Infrared spectroscopy allows chemists to identify functional groups by analysing characteristic bond absorptions, providing rapid, non-destructive evidence for key organic structures.

Purpose of IR Spectroscopy in Functional Group Identification

Infrared (IR) spectroscopy is an analytical technique used to determine which functional groups are present in an organic compound. At OCR A-Level, IR is not used to determine full molecular structures but to confirm or exclude specific functional groups listed in the specification.

IR spectra show absorption peaks caused by bond vibrations within molecules. Different bonds absorb IR radiation at characteristic wavenumbers, allowing chemists to match peaks to known functional groups using provided data.

Fundamental Principle of IR Absorption

IR radiation causes bonds to vibrate by stretching or bending. Absorption occurs only when the vibration causes a change in dipole moment, which is why some bonds give strong absorptions while others are weak or absent.

The horizontal axis of an IR spectrum is wavenumber (cm⁻¹), decreasing from left to right. The vertical axis shows percentage transmittance or absorption intensity.

Key Functional Groups in the OCR Specification

Only functional groups specified by OCR should be identified from IR spectra. Students are expected to recognise patterns rather than memorise entire spectra.

Alcohols: O–H Absorption

Alcohols contain the hydroxyl (–OH) functional group, which produces a highly recognisable absorption.

Key features for alcohols:

• Broad O–H stretch between 3200–3600 cm⁻¹

• Caused by extensive hydrogen bonding between molecules

• Often overlaps other peaks but remains distinguishable due to its width

An alcohol is identified in an IR spectrum by a broad O–H stretch in the 3200–3600 cm–1 region.

This IR spectrum (cyclohexanol) highlights the broad O–H stretching absorption caused by hydrogen bonding. The labelled C–O stretch sits in the fingerprint region and supports the alcohol assignment alongside the O–H band. Source

The broad nature of the peak is a crucial diagnostic feature. A sharp peak in this region would suggest a different functional group, not an alcohol.

Alcohols do not contain a C=O bond, so no strong absorption appears around 1700 cm⁻¹.

Aldehydes and Ketones: C=O Absorption

Both aldehydes and ketones contain the carbonyl (C=O) functional group, which gives a strong and sharp absorption.

Key features for aldehydes and ketones:

• Strong, sharp C=O stretch around 1650–1750 cm⁻¹

• One of the most intense peaks in most organic IR spectra

• Indicates presence of a carbonyl-containing compound

A C=O (carbonyl) stretch appears as a strong absorption near 1700 cm–1, so its presence strongly suggests an aldehyde, ketone, or carboxylic acid functional group.

This spectrum highlights a labelled C=O absorption in the ~1700 cm–1 region, illustrating the diagnostic carbonyl peak shape and position. Extra detail not required by the syllabus: the sample is photodegraded polyethylene, and the axes use German terms. Source

IR spectroscopy cannot distinguish between aldehydes and ketones on its own because both contain the same C=O bond. Further tests or data are required for differentiation.

Students should focus on recognising the presence of the carbonyl group rather than attempting detailed structural interpretation.

Carboxylic Acids: Combined O–H and C=O Features

Carboxylic acids contain both a carbonyl (C=O) and a hydroxyl (O–H) group, producing a distinctive combination of absorptions.

Key features for carboxylic acids:

• Very broad O–H stretch from approximately 2500–3300 cm⁻¹

• Strong C=O stretch around 1700 cm⁻¹

• O–H peak broader and lower than in alcohols due to strong hydrogen bonding

A carboxylic acid shows a very broad O–H absorption between 2500–3300 cm–1 and a strong C=O absorption around 1700 cm–1.

This IR spectrum (butanoic acid) shows the very broad O–H absorption extending across the 2500–3300 cm–1 region and the strong C=O peak around ~1700 cm–1. Seeing both together is the key pattern for identifying a carboxylic acid rather than an alcohol. Source

The extremely broad O–H absorption is the most diagnostic feature and often extends into the C–H region, making it easy to recognise.

Common Absorptions Seen in Most Organic Molecules

In addition to functional group peaks, most organic compounds show absorptions that are not diagnostic of specific functional groups but help confirm organic nature.

Common features include:

• C–H stretch near 2850–3000 cm⁻¹

• Present in alkanes, alcohols, aldehydes, ketones, and carboxylic acids

• Not used to distinguish functional groups at OCR level

These peaks should be recognised but not overinterpreted.

Using Provided Data in OCR Examinations

OCR examinations will always provide relevant IR absorption data. Students are not required to memorise exact wavenumbers but should be able to:

• Match observed peaks to supplied data

• Identify whether key functional groups are present or absent

• Use absence of peaks as evidence to eliminate possibilities

When analysing a spectrum:

• Look first for a C=O peak near 1700 cm⁻¹

• Then check for broad O–H absorptions

• Use combinations of peaks rather than isolated absorptions

Limitations of IR Spectroscopy at A-Level

IR spectroscopy has clear limitations that students must understand.

Key limitations:

• Cannot identify the full molecular structure

• Cannot distinguish between aldehydes and ketones

• Cannot identify position of functional groups within a molecule

• Only functional groups in the specification should be considered

IR spectroscopy is therefore best used alongside other analytical techniques such as mass spectrometry, but this is handled in a separate subtopic.

Exam-Focused Interpretation Skills

At OCR A-Level, success depends on recognising patterns, not analysing spectra in excessive detail.

Students should practise:

• Identifying broad versus sharp peaks

• Recognising strong absorptions

• Using logical elimination based on missing peaks

• Linking spectra directly to specification functional groups

Correct interpretation relies on careful observation and strict adherence to the functional groups defined by the specification, ensuring accurate and efficient analysis under exam conditions.