Lecture Notes: Understanding IR Spectrum and Bond Stretching

Introduction to IR Spectrum for Dibutylamines

• IR Spectrum Analysis: Used to identify signals related to bond stretches.
• Structure Example: Dibutylamine with a focus on carbon-hydrogen bond stretching.

Key Concepts in Wave Numbers

• SP3 Hybridized Carbon: Carbon-hydrogen bond stretch is expected to the right of 3000 cm⁻¹.
• Nitrogen-Hydrogen Bond Stretch: Found at approximately 3300 cm⁻¹, indicating a stronger bond than carbon-hydrogen due to a higher wave number.

Factors Affecting Bond Strength in IR

• Wave Number Dependents:
  • Force constant (k): Influences the wave number significantly.
  • Reduced mass: Has a similar value for N-H and C-H bonds.
• Energy Implications:
  • Energy (E) is proportional to frequency (ν) and wave number.
  • Stronger bonds require more energy to stretch, hence appear at higher wave numbers.

Primary vs. Secondary Amines in IR Spectrum

• Secondary Amines:
  • One signal at approximately 3300 cm⁻¹.
  • Nitrogen bonded to two carbons, indicating N-H bond stretch.
• Primary Amines:
  • Two signals due to symmetric (lower wave number) and asymmetric (higher wave number) stretching.

Symmetric vs. Asymmetric Stretching

• Symmetric Stretch: Bonds stretch in phase, requiring less energy.
• Asymmetric Stretch: Bonds stretch out of phase, requiring more energy, hence appearing at higher wave numbers.

Application to Carbon-Hydrogen and Acid Anhydrides

• CH2 Stretching:
  • Exhibits both symmetric and asymmetric stretching patterns.
  • Asymmetric stretching appears at slightly higher wave numbers.
• Acid Anhydride Example:
  • Strong signals around 1760 and 1800 cm⁻¹ in the double bond region.
  • Due to symmetric and asymmetric stretching of carbonyl groups.

Summary

• IR Spectrum Analysis: Essential for identifying bond types and strengths in molecules.
• Symmetric and Asymmetric Stretching: Key in understanding different signals in an IR spectrum.
• Practical Application: Helps in identifying functional groups and bond strengths in organic compounds.