Infrared Spectroscopy

Infrared (IR) spectroscopy is a powerful analytical technique that lets you identify functional groups in an unknown organic compound by looking at which frequencies of infrared radiation the molecule absorbs. It is fast, non-destructive, requires very little sample, and is a mainstay of every organic chemistry lab. At A-Level you are expected to interpret IR spectra, identify key absorption peaks, and combine IR with other techniques (mass spectrometry, NMR) to determine unknown structures.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 4.2.4 (a)–(b): principles of infrared spectroscopy, use of characteristic absorption frequencies to identify functional groups, interpretation of IR spectra, and the role of the fingerprint region.

1. The Principle — Bonds Vibrate

Every covalent bond behaves like a tiny spring connecting two masses. It can stretch (the two atoms moving closer together and further apart along the bond axis) and bend (the bond angle changing). Each type of vibration has a characteristic natural frequency, which depends on:

The masses of the two atoms (heavier atoms vibrate more slowly).
The strength of the bond (stiffer spring → higher frequency).

Mathematically, the frequency of a simple stretch is given by:

$\nu = \frac{1}{2\pi}\sqrt{\frac{k}{\mu}}$

where k is the bond force constant and μ is the reduced mass. You do not need this equation for the exam but it explains the trends: strong bonds (C=O, C≡C) absorb at high wavenumbers, weak bonds or heavy atoms (C–Cl, C–Br) absorb at low wavenumbers.

1.1 What Is "Absorption"?

When a bond is exposed to infrared radiation (wavelengths roughly 2.5–25 μm), it can absorb a photon whose energy exactly matches the spacing between its vibrational energy levels. The photon's energy is converted into extra vibrational motion. This shows up as a dip in the transmitted IR signal at that frequency.

The horizontal axis of an IR spectrum is wavenumber in cm⁻¹ (a weird but useful unit: wavenumber = 1 / wavelength in cm, so higher wavenumber = higher frequency = higher energy). The axis runs from 4000 cm⁻¹ on the left to 400 cm⁻¹ on the right — opposite to most other axes.

1.2 Selection Rule: Dipole Change

A bond only absorbs IR if its vibration causes a change in dipole moment. Perfectly non-polar bonds (H–H, symmetrical O=C=O stretch in one specific mode) are "IR silent". In practice, almost every bond in an organic molecule has some dipole and shows up in the IR.

Useful consequence: Homonuclear diatomics like N₂ and O₂ are IR-inactive, which is why they don't trap IR in the atmosphere. CO₂ is IR-active in its bending and asymmetric stretching modes — that is what makes it a greenhouse gas.

2. The Correlation Table — Key Functional Groups

The core skill at A-Level is reading off which functional groups are present from the position of absorption peaks. You must be able to use the OCR data sheet correlation table — or reproduce the key values from memory:

Bond	Location in molecule	Wavenumber / cm⁻¹	Appearance
O–H	Alcohol (free)	3550–3700	Sharp
O–H	Alcohol, H-bonded	3230–3550	Broad
O–H	Carboxylic acid	2500–3300	Very broad, often stretches most of the high-ν region
N–H	Amine, amide	3300–3500	Medium, sometimes two peaks (NH₂)
C–H	Alkane, alkene, arene	2850–3100	Medium
C≡C	Alkyne	2100–2260	Weak/medium
C≡N	Nitrile	2220–2260	Medium, sharp
C=O	Aldehyde, ketone, acid, ester, amide	1630–1820	Strong, sharp — very diagnostic
C=C	Alkene	1620–1680	Weak/medium
C–O	Alcohol, ester, ether	1000–1300	Strong

2.1 The Three Most Diagnostic Peaks

In an A-Level exam question, the peaks to look for first are:

C=O around 1700 cm⁻¹ — strong, sharp. If present, you have an aldehyde, ketone, acid, ester or amide. If absent, rule all five out.
Broad O–H somewhere between 2500 and 3550 cm⁻¹ — if narrow and around 3500, it is an alcohol. If very broad and stretching down to 2500, it is a carboxylic acid.
C–H just below 3000 cm⁻¹ — present in almost every organic molecule, so not very diagnostic on its own, but indicates the presence of CH bonds.

graph TD
  A[Look at spectrum] --> B{C=O around 1700?}
  B -->|Yes| C{O-H broad 2500-3300?}
  B -->|No| D[No carbonyl - look for alcohol/amine/alkene/CN]
  C -->|Yes| E[Carboxylic acid]
  C -->|No| F{O-H 3230-3550?}
  F -->|Yes| G[No - inconsistent, could be ester with residual moisture]
  F -->|No| H[Aldehyde, ketone, ester or amide]
  D --> I{O-H 3200-3600?}
  I -->|Broad| J[Alcohol]
  I -->|Two peaks around 3300-3500| K[Primary amine]
  I -->|Absent| L[Hydrocarbon or haloalkane]

2.2 Distinguishing Acid from Alcohol

Both contain –OH, but the carboxylic acid O–H peak is very broad and stretches from ~3300 down to ~2500 cm⁻¹, often overlapping and burying the C–H region. The alcohol O–H is narrower and confined to 3230–3550 (broad) or 3550–3700 (sharp, rare). The extra broadening in acids comes from strong hydrogen bonding in dimer form.

2.3 Distinguishing Aldehyde from Ketone

Both show C=O around 1700, but aldehydes have a characteristic weak, doubled C–H stretch around 2700–2900 cm⁻¹ from the H on the carbonyl carbon. If you see that, it is an aldehyde.

3. The Fingerprint Region

Below about 1500 cm⁻¹ lies the fingerprint region — a dense forest of small peaks from bending vibrations and C–C, C–N, C–O skeletal vibrations. Unlike the functional group region (1500–4000 cm⁻¹), the fingerprint region is:

Too complex to interpret peak-by-peak at A-Level.
Unique to each compound — it is the molecule's "fingerprint".

Lesson 9: Infrared Spectroscopy