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VICChemistryQuick questions
Unit 4: How are organic compounds categorised, analysed and used?
Quick questions on NMR spectroscopy and HPLC: VCE Chemistry Unit 4
15short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is proton (^1H) NMR?Show answer
A proton NMR spectrum gives four pieces of information for each set of equivalent hydrogens (each "environment") in the molecule:
What is carbon-13 (^13C) NMR?Show answer
Detects the natural ^13C isotope (1.1% abundance) instead of ^1H. The output is simpler:
What is how to interpret a ^1H NMR spectrum (workflow)?Show answer
1. Count the number of signals = number of H environments. 2. Read each chemical shift and look it up in your table to suggest the type of H.
What is high performance liquid chromatography (HPLC)?Show answer
HPLC separates components of a liquid mixture for both identification and quantification.
What is 4. Splitting follows the n+1 rule?Show answer
a hydrogen environment with n equivalent neighbouring hydrogens on adjacent carbons appears as an (n+1)-multiplet. So:
What is principle?Show answer
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.
What is use 1: identification?Show answer
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).
What is use 2: quantification?Show answer
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.
What is strengths?Show answer
separation and quantification of complex mixtures, can detect small amounts, works for non-volatile species that gas chromatography cannot handle.
What is limitations?Show answer
similar polarity compounds may co-elute, sample preparation can be involved, expensive instrument.
What is confusing number of H atoms with number of environments?Show answer
Ethanol has 6 hydrogens but only 3 environments (CH3, CH2, OH).
What is forgetting OH and NH do not always split neighbours?Show answer
They usually appear as broad singlets and do not split adjacent CH protons (rapid exchange).
What is reading peak heights instead of peak areas?Show answer
Always use the integration (area) for the H count, not the height.
What is mis-applying the n+1 rule?Show answer
The "n" is the number of H on adjacent carbons, not on the same carbon. CH2 protons on the same carbon are equivalent and do not split each other.
What is forgetting to scale the integration to molecular formula?Show answer
Integration ratios are relative; multiply by the total H count to get the actual numbers.