strength and length.
• When IR radiation passes through a compound, bonds absorb energy at characteristic frequencies, causing them to vibrate. These absorbed frequencies are detected and represented as peaks on an IR spectrum.

Analysing the IR Spectrum

An infrared spectrum is a graph plotting absorption against wavenumber (cm$⁻¹$). The position of each peak helps identify the functional groups present in the molecule.

Key Regions of the IR Spectrum

The Fingerprint Region (Below 1500 cm$⁻¹$)

• This region contains a complex pattern of peaks due to bond bending, rotations, and other deformations.
• Unique for each molecule: It acts like a "fingerprint" that can confirm the identity of a compound when compared to a known sample.
• Impurities: Additional peaks in this region compared to a known spectrum indicate the presence of impurities.

The Functional Group Region (Above 1500 cm⁻¹)

• This region is more straightforward to interpret, as it contains signals associated with specific functional groups.
• Key peaks in this region correspond to common bonds in organic molecules, making it possible to identify the structure of a compound.

Identifying Functional Groups Using IR Spectra

Common IR Absorption Ranges

$C-H$ Stretch:

• A peak just under 3000 cm⁻¹ is common for $C-H$ bonds in almost all organic molecules. $O-H$ Stretch:

• Alcohols: Broad peak between 3230 and 3550 cm⁻¹.

• Carboxylic Acids: Very broad peak between 2500 and 3000 cm⁻¹. $C=O$ Stretch (Carbonyl Group):

• A sharp peak between 1680 and 1750 cm⁻¹, indicating the presence of an aldehyde, ketone, carboxylic acid, or ester.

Using the Fingerprint Region for Molecule Identification

How to Use the Fingerprint Region

• This region is often too complex to identify individual functional groups.
• However, comparing the fingerprint region of an unknown spectrum with a known pure sample can confirm the identity of a molecule.
• If the spectra match exactly, the compounds are the same. Any additional peaks indicate impurities.

Infrared Absorption and Global Warming

Role of Greenhouse Gases

• The Earth absorbs sunlight (UV and visible radiation) and re-emits it as infrared radiation.
• Greenhouse gases like carbon dioxide ($CO₂$), methane ($CH₄$), and water vapour absorb this IR radiation effectively.
• The absorption of IR radiation by these gases traps heat in the atmosphere, contributing to global warming.

How IR Absorption Relates to Climate Change

• Increased levels of greenhouse gases lead to higher absorption of IR radiation.
• This increased IR absorption raises global temperatures, amplifying the greenhouse effect.
• The efficiency of a greenhouse gas in trapping heat is directly related to the amount of IR it can absorb.