Interpreting and explaining the Carbon-13 NMR spectrum of phenol C₆H₅OH

 Practise exam questions on the 13C NMR spectrum of phenol with answers!

TMS is the acronym for tetramethylsilane, formula Si(CH₃)₄, whose ¹³C atoms are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in ¹³C NMR spectroscopy and all other ¹³C resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the ¹³C atoms in an organic molecule - phenol here.

The molecular structure and naming of aromatic compounds

Interpreting the C-13 NMR spectrum of phenol

As you can see from the diagram above there are 4 different ¹³C chemical shift lines in the C-13 NMR spectrum of phenol indicating 4 different chemical environments of the 6 carbon atoms of phenol.

¹³C chemical shifts (a) to (d) on the C-13 NMR spectrum diagram for phenol.

Reference diagram for the benzene ring protons of phenol

Numbering of benzene ring carbon atoms.  Ring positions in monosubstituted benzene compounds. Note that C2 = C6 and C3 = C5 for ¹³C nmr shifts (each pair occupy the same chemical environment), this is an important point of symmetry for the ¹³c chemical shifts for these carbons, so only four ¹³C shifts for the benzene ring carbon atoms.

As a consequence of the symmetry of phenol:

Resonance¹³C  (b) is for the two chemically equivalent carbon atoms 2 and 6.

Resonance¹³C  (c) is for the two chemically equivalent carbon atoms 3 and 5.

The other two 13C NMR resonances are unique.

Carbon atom 1, resonance (a) chemical shift greatly affected by the electronegative oxygen atom of phenol group.

and carbon atom 4, resonance (d), similar to resonances b and c.

These resonance are not equivalent to each other or to any other carbon atoms in the benzene ring.

The carbon-13 NMR spectra provides direct evidence of 4 different carbon atom environments for the 6 carbon atoms in the phenol molecule, deduced from the presence of 4 different 13C NMR chemical shifts (ppm).

Key points about the ¹³C NMR Spectrum of Phenol

Phenol (C₆H₅OH) has six carbons, but due to symmetry in the monosubstituted benzene ring, some carbons are equivalent:

• C2 = C6 (ortho carbons) → equivalent by symmetry.
• C3 = C5 (meta carbons) → equivalent by symmetry.
• C4 (para carbon) → unique.
• C1 (ipso carbon, bonded to OH) → unique, strongly deshielded.

So in total, phenol shows 4 distinct ¹³C signals, not 6.

Table of ¹³C Chemical Shifts for Phenol

Sources: NIST WebBook (Phenol ¹³C NMR), ChemicalBook spectrum data, Organic Chemistry Data repositories.

Common Misconceptions

• Thinking phenol has 6 signals: Students often forget symmetry reduces the number of distinct carbons.
• Confusing the ipso carbon with a carbonyl: The δ ~150 ppm signal is due to C–OH, not a C=O group.
• Expecting integration to matter in ¹³C NMR: Unlike ¹H NMR, signal intensity does not directly reflect number of carbons.

Exam Revision Tips

• Always count symmetry: For monosubstituted benzenes, check which carbons are equivalent.
• Identify the ipso carbon (~150 ppm): This is diagnostic of phenols.
• Compare with benzene: Benzene has 1 signal (δ ~128 ppm); phenol has 4 signals due to substitution.
• Exam technique:
  1. State number of distinct signals (4).
  2. Assign ipso carbon.
  3. Group equivalent carbons (C2=C6, C3=C5).
  4. Relate chemical shifts to electronegativity and aromatic environment.

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 (13C NMR 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 13C NMR 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. How many ¹³C NMR signals does phenol normally show? A. 6   B. 5    C. 4    D. 1 Correct answer: C Explanation: Phenol has six carbon atoms, but two pairs share the same chemical environment (C2/C6 and C3/C5. Thus, four distinct ¹³C signals appear. Common misconception: Students often think “benzene gives one signal,” but phenol is not totally symmetric — the OH breaks equivalence and also students may think all 6 carbon atoms occupy different chemical environments. Q2. Where do aromatic carbons in phenol typically appear? A. 0–20 ppm      B. 20–40 ppm     C. 50–60 ppm     D. 110–160 ppm Correct answer: D Explanation: Aromatic carbons are strongly deshielded and appear in the 110–160 ppm region. Common misconception: Students sometimes place aromatic carbons at ~50 ppm, confusing them with carbons attached to oxygen. Q3. The carbon directly bonded to the OH group is usually: A. The most upfield aromatic carbon B. The most downfield aromatic carbon C. Invisible in ¹³C NMR D. A carbonyl carbon Correct answer: B Explanation: The carbon bonded to OH is strongly deshielded by the electronegative oxygen and appears furthest downfield (often ~155–160 ppm). Common misconception: Students think the OH carbon is “just another aromatic carbon,” but it is distinctly more downfield. Q4. What splitting pattern is normally observed for phenol's ¹³C signals? A. Doublets    B. Triplets    C. Quartets    D. Singlets Correct answer: D Explanation: Routine ¹³C NMR uses proton decoupling, so carbon signals appear as singlets. Common misconception: Students expect n+1 splitting like in ¹H NMR — but ¹³C spectra are decoupled. Q5. Where does the aromatic carbon bonded to OH typically appear? A. 20–40 ppm     B. 50–60 ppm     C. 110–130 ppm     D. 150–160 ppm Correct answer: D Explanation: The carbon attached to OH is strongly deshielded, appearing around 155–160 ppm. Common misconception: Students confuse this with alcohol C–O carbons (~50–60 ppm), forgetting phenol is aromatic. Q6. What happens to phenol's ¹³C NMR spectrum when D2O is added? A. The carbon bonded to OH disappears B. All aromatic carbons shift dramatically C. The spectrum remains essentially unchanged D. A new carbon signal appears at 200 ppm Correct answer: C Explanation: Replacing OH protons with OD via D2O does not significantly change the carbon environments. 13C NMR detects the chemical environments of 13C carbon atoms, not 1H protons, so the 13C NMR spectrum of phenol is unchanged. Common misconception: Students assume D2O affects ¹³C spectra like it does ¹H spectra — but it does not. Q7. A student claims phenol should show a carbonyl peak in ¹³C NMR because it contains oxygen. Why is this incorrect? A. Phenol contains no oxygen B. Oxygen always removes carbon signals C. Phenol contains no C=O group D. Carbonyl peaks only appear in ¹H NMR Correct answer: C Explanation: Phenol contains oxygen, but not a carbonyl group, so no peak appears in the 160–220 ppm carbonyl region. Common misconception: Students often assume “oxygen = carbonyl,” but many oxygen‑containing molecules lack C=O. 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.