C7H6O C6H5CHO C-13 nmr spectrum of benzaldehyde analysis of chemical shifts ppm interpretation of 13C chemical shifts ppm of benzaldehyde C13 13-C nmr doc brown's advanced organic chemistry revision notes

Advanced Organic pre-university/college Chemistry: 13C NMR spectrum of benzaldehyde C₆H₅CHO

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Interpreting & explaining the ¹³C NMR spectrum of benzaldehyde C₆H₅CHO

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C-13 NMR spectroscopy - spectra index

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Introductory note on the 13C NMR spectrum of benzaldehyde

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

The description does not involve the chemical shift δ spin-spin coupling effects for benzaldehyde 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 benzaldehyde molecule.

The most common solvent used for investigating the C13 NMR spectrum of compounds like benzaldehyde, is CDCl₃ and other deuterated solvents.

C7H6O C6H5CHO C-13 nmr spectrum of benzaldehyde analysis of chemical shifts ppm interpretation of 13C chemical shifts ppm of benzaldehyde 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 shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the ¹³C atoms in an organic molecule - benzaldehyde here.

benzaldehyde C₇H₆O, C₆H₅CHO , ,
The molecular structure and naming of aromatic compounds
Interpreting the C-13 NMR spectrum of benzaldehyde

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

C₆H₅CHO

Chemical shifts (a) to (d) for the aromatic ring carbon atoms on the C-13 NMR spectrum diagram for benzaldehyde.

13C resonance (a) is for benzene ring carbon 4, chemical shift δ 134.4 ppm.

13C resonance (b) is for benzene ring carbons 3 and 5, chemical shift δ 129.0 ppm.

C3 carbon = C5 carbon because of the symmetry of a mono-substituted benzene ring compound.

13C resonance (c) is for benzene ring carbons 2 and 6, chemical shift δ 29.7 ppm.

Again, C2 carbon = C5 carbon because of the symmetry of a mono-substituted benzene ring compound.

13C resonance (d) is for benzene ring carbon 1, chemical shift δ 136.5 ppm.

Note that most of the carbon ring carbon atoms give quite similar C-13 NMR chemical shifts, typical of aromatic compounds like benzaldehyde.

13C resonance (e) is for the carbon atom of the aldehyde group, chemical shift δ 192.3 ppm - significantly higher than the closely 'packed' benzene ring carbon atom resonances, this is typical of a highly electronegative atom (O) next to the carbon atom in question.

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

Key points about the 13C NMR spectrum of benzaldehyde

In the ¹³C NMR spectrum of benzaldehyde, the most diagnostic signal is the carbonyl carbon at ~δ 190–195 ppm, while the aromatic carbons resonate between δ 127–135 ppm.

The aldehyde carbonyl is highly deshielded, and the ring carbons show distinct chemical shifts depending on their position relative to the –CHO group.

Key Features of Benzaldehyde ¹³C NMR Spectrum

Carbonyl carbon (C=O):
- Appears far downfield at ~δ 191–194 ppm.
- Strongly deshielded due to the double bond and oxygen electronegativity.
- Diagnostic for aldehydes.
Aromatic carbons:
- Range: δ 127–135 ppm.
- Different positions (ortho, meta, para) give distinct signals.
- Substitution by the aldehyde group breaks symmetry, so six separate aromatic signals are observed.
No aldehyde proton carbon:
- Unlike ¹H NMR, the aldehyde carbon itself is observed directly at ~δ 191 ppm.

Table of Chemical Shifts

Carbon Environment δ (ppm) Notes

Carbonyl carbon (–CHO) ~191–194 Strongly deshielded, diagnostic aldehyde signal

Ortho carbons (2C) ~134–135 Adjacent to –CHO, slightly deshielded

Meta carbons (2C) ~129–130 Further from –CHO, less deshielded

Para carbon (1C) ~127–128 Opposite –CHO, distinct environment

Ipso carbon (C–CHO attachment) ~137–138 Directly bonded to carbonyl group, unique shift

Sources: Spectral Database for Organic Compounds (SDBS), ChemicalBook benzaldehyde ¹³C NMR data.

Common Misconceptions

Confusing aldehyde carbon with ketone carbon: Ketones also show downfield signals (~δ 200 ppm), but aldehydes are slightly upfield (~δ 190–195 ppm).
Assuming aromatic carbons are equivalent: In benzaldehyde, substitution breaks symmetry, so six distinct aromatic signals appear.
Overlooking ipso carbon: Students sometimes forget the unique chemical shift of the carbon directly bonded to the carbonyl group (~δ 137 ppm).
Thinking integration applies in ¹³C NMR: Unlike ¹H NMR, integration is not quantitative in routine ¹³C spectra.

Exam Revision Tips

Step 1: Spot the carbonyl carbon at ~δ 191 ppm. This is the most diagnostic aldehyde signal.
Step 2: Count aromatic signals (six distinct carbons). This confirms substitution pattern.
Step 3: Identify ipso carbon (~δ 137 ppm) as the one bonded directly to the carbonyl.
Step 4: Compare with similar compounds:
- Acetophenone (ketone) shows carbonyl ~δ 200 ppm.
- Benzoic acid shows carbonyl ~δ 170–180 ppm.
Step 5: Remember exam trick: Examiners often ask why aldehyde carbons appear further downfield than alcohol carbons (due to strong deshielding by C=O).

Summary for Students:

In benzaldehyde’s ¹³C NMR, the aldehyde carbonyl at ~δ 191 ppm is the key diagnostic peak. The six aromatic carbons appear between δ 127–135 ppm, with the ipso carbon slightly more downfield.

Avoid assuming symmetry, and remember that integration is not exam-relevant in ¹³C NMR.

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Links associated with benzaldehyde

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The H-1 NMR spectrum of Benzaldehyde

Index of notes on the chemistry of aromatic compounds

Index of notes on the chemistry of aldehydes and ketones

C-13 NMR spectroscopy index

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