Infrared Spectroscopy and Chromatography

Infrared Spectroscopy and Chromatography

Infrared (IR) Spectroscopy

Infrared spectroscopy measures the absorption of infrared radiation by molecules. Bonds absorb specific frequencies of IR radiation, causing them to vibrate (stretch, bend, or wag) more vigorously. Different bond types absorb at different frequencies, making IR a powerful tool for identifying functional groups.

How IR Spectroscopy Works

1. A beam of IR radiation (covering a range of frequencies) is passed through the sample

2. Certain frequencies are absorbed by the bonds in the molecule

3. A detector measures which frequencies were absorbed and how strongly

4. The result is plotted as % transmittance (y-axis) against wavenumber (x-axis, in cm⁻¹)

Wavenumber is the reciprocal of wavelength (1/λ) and is proportional to the frequency of radiation. Higher wavenumber = higher frequency = higher energy.

A dip (trough) in the spectrum indicates absorption at that wavenumber.

Key Absorption Frequencies

Bond	Wavenumber / cm⁻¹	Appearance	Found in
O–H (alcohol)	3200–3550	Broad	Alcohols
O–H (carboxylic acid)	2500–3300	Very broad	Carboxylic acids
N–H	3300–3500	Medium, sharp (2 peaks in 1° amine)	Amines, amides
C–H	2850–3100	Medium	Most organic compounds
C=O	1630–1820	Strong, sharp	Aldehydes, ketones, carboxylic acids, esters, amides
C=C	1620–1680	Medium	Alkenes
C–O	1000–1300	Strong	Alcohols, esters, ethers
O–H (water)	~3400	Broad	Water / wet samples
C≡C	2100–2260	Weak/medium	Alkynes
C≡N	2200–2260	Medium	Nitriles

Interpreting IR Spectra

Step 1: Look for broad O–H absorptions:

• Very broad 2500–3300 cm⁻¹ → carboxylic acid
• Broad 3200–3550 cm⁻¹ → alcohol

Step 2: Look for a strong C=O peak around 1700 cm⁻¹:

• C=O present + broad O–H → carboxylic acid
• C=O present + C–O around 1200 cm⁻¹ but no broad O–H → ester
• C=O present + no O–H → aldehyde or ketone

Step 3: Check for N–H peaks (3300–3500 cm⁻¹) → amine or amide

The fingerprint region (below ~1500 cm⁻¹) is complex and unique to each compound. It is used to confirm identity by comparison with a reference spectrum in a database, rather than for identifying individual bonds.

Monitoring Reactions by IR

IR spectroscopy can monitor the progress of a reaction by tracking the appearance or disappearance of functional group peaks:

Example: Oxidation of a primary alcohol to a carboxylic acid

• The broad O–H alcohol peak (3200–3550 cm⁻¹) decreases
• A C=O peak (~1710 cm⁻¹) appears
• A very broad O–H acid peak (2500–3300 cm⁻¹) appears

IR and Climate Change

Greenhouse gases (CO₂, H₂O, CH₄) absorb IR radiation emitted by the Earth's surface. The absorbed energy is re-emitted in all directions, warming the atmosphere (the greenhouse effect).

• CO₂ absorbs at ~2349 cm⁻¹ (asymmetric C=O stretch) and ~667 cm⁻¹ (O=C=O bend)
• H₂O absorbs broadly across several IR frequencies
• CH₄ absorbs at ~3020 cm⁻¹ (C–H stretch) and ~1306 cm⁻¹ (C–H bend)

Symmetric diatomic molecules (N₂, O₂) do not absorb IR radiation because their vibrations do not change the molecular dipole moment — they are not greenhouse gases.

Chromatography

Chromatography separates mixtures based on the different affinities of components for a stationary phase and a mobile phase.

Thin-Layer Chromatography (TLC)

• Stationary phase: Thin layer of silica gel or alumina on a glass/plastic plate
• Mobile phase: A solvent (or solvent mixture) that rises up the plate by capillary action

Procedure:

1. Draw a pencil baseline near the bottom of the plate

2. Spot the sample(s) and reference compounds on the baseline

3. Place the plate in a beaker with a shallow pool of solvent (below the baseline)

4. Allow the solvent to rise by capillary action until near the top

5. Remove, mark the solvent front, and visualise spots (UV light, iodine vapour, or ninhydrin for amino acids)

Rf value = distance moved by spot / distance moved by solvent front

Rf values are characteristic of a substance under specific conditions (solvent, temperature, stationary phase). They are used for identification by comparison with standards run alongside.

Gas Chromatography (GC)

• Stationary phase: A high-boiling-point liquid coated on an inert solid, packed inside a long coiled column
• Mobile phase: An inert carrier gas (helium or nitrogen)

Process:

1. The sample is injected and vaporised at the column inlet

2. Components are carried through the column by the carrier gas

3. Components interact differently with the stationary phase — those with stronger interaction take longer

4. Each component exits the column at a different retention time

5. A detector records a peak for each component

The chromatogram plots detector signal against retention time. The area under each peak is proportional to the amount of that component.

Retention time depends on:

• Boiling point of the component (higher bp → longer retention)
• Polarity of the component and stationary phase
• Column temperature (higher T → shorter retention times)

GC-MS couples gas chromatography with mass spectrometry — GC separates the mixture, then each component enters the MS for identification.

High-Performance Liquid Chromatography (HPLC)

• Stationary phase: Small silica particles packed in a column
• Mobile phase: A liquid solvent pumped at high pressure

HPLC is used for non-volatile, thermally unstable, or high-molecular-mass compounds that cannot be vaporised for GC.

Column Chromatography

Used for preparative separation (collecting pure fractions):

• A vertical glass column is packed with silica or alumina
• The mixture is loaded at the top and eluted with solvent
• Different components travel at different rates
• Fractions are collected at the bottom

Exam Tips

• Always quote wavenumber values from the data booklet — learn the general ranges but use exact values from the table in exams
• When identifying a compound, check the broad O–H region and the C=O region first — these narrow the options rapidly
• For chromatography, state the stationary phase, mobile phase, and what is measured (Rf or retention time)
• Rf values must be calculated to 2 decimal places and will always be between 0 and 1