Advanced Organic Chemistry: Interpreting the 13C NMR spectrum of benzoic acid C6H5COOH

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Interpreting and explaining the Carbon-13 NMR spectrum of benzoic acid C6H5COOH

[Author ©  Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy - analysing the C-13 NMR spectrum of benzoic acid [spectra page updated April 3rd 2026 *]

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Introduction to 13C NMR spectroscopy and 13C NMR spectra index

Some practice questions based on the 1H NMR spectrum of benzoic acid

Introductory note on the 13C NMR spectrum of benzoic acid

Students and teachers please note that my explanation of the carbon-13 NMR spectrum of benzoic acid is designed for advanced, but pre-university, chemistry courses.

The description does not involve the chemical shift δ spin-spin coupling effects for benzoic acid and the relative size of the carbon-13 NMR shifts does not give the ratio of the carbon atoms in the different non-equivalent chemical environments of the benzoic acid molecule.

The most common solvent used for investigating the 13C NMR spectrum of compounds like benzoic acid, is CDCl3 and other deuterated solvents.

13C nmr spectrum of benzoic acid C7H6O2 C6H5COOH analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of benzoic acid C13 13-C nmr doc brown's advanced organic chemistry revision notes 

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 - benzoic acid here.

Benzoic acid (benenecarboxylic acid),  C7H6O2,  C6H5COOH  ,  (c) doc b , (c) doc b , (c) doc b

The molecular structure and naming of carboxylic acids and derivatives

The molecular structure and naming of aromatic compounds

Interpreting the C-13 NMR spectrum of benzoic acid

As you can see from the diagram above there are 5 different 13C chemical shift lines in the C-13 NMR spectrum of benzoic acid indicating 5 different chemical environments of the 7 carbon atoms of benzoic acid.

C6H5COOH

(Note the 2 different colours indicating the aromatic benzene ring and hydroxyl group chemical environments of the 7 carbon atoms in benzoic acid).

BUT, strictly speaking, there are 5 different chemical environments, 4 for the benzene ring carbon atoms and 1 for the carboxyl group carbon atom.

13C chemical shifts (a) to (e) on the C-13 NMR spectrum diagram for benzoic acid.

Note:

(i) The close proximity of the 13C chemical shifts for the aromatic carbon atoms of the benzene ring - just about separated at high resolution.

(ii) Note the effect of two highly electronegative oxygen atoms on the carboxylic acid group carbon atom, considerably increasing its 13C chemical shift compared to the benzene ring carbon atoms. This carbon atom cannot be equivalent to any of the benzene ring carbon atoms.

(iii) Carbon atoms C1 and C4 are also not equivalent to each other or any other ring carbons.

(iv) Due to the symmetry of a monosubstituted benzene compound like benzoic acid and free rotation of the -COOH group, carbon atoms C2 and C6 are chemically equivalent to each other (same 13C NMR chemical shift) as are carbon atoms C3 and C5.

The carbon-13 NMR spectra provides direct evidence of 5 different carbon atom environments for the 7 carbon atoms in the benzoic acid molecule, deduced from the presence of 5 different 13C NMR chemical shifts (ppm).

Summary of the C-13 NMR spectrum of benzoic acid and extra comments

The ¹³C NMR spectrum of benzoic acid with precision and exam-aligned clarity.

This overview includes chemical shift assignments, structural origins, common pitfalls, and strategic tips for tackling exam questions.

Some practice questions based on the 1H NMR spectrum of benzoic acid

Key C-13 Chemical Shifts in NMR spectrum of Benzoic Acid

δ (ppm) Carbon Type Origin / Environment Notes
~172–174, 172.8 ppm Carboxylic acid C=O Highly deshielded due to electronegative O Furthest downfield; diagnostic for –COOH
~133–135, 129.4 ppm Aromatic C–COOH (C-1) Directly bonded to –COOH Deshielded by electron-withdrawing group
~129–131, 130.3 ppm Aromatic ortho (C-2, C-6) Adjacent to –COOH Slightly deshielded
~128–130, 128.5 ppm Aromatic meta (C-3, C-5) One carbon removed from –COOH Typical aromatic shift
~127–129, 133.8 ppm Aromatic para (C-4) Opposite –COOH Often slightly more shielded

Total signals: 5 distinct peaks for 7 carbon atoms due to symmetry (C-2 ≡ C-6, C-3 ≡ C-5)

Apart from C1, all the other five aromatic ring carbon atoms inhabit very similar 13C chemical environment in the benzoic acid molecule.

