Advanced Organic Chemistry: Interpreting the 1H NMR spectrum of benzoic acid C6H5COOH

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Interpreting and explaining the H-1 hydrogen-1 (proton) 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 1H proton NMR spectrum of benzoic acid [spectra page updated April 3rd 2026 *]

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Introduction to 1H NMR spectroscopy and 1H NMR spectra index

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

Introductory note on the 1H NMR spectra of benzoic acid

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

The chemical shift δ splitting pattern effects for benzoic acid are confined to a proton spin-spin coupling effects analysed using the n+1 rule for adjacent non-equivalent proton fields (n is the number of neighbouring protons in a non-equivalent different chemical environment for the benzoic acid molecule).

It is assumed that the integrated intensities of the δ chemical shifts give the ratio of the protons in the different non-equivalent chemical environments of the benzoic acid molecule.

The most common solvent used for investigating the 1H NMR spectrum of compounds like benzoic acid, is CDCl3 and other deuterated solvents to avoid confusion with a 1H NMR signal and 2D (2H) has a different chemical shift.

1H proton nmr spectrum of benzoic acid low/high resolution diagrams C7H6O2 C6H5COOH analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for benzoic acid explaining spin-spin coupling for line splitting doc brown's advanced organic chemistry revision notes

TMS is the acronym for tetramethylsilane, formula Si(CH3)4, whose protons are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in 1H NMR spectroscopy and all other proton resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - benzoic acid here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of benzoic acid represent the peaks of the intensity of the chemical shifts of (which are often groups of split lines at high resolution) AND the relative integrated areas under the peaks gives you the ratio of protons in the different chemical environments of the benzoic acid molecule.

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 H-1 NMR spectrum of benzoic acid

In terms of spin-spin coupling from the possible proton magnetic orientations, for benzoic acid I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH-CH2, - protons etc. but the proton resonance of the hydroxyl group O-H is not, nor does it split any C-H protons.

For relatively simple molecules, the low resolution H-1 NMR spectrum of benzoic acid is NOT a good starting point (diagram above) because 1H proton resonances of the benzene ring in aromatic compounds are often close together.

However, from high resolution spectra (higher than above diagram), you can show the 6 hydrogen atoms (protons) of benzoic acid occupy 4 different chemical environments.

C6H5COOH

Note the proton ratio 5:1 of the 2 colours of the 6 protons of benzoic acid in the benzene ring and carboxylic acid group chemically different proton environments

Chemical shifts (a) to (d) on the H-1 NMR spectrum diagram for benzoic acid.

Although there are 6 hydrogen atoms in the molecule, there are only 4 possible different chemical environments for the hydrogen atoms in benzoic acid molecule.

The integrated signal proton ratio 1:2:2:1 observed in the very high resolution H-1 NMR spectrum, corresponds with the structural formula of benzoic acid.

The high resolution 1H NMR spectrum of benzoic acid

The high resolution spectra of benzoic acid shows 4 groups of proton NMR chemical shift resonances and in the 1 : 2 : 2 : 1 ratio expected from the structural formula of benzoic acid.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of benzoic acid - since the peak' is at the apex of a band of H-1 NMR resonances due to spin - spin coupling field splitting effects - see high resolution notes on benzoic acid below.

So, using the chemical shifts and applying the n+1 rule to benzoic acid and make some predictions using some colour coding! (In problem solving you work the other way round!)

Resonance (a) 1H Chemical shift 12.1 ppm

The hydroxyl/carboxylic acid group proton O-H resonance appears a singlet.

There are no adjacent protons on the C1 carbon atom (see diagram below) to cause any splitting, so no application of the n+1 rule here.

Evidence for the presence of an isolated proton containing group in the molecule of benzoic acid

aromatic benzene ring carbon atom positions in phenol 1H proton NMR spectroscopy Ring positions in monosubstituted benzene compounds.

Resonance (b) 1H Chemical shift 8.12 ppm, CH ring protons

The C2 and C6 proton resonance is split by the C3 and C5 protons respectively, into a 1:1 doublet (n+1 = 2).

Evidence for the presence of a CH group in the molecule of benzoic acid - typical doublet from a monosubstituted benzene ring compound.

Note there is no proton on the ring carbon atom designated C1.

Resonance (c) 1H Chemical shift 7.45 ppm, CH ring protons

The C3 and C5 proton resonance is split, either side, by the C3 or C5 protons and by the C4 proton into a 1:2:1 triplet (n+2 = 3).

Resonance (d) 1H Chemical shift 7.62 ppm, CH ring proton.

The C4 proton resonance is split, either side, by the C3 and C5 protons into a 1:2:1 triplet (n+2 = 3).

