Lecture 9-6: Proton NMR - Final Aspects

Topics Covered

• Alcohol protons in proton NMR
• Application of proton NMR to solve the structure of an unknown organic compound
• Preview of Carbon-13 NMR

Alcohol Protons in NMR Spectrum

• Ethanol Example:
  • Expect 3 signals:
    • Unique methyl group
    • Unique CH2 group
    • Proton bonded to oxygen
  • OH group signal typically around 3 ppm (sometimes closer to 4).
  • OH proton is acidic and typically does not show splitting.
• Acidic Proton Chemistry:
  • Acidic protons, like OH, do not influence adjacent protons due to rapid exchange.
  • Addition of D2O can exchange OH proton with deuterium (OD) causing the OH signal to disappear in NMR.

Review of Key Proton NMR Concepts

• Chemical equivalence of hydrogen atoms (protons)
• Chemical shift scale: 0 to 13 ppm
• Signal splitting: Interaction of magnetic fields of adjacent hydrogens
• Alcohol protons:
  • Do not split adjacent protons
  • Exchange occurs with D2O

Steps for Structure Identification Using NMR

• Start with the Formula:
  • Determine hydrogen efficiency
  • Analyze what the formula indicates about possible functional groups
• IR Spectrum Analysis:
  • Identify obvious functional groups using IR correlation table
• NMR Spectrum Analysis:
  • Count and label the number of signals (A, B, C, etc.)
  • Match sum of protons with the given formula
  • Identify diagnostic chemical shifts (signals at 4.5 ppm or greater)
  • Use correlation tables to verify functional groups
  • Check for signal disappearance with D2O for acidic protons
  • Analyze splitting patterns to deduce structural fragments
• Assembly of Structure:
  • Combine structural fragments from splitting patterns to form complete molecule.

Next Steps

• Practice applying proton NMR to solve unknown structures in class.
• No quiz on this topic.
• Use provided formula, IR, and summarized NMR data to deduce structure.

Conclusion

• This lecture concludes the technical aspects of proton NMR.
• Introduction to Carbon-13 NMR will follow in upcoming sessions.