7B.2 - Infrared Spectroscopy

What An IR Spectrum Can Prove

Two colourless organic liquids may look identical in a sample vial. One could be an alcohol, one could be a ketone, and a third unknown could be a carboxylic acid. Infrared spectroscopy helps because different bonds absorb infrared radiation at characteristic wavenumber ranges when their vibrations interact with the radiation.

An IR spectrum is usually plotted as transmittance against wavenumber:

The key move is not "spot a peak and name a compound". The key move is:

absorption wavenumber → matching Data Booklet range → bond or functional group → conclusion with limits

For example, a strong absorption near $1710 \mathrm{cm^{-1}}$ is evidence for a $\ce{C=O}$ bond, but several carbonyl-containing functional groups can absorb in that region. A broad absorption from about $3300-2500 \mathrm{cm^{-1}}$ together with a $\ce{C=O}$ absorption is much stronger evidence for a carboxylic acid.

Using The Data Booklet Ranges

Use the IR absorption table in the Data Booklet. You do not need to memorise every number perfectly, but you do need to recognise the characteristic regions and quote the relevant range when data are supplied.

Some ranges overlap. That is why a good answer normally says more than "there is a peak at $1710 \mathrm{cm^{-1}}$". It names the evidence and rejects alternatives where the spectrum allows it.

Common-error contrast

$1710 \mathrm{cm^{-1}}$ does not automatically mean "ketone". It lies in the ketone $\ce{C=O}$ range, but it can also match a carboxylic-acid $\ce{C=O}$ range. If the spectrum also has a very broad $3300-2500 \mathrm{cm^{-1}}$ absorption, the better deduction is carboxylic acid.

Worked Deduction From An Unknown Spectrum

The spectrum below is a deterministic teaching spectrum for an unknown organic compound. Treat it as a data stimulus: read the labelled absorptions first, then decide what they prove.

[DIAGRAM: asset_name: IR Unknown Carboxylic Acid Spectrum; asset_slug: edexcel_a_level_chemistry_l041_ir_unknown_carboxylic_acid_spectrum; recommended_method: deterministic_chart; description: Infrared spectrum of an unknown organic compound showing a very broad O-H absorption from 3300 to 2500 cm^-1, a strong C=O absorption near 1710 cm^-1, and alkyl C-H absorptions near 2960 to 2850 cm^-1.]

Worked example

Question: An unknown organic compound has a very broad absorption from about $3300-2500 \mathrm{cm^{-1}}$, a strong absorption at $1712 \mathrm{cm^{-1}}$, and absorptions around $2960-2850 \mathrm{cm^{-1}}$. Deduce the functional group present and justify your answer.

Model route:

1. The very broad absorption from $3300-2500 \mathrm{cm^{-1}}$ matches the $\ce{O-H}$ stretch of a carboxylic acid.
2. The strong absorption at $1712 \mathrm{cm^{-1}}$ matches a $\ce{C=O}$ stretch, and it lies in the carboxylic-acid carbonyl range for an alkyl carboxylic acid.
3. The absorptions around $2960-2850 \mathrm{cm^{-1}}$ are consistent with alkane $\ce{C-H}$, so the molecule may contain an alkyl part.
4. The acid $\ce{O-H}$ and $\ce{C=O}$ evidence together support the presence of a carboxylic acid group, $\ce{-COOH}$.

Chemical interpretation: IR strongly supports a carboxylic-acid functional group here. It does not, by itself, tell you whether the compound is methanoic acid, ethanoic acid, propanoic acid, or another carboxylic acid. A full structure needs other evidence, such as molecular mass, formula, NMR data or comparison with a reference spectrum.

Guided practice

An unknown has a strong absorption at $1728 \mathrm{cm^{-1}}$, weak absorptions at $2825 \mathrm{cm^{-1}}$ and $2725 \mathrm{cm^{-1}}$, and alkyl $\ce{C-H}$ absorptions near $2960 \mathrm{cm^{-1}}$. There is no broad $\ce{O-H}$ absorption.

Complete the reasoning:

Expected conclusion: the spectrum gives evidence for an aldehyde functional group. The aldehyde $\ce{C-H}$ absorptions separate it from a ketone, which would have a carbonyl absorption but not the aldehyde $\ce{C-H}$ pair.

Predicting Absorptions From Structures

Prediction is the reverse of deduction. Start with the structure, list the bonds and functional groups present, then state the absorption range and any important shape.

Prediction route

structure → functional groups and key bonds → Data Booklet ranges → expected IR absorptions

Worked prediction: pentanal and pentan-2-one

Pentanal and pentan-2-one both contain a carbonyl group, so both should have a strong $\ce{C=O}$ absorption near $1700-1740 \mathrm{cm^{-1}}$. The distinction is the aldehyde hydrogen.

For pentanal, predict:

• $\ce{C=O}$: $1740-1720\,\mathrm{cm^{-1}}$
• aldehyde $\ce{C-H}$: $2900-2820$ and $2775-2700\,\mathrm{cm^{-1}}$
• alkyl $\ce{C-H}$: $2962-2853\,\mathrm{cm^{-1}}$

For pentan-2-one, predict:

• $\ce{C=O}$: $1720-1700\,\mathrm{cm^{-1}}$
• alkyl $\ce{C-H}$: $2962-2853\,\mathrm{cm^{-1}}$
• no aldehyde $\ce{C-H}$ pair expected

Chemical interpretation: the carbonyl peak alone says both are possible carbonyl compounds. The aldehyde $\ce{C-H}$ pair is the extra IR evidence that points to an aldehyde rather than a ketone.