Infrared spectroscopy and the absorptions of bonds and functional groups, the fingerprint region, mass spectrometry of organic molecules including the molecular ion and fragmentation, and the use of both to identify and determine the structure of compounds.

What this dot point is asking

CCEA wants you to explain how infrared spectroscopy identifies bonds and functional groups, recognise the fingerprint region, interpret the mass spectra of organic molecules including the molecular ion and fragmentation, and use both techniques together to determine the structure of a compound.

Infrared spectroscopy

The fingerprint region below about $1500\ \text{cm}^{-1}$ contains a complex pattern unique to each molecule, so a spectrum can be matched against a database to confirm identity. The reason bonds absorb at characteristic frequencies is that each bond behaves like a spring connecting two masses: stronger bonds and lighter atoms vibrate at higher frequencies, so they absorb at higher wavenumbers. This is why a C-H bond absorbs higher than a C-Cl bond. In practice, candidates use a data sheet of characteristic absorptions: the presence or absence of the broad O-H, the sharp C=O, and the C-H stretches is usually enough to identify the functional groups present, while the fingerprint region confirms the exact molecule.

Mass spectrometry of organic molecules

When the molecular ion breaks apart, it gives fragment ions. The mass differences between peaks reveal the groups lost, for example a loss of $15$ indicates a $\text{CH}_3$ group, a loss of $17$ indicates $\text{OH}$, a loss of $29$ indicates $\text{CHO}$ or $\text{C}_2\text{H}_5$, and a loss of $45$ indicates $\text{COOH}$. Equally useful is the mass of the fragment itself: a peak at $m/z = 15$ is $\text{CH}_3^+$, at $29$ is $\text{CHO}^+$ or $\text{C}_2\text{H}_5^+$, at $43$ is $\text{CH}_3\text{CO}^+$ (the acylium ion from a methyl ketone), and at $77$ is $\text{C}_6\text{H}_5^+$ (a phenyl group). Small peaks one or two mass units above the molecular ion (the M+1 and M+2 peaks) come from the heavier isotopes such as carbon-13 and chlorine-37, and the size of an M+2 peak can indicate the presence of chlorine or bromine.

Combining the techniques

Examples in context

Example 1. Monitoring vehicle emissions and breath alcohol. Modern roadside breath testers and laboratory emission analysers use infrared absorption: ethanol and carbon dioxide each absorb infrared at their own characteristic wavenumbers, so the instrument measures how much radiation of those frequencies is absorbed and converts it directly to a concentration. This is the everyday application of the idea that each bond absorbs at a fixed frequency, the same principle a CCEA candidate uses to read a laboratory spectrum.

Example 2. Confirming the structure of a synthesised product. When a chemist makes a new compound, mass spectrometry gives the relative molecular mass from the molecular ion peak and clues to the structure from the fragments, while infrared confirms which functional groups are present. Together they distinguish isomers that share a formula: ethanol and methoxymethane both have $\text{M}^+ = 46$, but only ethanol shows the broad O-H absorption and the $m/z = 31$ fragment. This combined-technique approach is exactly what CCEA structure-determination questions assess.

Try this

Q1. State what the molecular ion peak tells you about a compound. [1 mark]

• Cue. Its mass-to-charge ratio equals the relative molecular mass.

Q2. State the approximate wavenumber of the C=O absorption in infrared spectroscopy. [1 mark]

• Cue. About $1700\ \text{cm}^{-1}$.