Part 3: Infrared spectra and combined identification

Assign absorptions using the AQA data, compare fingerprints and combine independent clues without claiming more certainty than the evidence supports.

What an IR absorption means

Molecular bonds vibrate. Infrared radiation is absorbed when its frequency matches an allowed vibrational transition involving a change in dipole moment. Different bond environments absorb over characteristic wavenumber ranges. IR provides evidence about bonds and functional groups; it does not directly count every atom.

Wavenumber is measured in cm⁻¹. On a transmittance spectrum, absorption appears as a downward trough because less radiation passes through at that wavenumber. On an absorbance plot, absorption appears upwards. Read the axis labels before interpreting the trace.

Use the supplied absorption table

The ranges below are from AQA’s data booklet, Table A. Actual peak positions and shapes vary with molecular environment and measurement conditions. In an exam, quote the relevant measured absorption or range and name the bond responsible.

Functional groups narrow the field; fingerprints identify matches

The complex region below about 1500 cm⁻¹ contains many vibrations whose overall pattern helps distinguish compounds. Compare the unknown spectrum with a reference spectrum of a known pure compound under compatible conditions, looking for a matching pattern rather than a single shared trough.

Two isomers can have the same molecular mass and the same broad functional-group features but different fingerprints. Conversely, IR is not guaranteed to distinguish every possible stereochemical pair. The conclusion must fit what the technique and reference evidence actually show.

For an oxidation product expected to be a ketone, an unexpected broad OH absorption could indicate residual alcohol or water. It is evidence to investigate, not automatic proof of one specific contaminant. A lack of visible impurity peaks also does not establish absolute purity below all detection limits.

Worked identification: use every clue

An unknown has molecular formula C₃H₆O and a strong absorption near 1720 cm⁻¹, but no broad OH absorption. It gives a silver mirror with Tollens’ reagent. C=O is supported by IR and the positive oxidation test supports an aldehyde. Within this simple formula and candidate set, propanal, CH₃CH₂CHO, fits.

Propanone has the same molecular formula and also absorbs in the carbonyl region, but it does not give the normal positive Tollens’ test. Molecular mass and a carbonyl peak alone would leave both candidates possible.

For C₄H₈O with the same aldehyde evidence, both butanal and 2-methylpropanal remain possible. A reference fingerprint or additional structural evidence is needed. Stating that limitation is better chemistry than inventing a unique answer. NMR provides additional structural information later in the A-level course; it is a separate specification topic.

IR absorption also explains greenhouse behaviour

CO₂, methane and water vapour have IR-active vibrations and absorb some outgoing infrared radiation emitted by Earth’s warmed surface. They also emit infrared radiation. Increasing greenhouse-gas concentrations changes how energy escapes to space.

A permanent molecular dipole is not required: CO₂ is symmetric overall, but some vibrations change its dipole moment and interact with IR. Incoming sunlight and outgoing terrestrial infrared radiation are different parts of the energy picture. Do not say that greenhouse gases simply stop all radiation escaping.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources