4.2.4 Analytical techniques

Covalent bonds are not rigid sticks: they vibrate. When a molecule absorbs infrared radiation of the right frequency, the vibration of a bond can increase in energy.

Different bonds and molecular environments absorb at different wavenumbers, which is why an IR spectrum can give you evidence about which bonds and functional groups are present.

When you think about IR absorption, remember that suitable bond vibrations can take up particular frequencies of infrared radiation. Molecules containing bonds such as C=O, O–H and C–H can therefore absorb IR and gain vibrational energy.

That matters for greenhouse warming because Earth emits infrared radiation. Greenhouse gases absorb some of that outgoing IR and can re-emit energy in different directions, reducing the rate at which energy escapes to space. Keep the explanation about IR absorption and re-emission; do not say the gas simply “traps heat” with no mechanism.

When you identify these familiar functional groups from IR, look for combinations of the broad O–H region and the strong C=O absorption. An alcohol has an O–H absorption but no carbonyl; an aldehyde or ketone has C=O without the broad acid O–H; a carboxylic acid has both C=O and a very broad O–H absorption.

Use the OCR Data Sheet for the exact ranges in an exam. The shape matters too: the carboxylic-acid O–H absorption is broader and extends lower than the alcohol O–H region.

• alcohol: O–H present, C=O absent
• aldehyde/ketone: C=O present, acid O–H absent
• carboxylic acid: C=O + very broad O–H

You do not need to rely on memorised wavenumber ranges when the question supplies the OCR data. Match a significant absorption to the range, then ask whether that bond makes sense in the proposed structure.

Treat IR as evidence, not as a one-peak naming machine. A strong C=O absorption narrows the possibilities; the presence or absence of O–H and other supplied regions helps you decide which functional group fits.

IR can identify and measure substances because particular bonds absorb at characteristic wavenumbers. In air monitoring, an instrument can look for absorptions associated with pollutant molecules; in breath analysis, IR absorption can be used to estimate ethanol concentration.

The useful idea is selectivity: you choose wavelengths where the substance of interest absorbs and relate the amount of absorption to how much is present.

When you read an organic mass spectrum, the molecular ion represents the whole molecule after it has lost one electron. For the usual singly charged molecular ion, its m/z value gives the relative molecular mass, Mᵣ.

Look for the molecular-ion peak at the appropriate high-m/z end of the spectrum, but do not automatically assume the tallest peak is the molecular ion. The tallest peak is simply the base peak.

When the molecular ion breaks apart, some fragments keep the positive charge and appear as peaks. Use the m/z value to suggest a plausible charged fragment whose relative mass matches the peak.

I find it useful to work both ways: calculate the mass of a fragment you can see in the structure, or subtract a neutral loss from Mᵣ and ask what charged piece remains. Keep the charge on the fragment you are assigning to the peak.

For a structure problem, let each technique do the job it is good at. Elemental analysis can give an empirical formula; the molecular ion can give Mᵣ and hence the molecular formula; fragmentation suggests pieces of the carbon skeleton; IR tells you about bonds and functional groups.

Then make one structure that satisfies all the evidence. Before you finish, go back through the clues and check them against the structure you have drawn. If one fragment or IR absorption cannot come from your final molecule, the structure is not finished yet.