How to Match Carbonyl Carbon Signals to Functional Groups

Two unknown organic compounds both contain a carbonyl group, \(\ce{C=O}\). One produces a carbon-13 NMR signal at 174 ppm. The other produces one at 204 ppm.

Those two signals point towards different functional groups. But how do you make the match, and how much can you actually conclude from one peak?

For NSW HSC Chemistry, the key starting point is:

• 160-185 ppm: usually a carboxylic acid or ester carbonyl.
• 190-220 ppm: usually an aldehyde or ketone carbonyl.

The important catch is that these ranges identify a family of possible functional groups, not always one exact group. You need to combine the carbonyl signal with the other carbon environments, molecular structure, and any supporting proton NMR or infrared data.

Let’s build a method you can use when you’re given an unfamiliar spectrum.

01Start with the carbonyl carbon’s chemical shift

A carbonyl group contains a carbon atom double bonded to oxygen:

\[ \ce{C=O} \]

The carbon in this double bond is called the carbonyl carbon.

In carbon-13 NMR spectroscopy, this carbon usually produces a signal at a much higher chemical shift than ordinary saturated carbon atoms.

The most useful HSC-level ranges are:

Here, \(R\) and \(R'\) represent carbon-containing groups.

These are approximate reference ranges, not absolute boundaries. The chemical environment, other attached atoms, and molecular structure can shift the signal slightly.

Why do carbonyl carbons appear at higher chemical shifts?

Remember that electrons partially shield an atomic nucleus from an external magnetic field.

The carbonyl bond is polar because oxygen attracts electrons more strongly than carbon. This changes the electron distribution around the carbonyl carbon, leaving it relatively deshielded.

A deshielded carbon generally resonates at a higher chemical shift.

The additional oxygen attached to the carbonyl carbon in carboxylic acids and esters also changes its electronic environment. This produces a different chemical-shift range from the typical aldehyde and ketone carbonyls.

You don’t need to calculate the exact shift from electronegativity. Use the reference ranges and the surrounding structural information.

What do the different functional groups actually look like?

The chemical shift narrows down the possibilities. The structural formula tells you what distinguishes them.

Notice the differences:

• An aldehyde has a hydrogen attached directly to the carbonyl carbon.
• A ketone has two carbon-containing groups attached to the carbonyl carbon.
• A carboxylic acid has an \(\ce{-OH}\) group attached to the carbonyl carbon.
• An ester has an oxygen connecting the carbonyl carbon to another carbon-containing group.
• An amide has nitrogen directly attached to the carbonyl carbon.

There is an additional limitation worth knowing: amide carbonyls can also occur in the lower carbonyl region in broader chemical-shift tables. If your molecular formula contains nitrogen, don’t automatically classify a signal around 170 ppm as an acid or ester. Check whether an amide is possible.

The first decision rule: a signal around 200 ppm suggests an aldehyde or ketone, while a signal around 170 ppm suggests an acid or ester. Neither assignment should be treated as a complete structural identification.

02Understand what the carbon-13 NMR instrument measures

Before interpreting the spectrum, it helps to understand where the signals come from.

The experiment begins with a small amount of the organic compound dissolved in a suitable solvent. The solution is placed in a narrow glass tube.

The tube sits inside a strong magnetic field. A radiofrequency pulse excites the carbon-13 nuclei, and a receiver coil detects the changing magnetic signal produced as the nuclei return towards their original state.

A computer processes this signal to produce the NMR spectrum.

Why does this help identify a functional group?

Different carbon environments experience different electronic surroundings. Their nuclei therefore resonate at different frequencies.

The instrument converts these differences into chemical shifts, represented by \(\delta\) and measured in parts per million (ppm).

Chemical shifts are measured relative to a reference, commonly tetramethylsilane (TMS), which is assigned a value of 0 ppm.

On a conventional carbon-13 spectrum:

• Higher positive chemical shifts appear further downfield, towards the left.
• Lower chemical shifts appear further upfield, towards the right.
• Carbonyl carbons generally appear towards the high-ppm end.

A small measurement calculation

Suppose a carbon-13 NMR instrument operates at 100 MHz for carbon-13.

A carbon signal appears at 174 ppm relative to TMS.