Note that ortho (C2, C6), meta (C3, C5) and para (C4) are old notations for substituent positions in the aromatic ring of aromatic compounds.

You should know that for monosubstituted benzene ring compounds, the ring carbon atom attached to the substituent group is assigned as C1.

Common Misconceptions about the C-13 NMR spectrum of benzoic acid (see also below)

  • Expecting 7 signals for 7 carbons: Symmetry reduces the number of unique environments to 5.
  • Confusing aromatic shifts with alkene shifts: Aromatic carbons have narrower, more consistent ranges (~125–135 ppm).
  • Assuming the carboxylic acid C=O appears near aldehydes/ketones: Acid C=O is typically upfield of aldehyde/ketone C=O (which are ~190–220 ppm).
  • Overinterpreting signal intensity: Unlike ¹H NMR, integration is not reliable in ¹³C NMR due to variable relaxation times.

Exam Tips for questions involving the C-13 NMR spectrum of benzoic acid (see also above)

  • Start with symmetry: Count expected signals based on unique environments, not atom count.
  • Identify the C=O first: The ~172 ppm peak is a strong clue for carboxylic acids.
  • Use substitution logic: The carbon bonded to –COOH will be more deshielded than other ring carbons.
  • Compare with similar compounds: Practice with phenol, benzaldehyde, and methyl benzoate to sharpen recognition.
  • Ignore solvent peaks: CDCl3 appears around 77 ppm — don’t mistake it for a sample signal.
PRACTICE QUESTIONS based on the 13C NMR spectrum of benzoic acid  ANSWERS

You may have to sketch out some molecular structures to work out the answer.

1. How many distinct carbon environments are observed in the ¹³C NMR spectrum of benzoic acid?

  1. 4
  2. 5
  3. 6
  4. 7

Answer: B. 5

  • Feedback: Benzoic acid has 6 distinct carbon environments: 4 aromatic carbons and 1 carboxylic acid carbon.
  • Distractors:
    • A: Underestimate due to symmetry assumptions
    • C/D: Overestimate; no aliphatic carbons present, assuming all carbons are different, actually two pairs are equivalent
  • Tip: Use substitution symmetry to count unique carbon signals.

2. Which signal would be absent in the ¹³C NMR spectrum of methyl benzoate compared to benzoic acid?

  1. Aromatic carbon
  2. Carbonyl carbon
  3. Methyl carbon
  4. Carboxylic acid carbon

Answer: D. Carboxylic acid carbon

  • Feedback: Methyl benzoate has an ester group, not a –COOH group.
  • Distractors:
    • A/B/C: All present in methyl benzoate
  • Tip: Use functional group shifts to distinguish isomers.

3. Which carbon signal in benzoic acid is most affected by conjugation with the aromatic ring?

  1. Carboxylic acid carbon
  2. Ortho aromatic carbon (C2)
  3. Meta aromatic carbon (C3)
  4. Para aromatic carbon (C4)

Answer: B. Ortho aromatic carbon C2

  • Feedback: Ortho carbons are closest to –COOH and experience conjugation effects.
  • Distractors:
    • A: Conjugated but not aromatic
    • C/D: Less affected due to position
  • Tip: Substituent effects vary with position on the ring.

4. Which of the following compounds would show a methyl carbon signal near 20 ppm in its ¹³C NMR spectrum?

  1. Benzoic acid
  2. 2-hydroxybenzoic acid
  3. Benzaldehyde
  4. Methylbenzene

Answer: D. Methylbenzene

  • Feedback: Methylbenzene has a methyl group directly attached to the ring, appearing ~20 ppm.
  • Distractors:
    • A/B/C: No methyl group present
  • Tip: Methyl carbons appear upfield (~10–30 ppm).

