Skip to content

Spectroscopy and Synthesis -- Practice Problems

Spectroscopy and Synthesis — Practice Problems

10 MCQ questions covering IR spectroscopy, NMR spectroscopy, mass spectrometry, UV-Vis, and retrosynthetic analysis. Select an option to check your answer and view the explanation.

Spectroscopy

Retrosynthetic Analysis and Synthesis

Intuition

Spectroscopy is molecular fingerprinting: Different bonds absorb different frequencies of light. IR spectroscopy identifies functional groups, NMR reveals the carbon-hydrogen skeleton, and mass spectrometry gives molecular weight. Together, they let you determine an unknown molecule’s structure without destroying it.

Why it matters: Spectroscopy is used in drug development, forensics, environmental monitoring, and quality control. It is the primary tool for confirming that a synthesis produced the intended molecule.

The key insight: In ¹H NMR, the integration tells you how many protons, the splitting tells you what neighbours they have, and the chemical shift tells you what environment they are in.

Common Mistakes

Confusing NMR chemical shifts with coupling constants: Chemical shift (δ\delta, in ppm) indicates the electronic environment of a proton. Coupling constant (JJ, in Hz) indicates the interaction between nearby non-equivalent protons. They are different quantities with different units, a peak at δ=2.5\delta = 2.5 ppm with J=7J = 7 Hz means the chemical shift is 2.5 ppm and the splitting pattern has 7 Hz spacing.

Misidentifying IR absorption bands: The O-H stretch (broad, 2500-3300 cm−1^{-1}) and N-H stretch (sharp, 3300-3500 cm−1^{-1}) can overlap. The C=O stretch (strong, 1700 cm−1^{-1}) is one of the most reliable bands. Do not confuse C-H stretches (just below 3000 cm−1^{-1}) with O-H stretches, the shape (broad vs sharp) is the key distinguishing feature.

Forgetting protecting group compatibility in multi-step synthesis: Each protecting group has specific conditions for installation and removal. A THP ether is stable to base but labile to acid, if your next step requires acid, the protecting group will be removed prematurely. Always check that protecting group conditions are compatible with all reaction steps.

Cross-References

  • Spectroscopy: Detailed notes on NMR, IR, and mass spectrometry techniques.
  • Synthesis: Covers multi-step synthesis strategies and protecting group chemistry.
  • Practice Reaction Mechanisms: Interactive practice problems covering organic reaction mechanisms.