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Understanding IR Spectroscopy of Carbonyls

Aug 13, 2024

Lecture Notes: IR Spectroscopy of Carbonyl Compounds

Key Concepts

  • IR Spectrum: Importance of analyzing waves and frequencies to determine molecular structures.
  • Carbonyl Group: A functional group composed of a carbon atom double-bonded to an oxygen atom (C=O).

Ketones

  • Signal Strength: Strong IR signal due to significant separation of charge (oxygen partially negative, carbon partially positive).
  • Wave Number: Appears at approximately 1715 or 1720.
  • Conjugated Ketones: Lower wave number due to resonance; signal appears around 1680.

Resonance in Carbonyl Compounds

  • Effect of Resonance: Lowers the double bond character, weakening the carbonyl, decreasing bond strength (K), and thus, reducing wave number.
  • Resonance Hybrid: The actual structure is a mix between single and double bond characters.

Electronegativity and Carbonyls

  • Carboxylic Acid Derivatives: Electronegative atoms like oxygen can create resonance structures.
  • Inductive Effect vs. Resonance: Induction refers to electron withdrawal due to electronegativity, increasing bond strength and wave number.

IR Signals of Various Carbonyl Compounds

General Range

  • Wave Number Range: 1650 to 1850; average around 1750.

Specific Compounds

  • Esters: Inductive effect stronger than resonance; wave number around 1745.
  • Acid Anhydrides: Strong inductive effect, two signals due to symmetric and asymmetric stretches; wave numbers at 1760-1790 and approximately 1810.
  • Acyl Chlorides: Dominant inductive effect; stronger carbonyl, wave number around 1815.
  • Aldehydes vs. Ketones: Aldehydes have lower wave numbers (~1725) due to electron donation by alkyl groups; ketones are slightly lower (~1715-1720).
  • Carboxylic Acids: Strong hydrogen bonding weakens carbonyl, signal around 1710.
  • Amides: Resonance dominates, significantly weakening carbonyl; wave number around 1650-1690.

Competing Effects

  • Resonance lowers signal by decreasing bond strength.
  • Induction increases signal by enhancing bond strength.

Conclusion

Understanding the effects of resonance, inductive forces, electronegativity, and hydrogen bonding helps predict the IR signals for various carbonyl compounds, aiding in identifying molecular structures.

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