The proton NMR spectrum of benzoic is a bit tricky to interpret because the benzene ring protons give similar chemical shifts, especially the three protons furthest from the carboxylic acid group (chemical shifts c and d, protons of the C3/C5 and C4 carbon atoms).

Summary of the 1H proton NMR spectrum of benzoic acid and extra comments

The 1H NMR spectrum of benzoic acid with the clarity and structure you appreciate, especially for exam alignment and misconception-busting.

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

Key Proton Environments in the 1H proton NMR spectrum of the benzoic acid molecule

Chemical Shift (δ, ppm) Proton Type Origin Integration Ratio Notes
~11.0–13.0, 12.1 ppm Carboxylic acid –OH Highly deshielded due to H-bonding & C=O 1 Broad singlet; may be weak or absent
7.5–8.2, 8.12 ppm Aromatic H ortho (H-2, H-6) Adjacent to –COOH group 2 Doublet; deshielded by –COOH
7.4–7.6, 7.45 ppm Aromatic H meta (H-3, H-5) One carbon removed from –COOH 2 Triplet or overlapping multiplet
7.6–7.8, 7.62 ppm Aromatic H para (H-4) Opposite –COOH group 1 Triplet or singlet depending on symmetry

Total Integration Ratio: 1 (OH) : 5 (aromatic) → 6 protons total

All five aromatic 1H ring protons inhabit very similar 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 1H proton NMR spectrum of benzoic acid (see also below)

  • Assuming all aromatic protons are equivalent: In benzoic acid, the –COOH group breaks symmetry, leading to three distinct aromatic environments.
  • Expecting sharp OH peak: The acidic –OH is often broad, weak, or missing due to exchange with solvent or concentration effects.
  • Misinterpreting splitting patterns: Students may expect clean doublets/triplets, but overlapping multiplets are common due to similar chemical shifts.
  • Confusing integration: The OH proton is not always visible, so integration may appear as 5H instead of 6H.

Exam Tips for questions involving the 1H proton NMR spectrum of benzoic acid (see also above)

  • Start with integration: If you see a 5H aromatic pattern and a broad singlet ~12 ppm, suspect benzoic acid.
  • Use symmetry logic: Recognize that the –COOH group causes deshielding and splitting asymmetry in the ring.
  • Check solvent: In CDCl3, the –OH may be visible; in D2O, it disappears due to exchange — a useful diagnostic.
  • Compare with phenol or benzaldehyde: Practice distinguishing similar aromatic compounds by their OH/C=O shifts and integration.
  • Don’t overassign: Focus on group environments, not individual protons, unless high-resolution data is provided.

The splitting pattern from proton spin-spin coupling effects is analysed using the n+1 rule for adjacent non-equivalent proton fields (n is the number of neighbouring protons in a non-equivalent different chemical environment) and applied to the 1H NMR spectrum of benzoic acid.
Number of directly adjacent protons 1H causing splitting Splitting pattern produced from the n+1 rule on spin-spin coupling and the theoretical ratio of line intensities
0 means no splitting             1            
1 creates a doublet           1   1          
2 creates a triplet         1   2   1        
3 creates a quartet       1   3   3   1      
4 creates a quintet     1   4   6   4   1    
5 creates a sextet   1   5   10   10   5   1  
6 creates a septet 1   6   15   20   15   6   1
PRACTICE QUESTIONS based on the 1H NMR spectrum of benzoic acid  ANSWERS

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

If you think there is an error email me asap chem55555@hotmail.com

1. How many distinct proton environments are observed in the ¹H NMR spectrum of benzoic acid?

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

2. Which feature distinguishes benzoic acid from methyl benzoate in ¹H NMR?

  1. Aromatic multiplet
  2. Broad singlet at ~12 ppm
  3. Integration of 5 protons
  4. Singlet at ~3.7 ppm

3. What is the expected multiplicity of the –COOH proton in benzoic acid?

  1. Doublet
  2. Triplet
  3. Quartet
  4. Singlet

4. What integration ratio is expected for benzoic acid’s low resolution ¹H NMR spectrum?

  1. 3:1
  2. 4:1
  3. 5:1
  4. 6:1

5. Which compound is an isomer of benzoic acid and shows a different NMR profile?

  1. 2-hydroxybenzaldehyde
  2. 2-methylphenol
  3. 2-hydroxybenzoic acid
  4. Benzyl alcohol

