Determining the Number of Expected Signals in an NMR Spectrum

Key Concepts

• Chemically Equivalent Protons: Protons in the same environment and appear as one signal on an NMR spectrum.
• Symmetry: Important for determining chemical equivalence in molecules.

Examples

Methane

• Structure: CH₄
• Protons: 4 protons, all in the same environment.
• Expected Signals: 1 signal (all protons chemically equivalent).

Propane

• Protons:
  • Central carbon with 2 protons (same environment).
  • Methyl protons (6 total, from two groups, same environment by symmetry).
• Expected Signals: 2 signals (one from central carbon protons, one from methyl protons).

Complex Molecule (Example 1)

• Protons:
  • Methyl protons (same environment).
  • A proton on a carbon with a hydrogen (different environment).
• Expected Signals: 2 signals (one for methyl protons, one for the proton on the specified carbon).

Oxygen-Containing Molecule

• Protons:
  • Two protons next to oxygen (different environment from other protons).
  • Methyl protons (own environment).
  • Proton on oxygen (unique environment).
• Expected Signals: 4 signals (different environments for each set).

Symmetrical Alcohol

• Protons:
  • Two protons between CH₂ groups (same environment).
  • Protons near CH₂ and OH (same environment due to symmetry).
• Expected Signals: 3 signals.

Chiral Molecule

• Protons:
  • Chiral center affects equivalence of protons on methylene group.
• Expected Signals: 6 signals (due to chiral center affecting chemical environments).

Benzene

• Protons: 6 protons, all chemically equivalent.
• Expected Signals: 1 signal.

Oxygen-Benzene Compound

• Methyl Protons: Same environment.
• Ring Protons: Same environment due to symmetry.
• Expected Signals: 2 signals.

Cubane

• Protons: 8 protons, all equivalent.
• Expected Signals: 1 signal.

Summary

• Equivalence: Determine by the environment and symmetry; chiral centers can affect equivalence in CH₂ groups.
• Total Signals: Count distinct environments to determine the number of signals in the NMR spectrum.