Part 1: Infrared spectroscopy and its applications

Use absorption ranges as evidence for functional groups and understand why infrared absorption matters in analysis and the atmosphere.

Bonds absorb energy and vibrate more

Covalent bonds already vibrate. Absorbing appropriate infrared frequencies increases their vibrational energy; ordinary IR analysis does not break the bonds. Different bonds and molecular environments absorb in different ranges.

IR spectra commonly show percentage transmittance against wavenumber in cm⁻¹, so absorption appears as a downward trough; in absorbance plots it points upwards. Read the actual axes. Wavenumber scales often decrease from left to right. Use both the position and width of an absorption.

Use the supplied OCR data sheet

Combine presence and absence of absorptions

A broad O–H band at 3200–3600 cm⁻¹ without C=O supports an alcohol. A strong carbonyl absorption without either type of O–H can support an aldehyde or ketone. A carbonyl plus a very broad acid O–H band supports a carboxylic acid. Other carbonyl-containing groups exist, so use formula and context.

Worked comparison: candidates are propan-1-ol, propanal and propanoic acid. A spectrum with a strong band near 1720 cm⁻¹ and very broad absorption from about 2500–3300 cm⁻¹ supports propanoic acid. The C=O alone would not make that identification.

A complex lower-wavenumber fingerprint region can be compared with a reference spectrum, but a few selected functional-group peaks rarely prove one unique structure. Quote the range or value, name the bond, then infer the functional group.

Gas monitoring and infrared energy

IR instruments can identify and measure absorbing gases using calibrated absorption signals, including CO and NO in vehicle emissions and ethanol in breath. Calibration, interfering gases and sampling conditions affect quantitative interpretation.

CO₂, water vapour and methane absorb infrared radiation through molecular vibrations involving C=O, O–H and C–H bonds. Absorption and re-emission of outgoing terrestrial infrared affect Earth’s energy balance; increased greenhouse-gas concentrations contribute to warming. This motivates lower-carbon energy choices and emissions monitoring. The natural greenhouse effect and an increase in that effect are distinct.

Read a trace before searching for functional groups

Identify whether the vertical axis is transmittance or absorbance, then read the wavenumber axis. A low transmittance trough means more radiation has been absorbed there. A deeper trough does not automatically mean “more of that functional group” in an uncalibrated comparison: concentration, path length and the vibration’s absorption strength also affect the signal.

Use the supplied data sheet to match a region, not a single memorised number. Molecular environment shifts absorption positions within ranges. Note width as well as position: the broad alcohol O–H and very broad acid O–H features differ from a sharper carbonyl band.

The acid O–H region can overlap C–H absorptions near 3000 cm⁻¹. Do not require a clean separate trough for each bond before considering a carboxylic acid. Look for the broad envelope together with C=O and a compatible molecular formula.

Worked elimination of candidates using oxygen count

An illustrative compound has molecular formula C₄H₈O. Its IR evidence includes a strong carbonyl absorption near 1720 cm⁻¹ and no clear broad O–H feature. A carboxylic acid requires two oxygen atoms, so it is incompatible with this formula even before considering the missing acid O–H.

Aldehyde and ketone candidates remain. Butanal, 2-methylpropanal and butan-2-one all have C₄H₈O and a carbonyl group. The two selected IR observations alone do not distinguish these three structures. A complete answer can identify the compatible family possibilities while acknowledging that more evidence is needed.

By contrast, for a suitable C₄H₈O₂ candidate set with C=O and the characteristic very broad acid O–H region, a carboxylic acid is supported. That still does not locate branching without more evidence. Functional-group identification is not automatically whole-molecule identification.

Use a change in the spectrum as reaction evidence

For oxidation of a secondary alcohol to a ketone, an expected trend is loss of the broad alcohol O–H feature and appearance of a strong C=O absorption. If both are clearly present in the collected sample, possible explanations include residual alcohol or another OH-containing component such as water; do not declare complete conversion from the carbonyl peak alone.

For primary-alcohol oxidation, an aldehyde and a carboxylic acid both have C=O. The acid O–H region and other evidence help distinguish over-oxidation. Compare samples under suitable controlled conditions and with reference data if quantitative conclusions are required.

In gas monitoring or a breathalyser, a selected absorption signal must be calibrated against known samples to relate it to concentration. Interfering absorbers and sampling conditions need consideration. A correct bond assignment alone is not a calibrated concentration measurement.

Quick checks

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

Sources