Understanding IR Spectroscopy Revision Notes | A Level AQA Chemistry

1. Why Molecules Absorb Infrared Radiation

Every covalent bond in a molecule is constantly vibrating — stretching and bending even at room temperature. When infrared (IR) radiation hits a molecule, bonds absorb radiation at specific frequencies that match their natural vibration frequency, causing those vibrations to become more intense.

The key insight is that different bond types vibrate at different frequencies. A C=O bond is stiffer and involves different masses than an O–H bond, so it absorbs IR radiation at a different wavenumber. This is what makes IR spectroscopy so powerful: each bond type leaves a characteristic fingerprint on the spectrum.

Think of it like a tuning fork: each fork only resonates at its own frequency. A C–H bond only resonates with IR radiation near 3000 cm⁻¹, while a C=O bond resonates near 1700 cm⁻¹. The spectrum records which frequencies are absorbed, and from that you can read off which bonds are present.

2. Reading an IR Spectrum: The Two Key Regions

An IR spectrum is divided into two regions that serve different purposes in analysis. Knowing which region to look at first is a core exam skill.

In an exam, you will almost always be asked to work in the functional group region first — identifying which bonds are present from the position and shape of absorptions. The fingerprint region is too complex to assign individual peaks, but it is invaluable for confirming that two spectra belong to the same compound.

3. Characteristic Absorptions: Using the Data Sheet

In your AQA exam, you will always have access to the Chemistry Data Sheet, which lists characteristic IR absorption wavenumbers for key bond types. You do not need to memorise exact values — but you do need to know how to read the table and apply it confidently to a spectrum.

The table below summarises the most important absorptions you will encounter. Notice that the description of each absorption includes both its position and its shape — both details matter when assigning peaks.

Two features of the O–H absorption are worth paying close attention to. An alcohol O–H gives a broad absorption around 3200–3550 cm⁻¹, while a carboxylic acid O–H gives an even broader, more diffuse absorption that can extend all the way down to around 2500 cm⁻¹. The carboxylic acid spectrum will also show a strong C=O absorption near 1700–1725 cm⁻¹ — seeing both together is strong evidence for –COOH.

4. Identifying Functional Groups from a Spectrum

Now that you know what to look for, here is a reliable method for working through an IR spectrum in an exam. The process is systematic: scan the functional group region from left to right, note any significant absorptions, and cross-reference with the Data Sheet.

Worked example: Identifying Functional Groups from an IR Spectrum

An IR spectrum shows the following absorptions: a broad, strong absorption at approximately 3300 cm⁻¹, a medium absorption near 2950 cm⁻¹, and a strong, sharp absorption at 1715 cm⁻¹. Identify the functional groups present and suggest what type of compound this could be.

1. Broad, strong absorption at ~3300 cm⁻¹ — A broad, strong absorption in the range 3200–3550 cm⁻¹ is characteristic of an O–H stretch in an alcohol. If it were a carboxylic acid O–H, the absorption would be even broader and extend further down towards 2500 cm⁻¹. An N–H absorption at this position would typically be medium in intensity and may appear as a doublet for a primary amine.
2. Medium absorption at ~2950 cm⁻¹ — A medium absorption near 2950 cm⁻¹ indicates C–H stretching. This is present in virtually all organic molecules, so it confirms the compound is organic but does not distinguish between functional groups on its own.
3. Strong, sharp absorption at 1715 cm⁻¹ — A strong, sharp absorption at 1715 cm⁻¹ is characteristic of a C=O stretch. The exact position within the carbonyl region helps narrow down the functional group: aldehydes and ketones typically absorb around 1700–1750 cm⁻¹, while carboxylic acids absorb around 1700–1725 cm⁻¹.
4. O–H (alcohol, ~3300 cm⁻¹) + C=O (carbonyl, 1715 cm⁻¹) — The presence of both an alcohol O–H and a C=O group in the same molecule is consistent with a hydroxy carbonyl compound. However, if the O–H were broader and extended to lower wavenumbers, a carboxylic acid would be more likely. Here, the relatively narrow O–H at 3300 cm⁻¹ alongside the C=O suggests this could be a hydroxy ketone or hydroxy aldehyde, or alternatively the compound could be a carboxylic acid where the O–H is not as broad as typical — further evidence (such as NMR or the molecular formula) would be needed to confirm.
5. Functional groups identified: O–H (alcohol) and C=O (carbonyl) — In an exam, always state the bond type, its approximate wavenumber, and the functional group it indicates. For example: 'The broad absorption at ~3300 cm⁻¹ indicates an O–H bond, suggesting an alcohol group. The strong, sharp absorption at 1715 cm⁻¹ indicates a C=O bond, suggesting a carbonyl-containing functional group such as a ketone or aldehyde.'

Answer: The spectrum shows an O–H bond (alcohol, broad absorption ~3300 cm⁻¹) and a C=O bond (carbonyl, strong sharp absorption at 1715 cm⁻¹). The compound contains both an alcohol and a carbonyl functional group.

5. Identifying Impurities Using IR Spectroscopy

IR spectroscopy is not only used to identify compounds — it is also a powerful tool for detecting impurities. This is one of the most practically important applications of the technique and appears regularly in AQA exam questions.

The logic is straightforward: if a pure compound has a known IR spectrum, any additional absorption peaks in the spectrum of a sample indicate the presence of a different bond type — and therefore a different compound acting as an impurity. For example, if you synthesise an ester and the product spectrum shows a broad O–H absorption around 3300–3550 cm⁻¹, this suggests the sample is contaminated with unreacted alcohol starting material.

A particularly common scenario in AQA questions involves the oxidation of a primary alcohol to a carboxylic acid. If the reaction is incomplete, the product spectrum may show the C=O absorption of the carboxylic acid plus an additional O–H absorption around 3200–3550 cm⁻¹ from residual alcohol. That alcohol O–H peak is typically narrower and at higher wavenumber than the very broad carboxylic acid O–H absorption that extends down towards 2500 cm⁻¹.

Water is another common impurity that can be detected by IR spectroscopy. A broad O–H absorption in the spectrum of a compound that should not contain an O–H group may indicate that the sample is wet. This is why drying agents are used in organic synthesis before taking an IR spectrum of the product.

6. Comparing Spectra: Ethanol and Ethanoic Acid

One of the most instructive comparisons you can make is between the IR spectra of ethanol and ethanoic acid. Both contain an O–H group, but the spectra look noticeably different — and understanding why is exactly the kind of analysis AQA rewards at A Level.

The reason the carboxylic acid O–H absorption is so much broader and lower in wavenumber than an alcohol O–H is that carboxylic acids form strong hydrogen bonds in dimers — pairs of molecules held together by two O–H···O hydrogen bonds. This extensive hydrogen bonding weakens and broadens the O–H absorption significantly. In an exam, describing the O–H absorption of a carboxylic acid as merely ‘broad’ is not enough — you should say it is very broad and extends to lower wavenumbers than an alcohol O–H.

7. Exam Technique and Common Mistakes

IR spectroscopy questions follow predictable patterns in AQA exams. The marks are almost always awarded for three things: naming the bond, giving its approximate wavenumber, and linking it to a functional group. Missing any one of these three elements is the most common reason students drop marks.

When a question asks you to ‘use the IR spectrum to identify the functional groups present’, structure your answer like this: state the bond type, give the wavenumber range from the spectrum (not just from memory), describe the shape of the absorption, and name the functional group it indicates. This four-part structure consistently picks up full marks.

IR spectroscopy sits at the heart of organic analysis at A Level: every bond absorbs at a characteristic wavenumber, and reading those absorptions systematically — using the Data Sheet, noting peak shape, and combining evidence — allows you to identify functional groups, confirm compound identity, and detect impurities with confidence.