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.

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university advanced level chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These organic chemistry revision notes on spectroscopy (infrared spectrum of phenol, explanations of the detailed analysis and how to interpret the spectra notes) are suitable for use of pre-university students studying AQA advanced level chemistry, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE advanced level chemistry, CCEA advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry.

ANSWERS Advanced A-level chemistry - practise exam questions on the infrared spectrum of phenol If you think there are any errors, please email me asap at chem55555@hotmail.com I don't mind if students/teachers do a selected printout of these questions and answers. Q1. Phenol shows a broad O–H stretch. Where does this typically appear? A. 2500–3000 cm⁻¹    B. 3200–3600 cm⁻¹    C. 1500–1600 cm⁻¹    D. 600–800 cm⁻¹ Correct answer: B Explanation: Phenol's O–H stretch appears as a broad band around 3200–3600 cm⁻¹, similar to alcohols but often slightly sharper. Common misconception: Students confuse phenol O–H with carboxylic acid O–H, which is even broader and lower (2500–3000 cm⁻¹). Q2. Phenol shows strong absorptions from aromatic C=C stretching. Where are these found? A. 1000–1100 cm⁻¹   B. 1600 and 1500 cm⁻¹   C. 2800–3000 cm⁻¹   D. 3500–3700 cm⁻¹ Correct answer: B Explanation: Aromatic rings show two characteristic C=C stretches near 1600 cm⁻¹ and 1500 cm⁻¹. Common misconception: Students sometimes mistake these for carbonyl peaks, but C=O is typically around 1700 cm⁻¹. Q3. Which peak confirms the presence of an aromatic ring in phenol? A. Sharp peak at ~3030 cm⁻¹ B. Peak at 3500 cm⁻¹ C. Peak at 1700 cm⁻¹ D. Broad O–H stretch Correct answer: A Explanation: Aromatic C–H stretching appears around 3030 cm⁻¹, distinguishing aromatic rings from alkanes (2850–2960 cm⁻¹). Common misconception: Students often assume all C–H stretches look the same — aromatic C–H is slightly higher and sharper. Q4. Phenol shows a C–O stretching vibration. Where is this typically found? A. 1000–1300 cm⁻¹    B. 1500–1600 cm⁻¹    C. 2800–3000 cm⁻¹    D. 3500–3600 cm⁻¹ Correct answer: A Explanation: Phenol's C–O stretch appears in the 1000–1300 cm⁻¹ region, similar to alcohols but often slightly shifted due to aromatic conjugation. Common misconception: Students confuse this with C–O in ethers, which is often slightly higher (1050–1150 cm⁻¹). Q5. Which feature distinguishes phenol from simple alcohols in IR spectroscopy? A. Broad O–H stretch B. Aromatic C–H stretch at ~3030 cm⁻¹ C. C–O stretch at ~1050 cm⁻¹ D. Peak at 2900 cm⁻¹ Correct answer: B Explanation: Phenol contains an aromatic ring, giving a 3030 cm⁻¹ C–H stretch that alcohols do not have. Common misconception: Students often think the O–H stretch alone identifies phenol — but alcohols also have O–H. Q6. A strong peak at ~750 cm⁻¹ in phenol's IR spectrum is due to: A. Aromatic C–H bending   B. O–H stretching   C. C–O stretching   D. C=C stretching Correct answer: A Explanation: Aromatic rings show C–H out‑of‑plane bending between 650–900 cm⁻¹, often diagnostic of substitution patterns. Common misconception: Students often misassign this as an O–H bend, but O–H bending is typically around 650–750 cm⁻¹ and weaker. Q7. Which combination of peaks confirms phenol rather than benzene? A. Aromatic C–H stretch + aromatic C=C stretch B. Broad O–H stretch + C–O stretch C. C–H stretch + C=C stretch D. Peak at 1700 cm⁻¹ + peak at 3500 cm⁻¹ Correct answer: B Explanation: Phenol has O–H and C–O peaks that benzene lacks. Common misconception: Students sometimes think benzene has an O–H stretch — it does not. Q8. A student sees a broad peak at 3300 cm⁻¹ and concludes the compound is an alcohol. Why might this be incorrect? A. Phenol also shows a broad O–H stretch B. Alcohols never show broad O–H stretches C. Only phenols show broad O–H stretches D. Broad peaks always indicate carboxylic acids Correct answer: A Explanation: Phenol also shows a broad O–H stretch, so the peak alone cannot distinguish alcohols from phenols. Common misconception: Students often assume “broad O–H = alcohol,” forgetting phenols and acids also show broad O–H bands. Q9. Which peak would not appear in phenol's IR spectrum? A. Aromatic C=C stretch   B. Broad O–H stretch    C. C–O stretch    D. C=O stretch Correct answer: D Explanation: Phenol contains no carbonyl group, so it cannot show a C=O stretch around 1700 cm⁻¹. Common misconception: Students sometimes assume “oxygen‑containing molecules have C=O,” which is incorrect. If you think there are any errors, please email me asap at chem55555@hotmail.com I don't mind if students/teachers do a selected printout of these questions and answers.