6. Which signal would disappear upon D2O shake in benzoic acid’s spectrum?

  1. Aromatic multiplet
  2. Broad singlet at ~12 ppm
  3. Singlet at ~3.7 ppm
  4. Doublet at ~1.2 ppm

7. Why do benzoic acid’s aromatic protons appear as a multiplet?

  1. All are equivalent
  2. Coupling between non-equivalent protons
  3. Shielding by –COOH
  4. Rapid exchange

ANSWERS to the questions based on the 1H NMR spectrum of benzoic acid

If you think there is an error email me asap chem55555@hotmail.com

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

If you think there is an error email me asap chem55555@hotmail.com

1. How many distinct proton environments are observed in the ¹H NMR spectrum of benzoic acid?

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

Answer: B. 4

  • Feedback: Benzoic acid has four distinct environments: one for the carboxylic acid proton (~12 ppm), and three for the aromatic protons due to symmetry.
  • Distractors:
    • A: Misses aromatic complexity
    • C/D: Overestimate due to misinterpreting symmetry, C2 = C6 and C3 = C5
  • Tip: Use molecular symmetry to group equivalent protons.

2. Which feature distinguishes benzoic acid from methyl benzoate in ¹H NMR?

  1. Aromatic multiplet
  2. Broad singlet at ~12 ppm
  3. Integration of 5 protons
  4. Singlet at ~3.7 ppm

Answer: B. Broad singlet at ~12 ppm

  • Feedback: Benzoic acid has a –COOH proton; methyl benzoate lacks this but has a –OCH3 singlet at ~3.7 ppm.
  • Distractors:
    • A/C: Shared features
    • D: Present in methyl benzoate only
  • Tip: Look for exchangeable protons to identify acids.

3. What is the expected multiplicity of the –COOH proton in benzoic acid?

  1. Doublet
  2. Triplet
  3. Quartet
  4. Singlet

Answer: D. Singlet

  • Feedback: The –COOH proton does not couple with neighbours due to rapid exchange and there is no immediate neighbouring proton i.e. no H on C1 of benzene ring.
  • Distractors:
    • A–C: Coupling is possible due to neighbouring C-H protons, which doesn’t occur for exchangeable protons
  • Tip: Acidic protons typically appear as broad singlets.

4. What integration ratio is expected for benzoic acid’s low resolution ¹H NMR spectrum?

  1. 3:1
  2. 4:1
  3. 5:1
  4. 6:1

Answer: C. 5:1

  • Feedback: Five aromatic protons and one carboxylic acid proton
  • Distractors:
    • A/B/D: miscounting protons and misinterpreting symmetry
  • Tip: Integration reflects relative proton counts, not environments.

5. Which compound is an isomer of benzoic acid and shows a different NMR profile?

  1. 2-hydroxybenzaldehyde
  2. 2-methylphenol
  3. 2-hydroxybenzoic acid
  4. Benzyl alcohol

Answer: A. 2-hydroxybenzaldehyde

  • Feedback: Only A has the same molecular formula
  • Distractors:
    • B/C/D: Not isomers (different molecular formula)
  • Tip: Isomers have same formula, different connectivity → different NMR.

6. Which signal would disappear upon D2O shake in benzoic acid’s spectrum?

  1. Aromatic multiplet
  2. Broad singlet at ~12 ppm
  3. Singlet at ~3.7 ppm
  4. Doublet at ~1.2 ppm

Answer: B. Broad singlet at ~12 ppm

  • Feedback: The –COOH proton exchanges with D₂O → signal disappears.
  • Distractors:
    • A/C/D: Non-exchangeable protons remain
  • Tip: D2O removes signals from exchangeable protons.

7. Why do benzoic acid’s aromatic protons appear as a multiplet?

  1. All are equivalent
  2. Coupling between non-equivalent protons
  3. Shielding by –COOH
  4. Rapid exchange

Answer: B. Coupling between non-equivalent protons

  • Feedback: Substitution breaks symmetry → non-equivalent protons → complex splitting.
  • Distractors:
    • A: False equivalence
    • C/D: Misapplied concepts
  • Tip: Use substitution pattern to predict splitting.

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Links associated with benzoic acid

The infrared spectrum of benzoic acid

The mass spectrum of benzoic acid

The C-13 carbon-13 NMR spectrum of benzoic acid

INDEX of all AROMATIC COMPOUND chemistry revision notes

The physical and chemical properties of benzoic acid and selected derivatives

H-1 proton NMR spectroscopy index

(Please read 8 points at the top of the 1H NMR index page)

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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Index of advanced (pre-university) organic chemistry revision notes

 The chemistry of alkanes and the petrochemical industry

 The chemistry of alkenes

 The chemistry of organic halogen compounds

 The chemistry of alcohols

 The chemistry of aldehydes and ketones

 The chemistry of carboxylic acids and derivatives

 The chemistry of organo-nitrogen compounds

 The chemistry of aromatic compounds

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