Mass spectrometry of organic compounds (molecular ion and fragmentation), infrared spectroscopy and characteristic absorptions, and using spectra to deduce structures and monitor functional groups.

What this topic is asking

Eduqas topic C3.5 introduces the two analytical techniques used at AS level: mass spectrometry applied to organic molecules (the molecular ion and fragmentation) and infrared spectroscopy (characteristic bond absorptions). You must use spectra to find relative molecular masses, identify fragments, recognise functional groups and deduce structures. The NMR and chromatography techniques are added in the Organic Chemistry and Analysis section.

Mass spectrometry of organic molecules

Common losses to memorise: $15$ ($\text{CH}_3$), $17$ ($\text{OH}$), $18$ (water), $29$ ($\text{CHO}$ or $\text{C}_2\text{H}_5$) and $45$ ($\text{COOH}$). An $\text{M}+1$ peak arises from the small natural abundance of carbon-13.

Infrared spectroscopy

Covalent bonds absorb infrared radiation at characteristic frequencies, vibrating by stretching or bending. The wavenumber (in $\text{cm}^{-1}$) of an absorption identifies the bond. Eduqas provides a data table; the key absorptions to recognise are the broad O-H of alcohols and acids, the sharp C=O of carbonyl compounds, and the N-H of amines.

Using spectra together

The strength of these techniques is combining them. The mass spectrum fixes the molecular mass and reveals fragments; the infrared spectrum identifies the functional groups present. Together they narrow many candidate structures to one, which is exactly the synoptic skill Component 3 rewards.

Examples in context

Example 1. Detecting drugs in sport. Mass spectrometry identifies banned substances from their molecular ion and characteristic fragmentation pattern, even at tiny concentrations, because the fragmentation acts as a chemical fingerprint.

Example 2. Monitoring a reaction by infrared. As a primary alcohol is oxidised to a carboxylic acid, the appearance of a strong C=O absorption near $1700\ \text{cm}^{-1}$ confirms the reaction has occurred, letting chemists follow it without isolating the product.

Try this

Q1. In a mass spectrum, what does the molecular ion peak tell you? [1 mark]

• Cue. Its $m/z$ value gives the relative molecular mass of the compound.

Q2. State the bond responsible for a strong absorption at about $1700\ \text{cm}^{-1}$ in an infrared spectrum. [1 mark]

• Cue. The carbonyl bond, C=O (found in aldehydes, ketones, carboxylic acids and esters).