The relationship between chemical shift and frequency difference is:

\[ \Delta \nu = \delta \times 10^{-6}\times \nu_0 \]

Where:

• \(\Delta \nu\) is the frequency difference in hertz (Hz).
• \(\delta\) is the chemical shift in ppm.
• \(\nu_0\) is the operating frequency in hertz.

Substituting:

\[ \begin{aligned} \Delta \nu &=174\times10^{-6}\times100\times10^6\\ &=17\,400\text{ Hz}\\ &=17.4\text{ kHz} \end{aligned} \]

The signal is therefore separated from the reference resonance by 17.4 kHz.

This illustrates what ppm represents: a chemical-shift difference expressed relative to the instrument’s operating frequency. It is not simply a frequency measured in hertz.

03Don’t confuse signal count, integration, chemical shift, and splitting

These four features provide different types of information.

Confusing them can lead to an incorrect functional-group assignment, even when you recognise the carbonyl region correctly.

Let’s examine the two features that cause the most confusion.

Signal count: one peak does not always mean one carbonyl group

A carbon-13 NMR signal represents a carbon environment, not necessarily one individual carbon atom.

For example, consider succinic acid:

\[ \ce{HOOC-CH2-CH2-COOH} \]

The molecule contains four carbon atoms, including two carbonyl carbons.

However, the molecule is symmetrical. The two carbonyl carbons experience equivalent chemical environments, as do the two \(\ce{CH2}\) carbons.

Consequently, the molecule produces only two distinct carbon-13 signals.

One signal represents both carbonyl carbons.

This is why you must count distinct environments rather than simply count the carbon atoms or carbonyl groups.

For a more detailed method of recognising equivalent carbons, see How to Count Carbon Environments in a Carbon-13 NMR Spectrum.

Integration: why carbon-13 peak height is unreliable

In proton NMR, integration provides information about the relative number of hydrogen atoms contributing to each signal.

For example, an integration ratio of 3:2:3 suggests three different proton environments containing 3, 2, and 3 hydrogen atoms respectively.

It is tempting to apply the same reasoning to carbon-13 NMR.

Imagine a signal representing two equivalent carbonyl carbons. Shouldn’t it be twice as intense as a signal representing one carbon atom?

Not necessarily.

Ordinary carbon-13 NMR signal intensities are affected by factors such as nuclear relaxation times and nuclear Overhauser effects. As a result, peak heights and areas do not reliably represent the number of carbon atoms contributing to each signal.

For HSC questions, use the number of distinct signals and the molecular structure to determine carbon environments.

Do not use carbon-13 peak height as a substitute for proton-style integration.

Splitting: why the normal carbon-13 spectrum looks different

In proton NMR, splitting occurs because hydrogen nuclei interact with neighbouring hydrogen nuclei.

You may already know the simple \(n+1\) rule:

• No neighbouring hydrogens: singlet.
• One neighbouring hydrogen: doublet.
• Two neighbouring hydrogens: triplet.
• Three neighbouring hydrogens: quartet.

However, standard carbon-13 NMR spectra are normally broadband proton-decoupled.

This means the observable coupling between carbon-13 nuclei and attached hydrogen nuclei is removed during the experiment.

Each distinct carbon environment therefore normally produces one singlet.

There is an important experimental trade-off here.

If proton-carbon coupling is retained, a carbon attached to three hydrogens can produce a quartet, a carbon attached to two hydrogens can produce a triplet, and a carbon attached to one hydrogen can produce a doublet.

Broadband decoupling simplifies the spectrum by removing these patterns, but it also removes that attached-hydrogen information.

Therefore, for ordinary HSC carbon-13 spectra, do not apply the proton NMR \(n+1\) rule to identify carbon environments.

If a question explicitly gives a proton-coupled carbon-13 spectrum, interpret its splitting separately.

04A reliable method for matching carbonyl signals

When you receive an unfamiliar carbon-13 NMR spectrum, follow this process.

Step 1: Find the carbonyl region.

Look towards the high-ppm end of the spectrum, generally between 160 and 220 ppm.

Identify the signal or signals that could represent carbonyl carbons.

Step 2: Compare the chemical shift with the reference ranges.