5. Which substitution pattern leads to fewer distinct aromatic carbon signals in ¹³C NMR?

  1. Trisubstituted benzene (e.g. 1,2,4 substitution)
  2. Ortho-disubstituted benzene (1,2 substitution)
  3. Meta-disubstituted benzene (1,3 substitution)
  4. Para-disubstituted benzene (1,4 substitution)

Answer: D. Para-disubstituted benzene (1,4 substitution)

  • Feedback: Para substitution increases symmetry, reducing the number of unique carbon environments.
  • Distractors:
    • A/B/C: Lower symmetry → more signals
  • Tip: Greater symmetry = fewer signals.
  • ortho, meta and para are an older notation based on substitution in a benzene ring.

6. Which feature distinguishes benzoic acid from 2-hydroxybenzoic acid in ¹³C NMR?

  1. Number of aromatic signals
  2. Presence of methyl carbon
  3. Presence of hydroxyl carbon
  4. Presence of carboxylic acid carbon

Answer: C. Presence of hydroxyl carbon

  • Feedback: 2-hydroxybenzoic acid (salicylic acid) has an –OH group ortho (C2) to –COOH, giving an extra signal ~150–160 ppm.
  • Distractors:
    • A: Both have similar aromatic regions
    • B: Neither has methyl
    • D: Both have –COOH
  • Tip: Look for phenolic –OH signals to identify 2-hydroxybenzoic acid (salicylic acid).

ANSWERS to the questions based on the 13C NMR spectrum of benzoic acid

Key words & phrases: isomer of C7H6O2 C6H5COOH Interpreting the C-13 NMR spectra of benzoic acid, C-13 nmr spectrum of benzoic acid, understanding the carbon-13 nmr spectrum of benzoic acid, explaining the line pattern in the high resolution C-13 nmr spectra of benzoic acid, revising the C-13 nmr spectrum of benzoic acid, ppm chemical shifts of the C-13 nmr spectrum of benzoic acid, how to construct the diagram of the C-13 nmr spectrum of benzoic acid, how to analyse the chemical shifts in the carbon-13 NMR spectrum of benzoic acid deducing the chemical environment of all the carbon atoms in benzoic acid examining the c13 nmr spectrum of  benzoic acid analysing the 13-c nmr spectrum of benzoic acid how do you sketch and interpret the C-13 NMR spectrum of benzoic acid interpreting interpretation of the C-13 NMR spectrum of benzoic acid assignment of chemical shifts in the 13C NMR spectrum of benzoic acid type functional group functional group aromatic carboxylic acid benzenecarboxylic acid Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the benzoic acid molecule in terms of its molecular structure. Listing data of all the chemical shift peaks in ppm in the carbon-13 NMR spectrum of benzoic acid. How to explain the C-13 NMR spectrum of benzoic acid. How to deduce the number of different carbon atom environments in the benzoic acid molecule from its carbon-13 NMR spectrum to help work out the molecular structure of the benzoic acid molecule. The uses and distinctive features of the carbon-13 NMR spectrum of the benzoic acid molecule explained What does the H-1 proton NMR spectrum tell us about the structure and properties of the benzoic acid molecule? How do you interpret the chemical shifts of the C-13 NMR spectrum of benzoic acid How to interpret the C-13 NMR spectrum of benzoic acid Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the  number of different carbon atom environments in the benzoic acid molecule from its carbon-13 NMR spectrum to help work out the molecular structure of the benzoic acid molecule? The uses and distinctive features of the carbon-13 NMR spectrum of the benzoic acid molecule explained. What do the number and values of the chemical shifts from the c-13 carbon-13 NMR spectrum tell us about the benzoic acid molecule? explaining the decoupled carbon-13 NMR spectrum of benzoic acid  with a detailed interpretation diagram of all the C-13 chemical shifts and intensities How do you interpret the chemical shifts of the C-13 NMR spectrum of benzoic acid How to interpret the C-13 NMR spectrum of benzoic acid Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the  number of different carbon atom environments in the benzoic acid molecule from its carbon-13 NMR spectrum to help work out the molecular structure of the benzoic acid molecule? The uses and distinctive features of the carbon-13 NMR spectrum of the benzoic acid molecule explained. What do the number and values of the chemical shifts from the c-13 carbon-13 NMR spectrum tell us about the benzoic acid molecule? explaining the decoupled carbon-13 NMR spectrum of benzoic acid  with a detailed interpretation diagram of all the C-13 chemical shifts and intensities

