Interpreting and explaining the infrared spectrum of phenol C₆H₅OH

Spectra obtained from a liquid film of phenol. The right-hand part of the of the infrared spectrum of phenol, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of phenol and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of phenol.

The molecular structure and naming of aromatic compounds

 

Interpretation of the infrared spectrum of phenol

The most prominent infrared absorption lines of phenol

The most characteristic absorption band is the broad O-H stretching vibration bands at wavenumbers ~3550 to 3230 cm^-1, the breadth is caused by interference of hydrogen bonding interactions, common to all hydrogen bonded molecules with a hydroxyl group (diagram above).

The intermolecular hydrogen bond C₆H₅-O^δ--H^δ+ǁǁǁ:O^δ--C₆H₅ in phenol.

Aromatic benzene ring (NOT C=C) stretching vibrations occur with strong absorptions at 1600 to 1440 cm^-1.

There are C-O stretching vibration absorption bands at wavenumbers 1410 to 1310 and 1230 to 1140 cm^-1.

The absence of other specific functional group bands will show that a particular functional group is absent from phenol's molecular structure.

Key IR Features of Phenol in a bit more detail

Phenol combines features of both aromatic compounds and alcohols, so its IR spectrum is quite diagnostic:

• Broad O–H stretch (3200–3600 cm⁻¹)
  • Broader than simple alcohols due to strong hydrogen bonding.
  • Sometimes overlaps with aromatic C–H stretches.
• Aromatic C–H stretches (3000–3100 cm⁻¹)
  • Sharper than aliphatic C–H stretches (<3000 cm⁻¹).
• C=C aromatic ring stretches (1500–1600 cm⁻¹)
  • Medium to strong, often appearing as multiple peaks.
• C–O stretch (around 1220 cm⁻¹)
  • Distinguishes phenols from aliphatic alcohols (which show C–O stretches at 1050–1150 cm⁻¹).
• Out-of-plane C–H bending (750–850 cm⁻¹)
  • Useful for identifying substitution patterns on the benzene ring.

Prominent Wavenumbers Table

Sources: NIST Chemistry WebBook, Chemistry LibreTexts, Quimica Organica.

Common Misconceptions

• Confusing phenol with alcohols:
  Students often assume the O–H stretch is identical to ethanol. In phenol, hydrogen bonding and aromatic conjugation broaden and shift the band.
• Overlooking the C–O stretch:
  Many learners focus only on O–H and aromatic peaks, forgetting the diagnostic ~1220 cm⁻¹ band.
• Misinterpreting aromatic C–H stretches:
  These appear just above 3000 cm⁻¹, not below (which is typical for aliphatic C–H).

Exam Revision Tips

• Compare alcohol versus phenol spectra:
  Alcohol O–H stretch is broad but usually centered higher (~3300 cm⁻¹), while phenol’s overlaps with aromatic signals.
• Always mention hydrogen bonding:
  Examiners expect recognition of broadening due to intermolecular forces.
• Spot the C–O stretch at ~1220 cm⁻¹:
  A key discriminator in exam questions.
• Use substitution pattern clues:
  Out-of-plane bending (750–850 cm⁻¹) helps distinguish ortho, meta, para substitution in aromatic chemistry.
• Practice with comparative overlays:
  Align phenol’s IR with ethanol and benzene to highlight differences—this is a common exam-style question.

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