Use the following decision:

• 160-185 ppm: carboxylic acid or ester.
• 190-220 ppm: aldehyde or ketone.

If an amide is a possible structure, consider its carbonyl environment too.

Step 3: Count the other carbon environments.

Look at the remaining signals.

Some useful supporting ranges are:

These ranges overlap and are approximate, but they help you identify the rest of the structure.

For example, a signal around 55 ppm may indicate an \(\ce{O-CH3}\) group in an ester.

Step 4: Check other available evidence.

If proton NMR is provided, look for:

• An aldehyde proton, commonly around 9-10 ppm.
• A carboxylic acid proton, commonly around 9-13 ppm.
• Hydrogen atoms attached to a carbon bonded to oxygen, often producing signals further downfield than ordinary alkyl hydrogens.

If infrared data is available, look for:

• A strong carbonyl absorption.
• A very broad carboxylic acid O-H absorption.
• N-H absorption if an amide is a possible structure.

Step 5: State your conclusion carefully.

A good HSC response connects the evidence rather than simply naming a functional group.

For example:

“The signal at 174 ppm is consistent with a carboxylic acid or ester carbonyl. The additional signal at 52 ppm suggests a carbon bonded directly to oxygen, while the proton NMR signal at 3.7 ppm integrates to three hydrogens and is consistent with an \(\ce{O-CH3}\) group. Together, these observations support an ester.”

That is much stronger than simply writing “174 ppm = ester”.

05Worked examples

06Questions and solutions

For the following questions, assume that the carbon-13 spectra are normal broadband proton-decoupled spectra with well-resolved signals, unless stated otherwise.

All chemical shifts are illustrative data. Treat the supplied proton integrations as relative hydrogen counts. Small variations in actual chemical shifts are normal.

Question 1

An unknown compound produces a carbon-13 NMR signal at 181 ppm.

Its infrared spectrum also contains a very broad absorption between 2500 and 3000 cm\(^{-1}\), as well as a strong carbonyl absorption.

Which carbonyl-containing functional group is most likely present?

Explain why the carbon-13 signal alone would not be sufficient to make this identification.

Solution 1

The compound most likely contains a carboxylic acid functional group.

The carbon-13 signal at 181 ppm falls within the typical 160-185 ppm range for carboxylic acids and esters.

Therefore, the NMR signal alone cannot distinguish between these two functional groups.

However, the very broad infrared absorption between 2500 and 3000 cm\(^{-1}\) is characteristic of the O-H bond in a carboxylic acid.

The strong carbonyl absorption supports the presence of the \(\ce{C=O}\) bond.

Combining the two techniques provides stronger evidence than either measurement alone.

The important distinction is that carbon-13 NMR identifies the carbon environment, while infrared spectroscopy provides additional information about the bonds present.

Question 2

An unknown organic compound has the molecular formula \(\ce{C4H8O}\).

Its carbon-13 NMR spectrum contains four signals at:

• 207 ppm
• 36 ppm
• 29 ppm
• 8 ppm

Its proton NMR spectrum contains:

Determine whether the compound contains an aldehyde or ketone functional group. Propose its molecular structure and justify your answer.

Solution 2

The compound is butan-2-one, which contains a ketone functional group.

The carbon-13 signal at 207 ppm lies within the 190-220 ppm range associated with aldehyde and ketone carbonyls.

However, this signal alone does not distinguish between an aldehyde and a ketone.

The proton NMR data provides the additional evidence.

The 3H singlet at 2.1 ppm is consistent with a methyl group directly attached to a carbonyl carbon:

\[ \ce{CH3CO-} \]

There are no hydrogen atoms on an adjacent carbon that would split this methyl signal.

The 2H quartet and 3H triplet indicate an ethyl group:

\[ \ce{-CH2CH3} \]

The \(\ce{CH2}\) group is adjacent to the carbonyl, explaining its relatively downfield position at 2.4 ppm.

Combining the two groups gives:

\[ \ce{CH3COCH2CH3} \]

This is butan-2-one.

An aldehyde would contain a hydrogen directly attached to the carbonyl carbon and would typically produce an additional proton signal around 9-10 ppm.

No such signal is present.

