Carbonyl - compounds - IR, Lecture notes of Organic Chemistry

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IR – Spectroscopy

Part II

Carbonyl - compounds

For simple aldehydes and ketones, the stretching vibration of the carbonyl group has a strong infrared absorption between 1710 and 1740 cm-^1. Alkyl substituents are E lectron D onating G roups

• inductive effect lower the bond strength of C=O ketone carbonyls have slightly lower stretching frequencies, 1715 ± 7 cm
  • , compared with aldehydes, 1730 ± 7 cm-1.

Ketones and aldehydes

1740 cm- 1715 cm-

Ketones and aldehydes – conjugated systems

Conjugation of C=O bond with a double bond or phenyl ring lowers the stretching frequency.

Ketones and aldehydes – influence of EDG and EWG

Ketones and aldehydes – influence of EDG and EWG

Ketones– ring size

Incorporation of the carbonyl group into a small ring, increase the stretching frequency.

Carbonyl compounds

Acid chlorides – aromatic - benzoyl chloride

1775 cm-1^ 1733cm-

Carboxylic anhydrides

Anhydrides C=O stretch : 2 bands separated by 60 - 30 cm-1) Acyclic anhydrides, 1750 and 1820 cm-1; 6-membered ring, 1750 and 1820; 5-membered ring, 1785 and 1865

Carboxylic acid – hydrogen bonds

Spectrum of carboxylic acid in solution– dimeric and monomeric

Carboxylic acid – hydrogen bonds

Spectrum of liquid carboxylic acid (pure substance) – only dimer is observed

Amides bond properites

Conjugation of C=O bond with an EDG : (lone pair, double bond or phenyl ring) lowers the stretching frequency.

Secondary Amides

ν NH 3326 cm-^ ν C=O 1638 cm- δ NH (in plane) 1554cm- ω NH (out of plane) 711cm-1^645 One stretching band ν C-N 1450 cm- ω CH aromatic (out of plane) 711cm-1^645