Quick questions on NMR spectroscopy and HPLC: VCE Chemistry Unit 4

A proton NMR spectrum gives four pieces of information for each set of equivalent hydrogens (each "environment") in the molecule:

HPLC separates components of a liquid mixture for both identification and quantification.

a hydrogen environment with n equivalent neighbouring hydrogens on adjacent carbons appears as an (n+1)-multiplet. So:

A liquid (the mobile phase) is pumped at high pressure (5 to 400 bar) through a column packed with a solid (the stationary phase). The sample is injected, and components travel through the column at rates that depend on their partition between the two phases. More polar species in a non-polar (reversed-phase) column move quickly; less polar species spend more time on the stationary phase and move slowly.

Compare the retention time of an unknown peak to that of a known standard run under the same conditions. Matching Rt is consistent with the same compound (but not proof; many compounds may share an Rt).

The area under the peak is proportional to the amount of that component injected. A calibration curve of peak area against known concentrations of pure standard converts the unknown's peak area into a concentration.

separation and quantification of complex mixtures, can detect small amounts, works for non-volatile species that gas chromatography cannot handle.

similar polarity compounds may co-elute, sample preparation can be involved, expensive instrument.

A compound shows two singlets in its $^1$H NMR at $\delta = 2.1$ (3H) and $\delta = 3.7$ (3H). It also has $M^+ = 74$. (a) Suggest a structure.

Ethanol gives three peaks in $^1$H NMR. (a) State the chemical shift, multiplicity and integration ratio for each. (b) Explain the splitting using n+1.

HPLC of a mixture of acetaminophen and caffeine shows peaks at $R_t = 3.2 \, \text{min}$ (area $4200$) and $R_t = 5.8 \, \text{min}$ (area $8800$). Standards give calibration slopes $42.0$ and $22.0$ ($\text{area/(mg/L)}$). (a) Identify which $R_t$ is which.