12.7 Interpreting Infrared Spectra

Infrared spectroscopy is a powerful tool for identifying functional groups in organic molecules. By analyzing the absorption of specific wavelengths, chemists can deduce structural information about compounds, distinguishing between similar molecules and deducing complex structures.

Understanding IR spectra is crucial for characterizing organic compounds. From O-H stretching in alcohols to C=O vibrations in carbonyls, each functional group has a unique spectral fingerprint. This knowledge enables chemists to unravel molecular structures and confirm synthetic products.

Interpreting Infrared Spectra

Functional groups in IR spectra

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• O-H stretching vibrations
  • Alcohols and phenols absorb in the range of 3200-3600 cm$^{-1}$ (methanol, ethanol)
  • Carboxylic acids exhibit a broad absorption band from 2500-3300 cm$^{-1}$ due to hydrogen bonding (acetic acid, benzoic acid)
• N-H stretching vibrations
  • Amines and amides display absorption bands between 3300-3500 cm$^{-1}$ (methylamine, acetamide)
• C-H stretching vibrations
  • Alkanes show absorption peaks in the range of 2850-3000 cm$^{-1}$ (hexane, cyclohexane)
  • Alkenes absorb between 3010-3100 cm$^{-1}$ due to the presence of sp$^2$ hybridized carbons (ethene, 1-butene)
  • Aromatic compounds exhibit C-H stretching vibrations from 3000-3100 cm$^{-1}$ (benzene, toluene)
  • Alkynes have a characteristic absorption peak around 3300 cm$^{-1}$ resulting from the C-H stretching of the sp hybridized carbon (ethyne, 1-butyne)
• C=O stretching vibrations
  • Aldehydes and ketones absorb strongly between 1690-1760 cm$^{-1}$ (acetaldehyde, acetone)
  • Carboxylic acids show a strong C=O stretching band from 1700-1730 cm$^{-1}$ (formic acid, propionic acid)
  • Esters display a characteristic absorption peak in the range of 1735-1750 cm$^{-1}$ (ethyl acetate, methyl benzoate)
  • Amides exhibit C=O stretching vibrations between 1640-1690 cm$^{-1}$ (formamide, acetamide)
• C=C stretching vibrations
  • Alkenes absorb in the range of 1620-1680 cm$^{-1}$ due to the presence of C=C double bonds (1-pentene, cyclopentene)
  • Aromatic compounds show characteristic absorption bands between 1450-1600 cm$^{-1}$ resulting from the conjugated C=C bonds (naphthalene, anthracene)
• C≡C stretching vibrations
  • Alkynes exhibit a strong absorption peak in the range of 2100-2260 cm$^{-1}$ due to the C≡C triple bond (1-hexyne, diphenylacetylene)
• C-O stretching vibrations
  • Alcohols, ethers, and esters display absorption bands between 1050-1300 cm$^{-1}$ corresponding to the C-O single bond stretching (ethanol, diethyl ether, ethyl acetate)

Comparison of similar compounds

• Alcohols vs phenols
  • Alcohols exhibit a strong, broad O-H stretching peak around 3300-3400 cm$^{-1}$ due to intermolecular hydrogen bonding (1-propanol, 2-butanol)
  • Phenols display a sharp O-H stretching peak between 3200-3600 cm$^{-1}$ and characteristic aromatic C=C stretching peaks around 1450-1600 cm$^{-1}$ (phenol, 4-methylphenol)
• Aldehydes vs ketones
  • Aldehydes show unique C-H stretching vibrations from 2700-2900 cm$^{-1}$ and a strong C=O stretching peak around 1720-1740 cm$^{-1}$ (propanal, benzaldehyde)
  • Ketones lack the aldehyde C-H stretching vibrations and exhibit a strong C=O stretching peak between 1705-1725 cm$^{-1}$ (2-butanone, cyclohexanone)
• Primary vs secondary vs tertiary alcohols
  1. Primary alcohols have a strong, broad O-H stretching peak and a strong C-O stretching peak around 1050 cm$^{-1}$ (1-butanol, 1-hexanol)
  2. Secondary alcohols display a strong, broad O-H stretching peak and a medium intensity C-O stretching peak around 1100 cm$^{-1}$ (2-propanol, 2-pentanol)
  3. Tertiary alcohols exhibit a strong, broad O-H stretching peak and a weak C-O stretching peak around 1150 cm$^{-1}$ (2-methyl-2-propanol, 2-methyl-2-butanol)

Structural deduction from IR data

• Identify the presence or absence of key functional groups based on their characteristic absorption bands (carboxylic acid in acetic acid, ester in ethyl acetate)
• Determine the relative number of hydrogens attached to sp$^3$, sp$^2$, and sp hybridized carbons by comparing the intensities of C-H stretching peaks (more sp$^3$ C-H in hexane compared to 1-hexene)
• Distinguish between conjugated and non-conjugated systems by observing the shift in C=O and C=C stretching frequencies
  • Conjugation lowers the frequency of C=O and C=C stretching vibrations (conjugated C=O in benzoic acid vs non-conjugated C=O in acetic acid)
• Recognize the presence of hydrogen bonding by observing the broadening and shifting of O-H and N-H stretching bands (broad O-H stretching in ethanol due to intermolecular hydrogen bonding)
• Identify the presence of symmetry in molecules by the absence of certain vibrational modes
  • Centrosymmetric molecules lack IR active asymmetric stretching and bending vibrations (no IR active asymmetric stretching in carbon dioxide)

Principles of IR Spectroscopy

• IR spectroscopy is based on the absorption of electromagnetic radiation by molecules, causing molecular vibrations
• The Beer-Lambert law relates the absorption of light to the concentration of the absorbing species and path length
• Fourier transform infrared spectroscopy (FTIR) is a modern technique that improves the quality and speed of IR measurements
• Different functional groups absorb IR radiation at characteristic frequencies, allowing for their identification in organic compounds