ANSWERS to the practice questions based on the 13C NMR spectrum of benzoic acid

1. How many distinct carbon environments are observed in the ¹³C NMR spectrum of benzoic acid?

  1. 4
  2. 5
  3. 6
  4. 7

Answer: B. 5

  • Feedback: Benzoic acid has 6 distinct carbon environments: 4 aromatic carbons and 1 carboxylic acid carbon.
  • Distractors:
    • A: Underestimate due to symmetry assumptions
    • C/D: Overestimate; no aliphatic carbons present, assuming all carbons are different, actually two pairs are equivalent
  • Tip: Use substitution symmetry to count unique carbon signals.

2. Which signal would be absent in the ¹³C NMR spectrum of methyl benzoate compared to benzoic acid?

  1. Aromatic carbon
  2. Carbonyl carbon
  3. Methyl carbon
  4. Carboxylic acid carbon

Answer: D. Carboxylic acid carbon

  • Feedback: Methyl benzoate has an ester group, not a –COOH group.
  • Distractors:
    • A/B/C: All present in methyl benzoate
  • Tip: Use functional group shifts to distinguish isomers.

3. Which carbon signal in benzoic acid is most affected by conjugation with the aromatic ring?

  1. Carboxylic acid carbon
  2. Ortho aromatic carbon (C2)
  3. Meta aromatic carbon (C3)
  4. Para aromatic carbon (C4)

Answer: B. Ortho aromatic carbon C2

  • Feedback: Ortho carbons are closest to –COOH and experience conjugation effects.
  • Distractors:
    • A: Conjugated but not aromatic
    • C/D: Less affected due to position
  • Tip: Substituent effects vary with position on the ring.

4. Which of the following compounds would show a methyl carbon signal near 20 ppm in its ¹³C NMR spectrum?

  1. Benzoic acid
  2. 2-hydroxybenzoic acid
  3. Benzaldehyde
  4. Methylbenzene

Answer: D. Methylbenzene

  • Feedback: Methylbenzene has a methyl group directly attached to the ring, appearing ~20 ppm.
  • Distractors:
    • A/B/C: No methyl group present
  • Tip: Methyl carbons appear upfield (~10–30 ppm).

5. Which substitution pattern leads to fewer distinct aromatic carbon signals in ¹³C NMR?

  1. Trisubstituted benzene (e.g. 1,2,4 substitution)
  2. Ortho-disubstituted benzene (1,2 substitution)
  3. Meta-disubstituted benzene (1,3 substitution)
  4. Para-disubstituted benzene (1,4 substitution)

Answer: D. Para-disubstituted benzene (1,4 substitution)

  • Feedback: Para substitution increases symmetry, reducing the number of unique carbon environments.
  • Distractors:
    • A/B/C: Lower symmetry → more signals
  • Tip: Greater symmetry = fewer signals.
  • ortho, meta and para are an older notation based on substitution in a benzene ring.

6. Which feature distinguishes benzoic acid from 2-hydroxybenzoic acid in ¹³C NMR?

  1. Number of aromatic signals
  2. Presence of methyl carbon
  3. Presence of hydroxyl carbon
  4. Presence of carboxylic acid carbon

Answer: C. Presence of hydroxyl carbon

  • Feedback: 2-hydroxybenzoic acid (salicylic acid) has an –OH group ortho (C2) to –COOH, giving an extra signal ~150–160 ppm.
  • Distractors:
    • A: Both have similar aromatic regions
    • B: Neither has methyl
    • D: Both have –COOH
  • Tip: Look for phenolic –OH signals to identify 2-hydroxybenzoic acid (salicylic acid).

Links associated with benzoic acid

The mass spectrum of benzoic acid

The H-1 proton NMR spectrum of benzoic acid

The infrared spectrum of benzoic acid

INDEX of all AROMATIC COMPOUND chemistry revision notes

The physical and chemical properties of benzoic acid and selected derivatives

C-13 NMR spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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