Lecture Notes on IR Spectroscopy of Carbonyl Compounds

Key Concepts

• IR Spectroscopy: A technique used to identify functional groups in molecules by detecting vibrations of bonds.
• Carbonyl Group: A functional group composed of a carbon atom double-bonded to an oxygen atom (C=O).
• Wave Number: The number of waves per unit distance, used in IR spectroscopy to identify the frequency of a bond vibration.

Carbonyl Compounds in IR Spectroscopy

Ketones

• IR Signal: Strong signal due to large dipole moment change.
• Wave Number: ~1715-1720 cm⁻¹
• Conjugated Ketones: Resonance decreases double bond character, reducing wave number to ~1680 cm⁻¹ due to weakening of the bond.

Resonance and Induction

• Resonance: Delocalization of electrons that weakens double bond character, lowering wave numbers.
• Induction: Electronegativity differences increase bond strength, raising wave numbers.

Carboxylic Acid Derivatives

• General Structure: Carbonyl group bonded to an electronegative atom (e.g., oxygen) or group.
• Resonance Effect: Decreases bond strength and wave number.
• Inductive Effect: Increases bond strength and wave number.

Specific Compounds

1. Esters

   • Wave Number: ~1745 cm⁻¹
   • Effect: Induction slightly stronger than resonance.

2. Acid Anhydrides

   • Symmetrical Stretch Wave Number: ~1760-1790 cm⁻¹
   • Asymmetrical Stretch Wave Number: ~1810 cm⁻¹
   • Inductive Effect: Stronger than resonance.

3. Acyl/Acid Chlorides

   • Wave Number: ~1800-1815 cm⁻¹
   • Chlorine's Electronegativity: Dominates resonance effect.

4. Aldehydes

   • Wave Number: ~1725 cm⁻¹
   • Inductive Effect: Minimal due to absence of electronegative atom, alkyl group electron donation.

5. Carboxylic Acids

   • Wave Number: ~1710 cm⁻¹ (Dimeric Form)
   • Hydrogen Bonding: Weakens carbonyl.

6. Amides

   • Wave Number: ~1650-1690 cm⁻¹
   • Resonance Dominates: Weakens carbonyl more than induction.

Summary

• IR spectroscopy is crucial for identifying carbonyl compounds by their IR signals.
• Resonance and induction are competing effects influencing the IR signals of carbonyl compounds.
• Understanding these concepts allows for prediction of wave numbers and carbonyl strength in various compounds.