Mass spectrometry, infrared spectroscopy, proton and carbon-13 NMR, and chromatographic separation in structure determination.

What this dot point is asking

WJEC wants you to use mass spectrometry, infrared spectroscopy, proton and carbon-13 NMR, and chromatography together to determine the structure of an unknown organic compound.

The answer

Mass spectrometry

Infrared spectroscopy

NMR

Proton ($^1\text{H}$) NMR gives the number of different proton environments (number of peaks), the relative numbers in each (integration), and neighbouring protons (splitting, by the $n+1$ rule). Carbon-13 NMR gives the number of different carbon environments.

Chromatography

Chromatography separates a mixture between a stationary and a mobile phase; components are identified by their retention factor ($R_f$) in TLC or retention time in gas chromatography, often coupled to mass spectrometry (GC-MS) for identification.

A strategy for an unknown

Markers reward a logical order of reasoning. Work from the mass spectrum first to fix the molecular mass and likely formula, then use infrared to identify functional groups, then use NMR to count and place the carbon and hydrogen environments. Each technique narrows the possibilities until only one structure remains. For example, a molecular ion at $46$ with a broad $\text{O}-\text{H}$ infrared band and two proton environments points to ethanol rather than its isomer methoxymethane, which would show only one proton environment.

NMR splitting and the n+1 rule

In high-resolution proton NMR, a peak is split into a number of lines equal to one more than the number of hydrogens on the adjacent carbon (the $n+1$ rule). A $\text{CH}_3$ next to a $\text{CH}_2$ appears as a triplet ($2+1$), and the $\text{CH}_2$ appears as a quartet ($3+1$). This pattern reveals which groups are next to each other, which is why the ethyl group gives its characteristic triplet-and-quartet signature.

Examples in context

Drug testing by GC-MS. Forensic and anti-doping labs separate complex mixtures by gas chromatography then identify each component by its mass spectrum, the combined power of the two techniques. MRI from NMR. Medical magnetic resonance imaging is built on the same nuclear magnetic resonance physics used to read proton NMR spectra in the lab.

Try this

Q1. State what the molecular ion peak in a mass spectrum tells you. [1 mark]

• Cue. The relative molecular mass of the compound.

Q2. State the approximate infrared absorption of a $\text{C}=\text{O}$ bond. [1 mark]

• Cue. Around $1700$ cm$^{-1}$.

Q3. State what the number of peaks in a proton NMR spectrum represents. [1 mark]

• Cue. The number of different proton (hydrogen) environments.

Q4. State the splitting pattern expected for the $\text{CH}_2$ protons in an ethyl group. [1 mark]

• Cue. A quartet ($3+1$, from the three hydrogens on the adjacent $\text{CH}_3$).

Q5. A mass spectrum loses $15$ from the molecular ion. Suggest the group lost. [1 mark]

• Cue. A methyl group, $\text{CH}_3$.