The tempting mistake is to identify a ketone from 207 ppm alone. The more reliable conclusion comes from combining the carbonyl shift with the absence of an aldehyde proton and the specific proton splitting pattern.

Question 3

An unknown compound has the molecular formula \(\ce{C3H7NO}\).

Its carbon-13 NMR spectrum contains three signals:

Additional evidence includes:

• A quartet at 2.2 ppm integrating to 2H.
• A triplet at 1.0 ppm integrating to 3H.
• A broad, exchangeable proton signal integrating to 2H.
• Infrared evidence of N-H bonds.

Propose the most likely carbonyl-containing functional group and molecular structure.

Explain why the signal at 174 ppm should not automatically be classified as a carboxylic acid or ester.

Solution 3

The compound is most likely propanamide, which contains an amide functional group.

Its structural formula is:

\[ \ce{CH3CH2CONH2} \]

The carbonyl signal at 174 ppm lies in the lower carbonyl region.

Although this region is commonly associated with carboxylic acids and esters in simplified HSC reference tables, amide carbonyls can also appear in this region.

The molecular formula contains nitrogen, which makes an amide a possible structure.

The infrared evidence confirms the presence of N-H bonds.

The proton NMR spectrum provides further support.

The quartet integrating to 2H and triplet integrating to 3H indicate an ethyl group:

\[ \ce{CH3CH2-} \]

The broad, exchangeable signal integrating to 2H is consistent with an \(\ce{NH2}\) group.

Combining these features gives:

\[ \ce{CH3CH2CONH2} \]

The carbonyl carbon is directly bonded to nitrogen, which identifies the compound as an amide.

The misconception here is treating a chemical-shift range as an exclusive fingerprint. Chemical-shift ranges can overlap, so the molecular formula and other spectral evidence must also be considered.

Question 4

An unknown compound has the molecular formula \(\ce{C4H8O2}\).

Two possible structures are:

• Candidate A: butanoic acid, \(\ce{CH3CH2CH2COOH}\).
• Candidate B: methyl propanoate, \(\ce{CH3CH2COOCH3}\).

The unknown produces the following carbon-13 NMR signals:

Its proton NMR spectrum contains:

• A triplet at 2.35 ppm, integrating to 2H.
• A multiplet at 1.6 ppm, integrating to 2H.
• A triplet at 0.95 ppm, integrating to 3H.
• A broad signal at 11.4 ppm, integrating to approximately 1H.

Identify the correct candidate.

Explain why the carbonyl signal is not sufficient on its own and use the remaining spectral evidence to justify your answer.

Solution 4

The correct candidate is A, butanoic acid.

The signal at 180 ppm is consistent with a carboxylic acid or ester carbonyl.

Therefore, the carbonyl signal alone cannot distinguish the two proposed structures.

The other carbon signals provide the next piece of evidence.

The signals at 36, 18, and 13 ppm are consistent with three different saturated carbon environments.

Importantly, there is no signal between 50 and 90 ppm indicating a carbon directly bonded to oxygen, such as the methoxy carbon in methyl propanoate.

The proton NMR spectrum supports this conclusion.

The triplet at 2.35 ppm represents the \(\ce{CH2}\) group adjacent to the carbonyl carbon.

The multiplet at 1.6 ppm represents the middle \(\ce{CH2}\) group, while the triplet at 0.95 ppm represents the terminal \(\ce{CH3}\) group.

Together, these form:

\[ \ce{CH3CH2CH2-} \]

The broad signal at 11.4 ppm is consistent with the acidic O-H proton.

This gives the complete structure:

\[ \ce{CH3CH2CH2COOH} \]

Methyl propanoate would instead contain an \(\ce{O-CH3}\) group, producing a carbon-13 signal in the 50-90 ppm region and a three-hydrogen proton signal further downfield, commonly around 3.7 ppm.

Neither feature is present.

The important reasoning is that both candidates contain a carbonyl group in a similar chemical-shift region. The distinction comes from the other carbon environments and the presence of the acidic proton.

Question 5

An unknown organic compound has the molecular formula \(\ce{C4H6O4}\).

