Infrared spectroscopy is a powerful tool for identifying functional groups in organic compounds. By analyzing absorption patterns, we can determine the presence of alkenes, alkynes, alcohols, and alkanes based on their unique vibrational frequencies.

Carbonyl compounds like aldehydes, ketones, carboxylic acids, and esters have distinct IR absorption patterns. These patterns help distinguish between similar structures and provide valuable information about a molecule's composition and bonding.

Infrared Spectroscopy and Functional Group Identification

IR absorptions of functional groups

• Alkenes ($C=C$)
  • Weak absorption around 1600-1680 cm$^{-1}$ caused by stretching of the carbon-carbon double bond
  • Out-of-plane bending of the C-H bonds at 800-1000 cm$^{-1}$ (cis-2-butene)
• Alkynes ($C\equiv C$)
  • Weak absorption at 2100-2260 cm$^{-1}$ resulting from stretching of the carbon-carbon triple bond
  • Absence of signal at 3300 cm$^{-1}$ which distinguishes from terminal alkynes (2-butyne)
• Alcohols ($O-H$)
  • Strong, broad absorption at 3200-3600 cm$^{-1}$ attributed to stretching of the O-H bond
  • Stretching of the C-O bond at 1050-1150 cm$^{-1}$ (ethanol)
• Alkanes ($C-H$)
  • Strong absorption at 2850-3000 cm$^{-1}$ caused by stretching of the C-H bonds
  • Bending vibrations of the C-H bonds at 1450-1470 cm$^{-1}$ and 1370-1390 cm$^{-1}$ (propane)

Functional group analysis in IR spectra

• Identify functional groups by their characteristic absorption frequencies
  • Compare observed peaks to known absorption ranges for each functional group (alcohols: 3200-3600 cm$^{-1}$)
• Absence of expected peaks indicates the absence of the corresponding functional group
  • No strong, broad absorption at 3200-3600 cm$^{-1}$ suggests the compound lacks an $O-H$ group (hexane)
• Presence of unexpected peaks may indicate the presence of additional functional groups
  • A strong peak at 1700-1750 cm$^{-1}$ suggests the compound contains a carbonyl group (acetone)
• Intensity of absorption peaks is related to the change in dipole moment during molecular vibrations

Carbonyl Compounds and Their IR Absorptions

Carbonyl compounds in IR spectroscopy

• Aldehydes ($R-CHO$)
  • Strong stretching of the $C=O$ bond at 1720-1740 cm$^{-1}$
  • Weak stretching of the C-H bond at 2700-2800 cm$^{-1}$ and 2820-2900 cm$^{-1}$ (propanal)
• Ketones ($R-CO-R'$)
  • Strong stretching of the $C=O$ bond at 1705-1725 cm$^{-1}$
  • Absence of C-H stretching at 2700-2900 cm$^{-1}$ which distinguishes from aldehydes (2-butanone)
• Carboxylic acids ($R-COOH$)
  • Strong, broad stretching of the $O-H$ bond at 2500-3300 cm$^{-1}$
  • Strong stretching of the $C=O$ bond at 1700-1730 cm$^{-1}$
  • Stretching of the C-O bond at 1210-1320 cm$^{-1}$ (acetic acid)
• Esters ($R-COO-R'$)
  • Strong stretching of the $C=O$ bond at 1735-1750 cm$^{-1}$
  • Stretching of the C-O bond at 1000-1300 cm$^{-1}$
  • Absence of O-H stretching which distinguishes from carboxylic acids (ethyl acetate)

Principles of Infrared Spectroscopy

• Infrared spectroscopy is a type of absorption spectroscopy that uses electromagnetic radiation in the infrared region
• It measures the absorption of infrared light by molecules, which causes molecular vibrations
• Fourier transform infrared spectroscopy (FTIR) is a modern technique that improves the quality and speed of IR spectra collection