Lecture on IR Spectroscopy and Hybridization

Introduction

• Review of carbon-hydrogen bond.
• Calculated approximate wavenumber for C-H bond stretch: slightly over 3000 wavenumbers.
• Wavenumber depends on the hybridization state of carbon.

Carbon Hybridization and IR Signals

sp-hybridized Carbon

• sp-hybridized carbon (triple bond):
  • C-H bond stretch signal at ~3300 wavenumbers.
  • 50% s character; electron density closest to the nucleus.
  • Shortest and strongest bond.

sp2-hybridized Carbon

• sp2-hybridized carbon (double bond):
  • C-H bond stretch signal at ~3100 wavenumbers.
  • 33% s character.
  • Stronger than sp3 but weaker than sp hybridized bonds.

sp3-hybridized Carbon

• sp3-hybridized carbon (single bond):
  • C-H bond stretch signal at ~2900 wavenumbers.
  • 25% s character.
  • Longest and weakest bond among the three.

Explanation of Different Wavenumbers

• Hybridization affects bond strength and hence, the force constant (k) of the bond.
• Stronger bond → Higher k → Higher frequency and wavenumber.

Comparing IR Spectra

Alkanes vs. Alkynes

• Alkanes (e.g., Decane):

  • Only sp3-hybridized C-H bonds appear under 3000 wavenumbers.
  • Diagnostic region shows only sp3 C-H bonds.

• Alkynes (e.g., 1-Octyne):

  • Line at 3000 distinguishes sp3-H and sp-H bonds.
  • Signal close to 3300 wavenumbers indicates sp-H bond (triple bond).
  • Additional signal at ~2100 wavenumbers indicates carbon-carbon triple bond stretch.

Alkenes (e.g., 1-Hexene)

• 3100 wavenumber signal indicates sp2-H bond presence.
• 1650 wavenumber signal for carbon-carbon double bond stretch.

Arenes (e.g., Toluene)

• Line around 3000 shows sp3 vs. sp2 hybridization.
• Aromatic C-C double bonds show below 1600 wavenumbers.
• Subtle differences in spectra help distinguish from alkenes.

Conclusion

• Understanding hybridization assists in IR spectrum analysis.
• Identifying bond types and hybridization states is crucial for interpreting signals.