Its normal broadband proton-decoupled carbon-13 NMR spectrum contains exactly two well-resolved signals:

• 174 ppm
• 29 ppm

The signal at 174 ppm is approximately twice as tall as the signal at 29 ppm.

Its proton NMR spectrum contains:

• A singlet at 2.6 ppm, integrating to 4H.
• A broad signal at 11.3 ppm, reported as integrating to 2H.

Two possible structures are proposed:

Candidate A: Succinic acid

\[ \ce{HOOCCH2CH2COOH} \]

Candidate B: Dimethyl oxalate

\[ \ce{CH3OCOCOOCH3} \]

Assume that the reported proton integrations are normalised relative integrations, including the two exchangeable acid protons.

Answer the following:

1. Which candidate is consistent with the complete spectral evidence?
2. How many carbon atoms contribute to each carbon-13 signal?
3. Does the greater height of the 174 ppm signal establish that it represents two carbon atoms? Explain.
4. Calculate the degree of unsaturation of the molecular formula and explain how it supports your proposed structure.
5. Explain why simply counting carbon-13 signals would not distinguish the two candidates.

Solution 5

The unknown compound is succinic acid (Candidate A).

This conclusion requires combining molecular symmetry, chemical shifts, proton integration, and the limitations of carbon-13 peak intensity.

1. Identify the correct structure.

Succinic acid has the structural formula:

\[ \ce{HOOCCH2CH2COOH} \]

It contains two carboxylic acid groups connected by two methylene groups.

The molecule is symmetrical, meaning the two carbonyl carbons experience equivalent chemical environments.

The two methylene carbons also experience equivalent environments.

Consequently, the molecule produces exactly two carbon-13 signals.

This matches the observed spectrum.

2. Determine how many carbons contribute to each signal.

The 174 ppm signal represents two equivalent carbonyl carbons.

Both belong to the two carboxylic acid groups.

The 29 ppm signal represents two equivalent methylene carbons.

The complete carbon count is therefore:

The molecule contains four carbon atoms but only two distinct carbon environments.

3. Interpret the peak-height difference.

No, the greater height of the 174 ppm signal does not establish that it represents two carbon atoms.

Ordinary carbon-13 NMR signal intensities are affected by relaxation times and nuclear Overhauser effects.

Therefore, a signal representing two equivalent carbon atoms is not guaranteed to be twice as intense as a signal representing one carbon atom.

In this particular structure, we know that the 174 ppm signal represents two carbon atoms because of the molecular symmetry and the other spectral evidence, not because of its height.

This distinction is important: the peak intensity is not a reliable carbon-counting tool.

4. Calculate the degree of unsaturation.

The degree of unsaturation (DBE) can be calculated using:

\[ \mathrm{DBE}=\frac{2C+2+N-H-X}{2} \]

Where:

• \(C\) is the number of carbon atoms.
• \(H\) is the number of hydrogen atoms.
• \(N\) is the number of nitrogen atoms.
• \(X\) is the number of halogen atoms.

Oxygen atoms are not included in this calculation.

For \(\ce{C4H6O4}\):

\[ \begin{aligned} \mathrm{DBE}&=\frac{2(4)+2-6}{2}\\ &=\frac{4}{2}\\ &=2 \end{aligned} \]

The degree of unsaturation is 2.

Succinic acid contains two carbonyl double bonds, accounting for both degrees of unsaturation.

There are no additional rings or carbon-carbon double bonds required.

Notice that dimethyl oxalate also contains two carbonyl double bonds. Therefore, the molecular formula and degree of unsaturation alone cannot distinguish the candidates.

5. Explain why the signal count alone is insufficient.

This is the central difficulty of the question.

Both proposed structures are symmetrical.

In succinic acid:

• The two carbonyl carbons are equivalent.
• The two methylene carbons are equivalent.

This produces two carbon-13 signals.

In dimethyl oxalate:

• The two carbonyl carbons are equivalent.
• The two methoxy carbons are equivalent.

This also produces two carbon-13 signals.

Therefore, both candidates can produce two distinct carbon environments, despite having different functional groups.

The chemical shifts and proton NMR data resolve the ambiguity.

For succinic acid, the signal at 29 ppm is consistent with methylene carbons adjacent to carbonyl groups.

For dimethyl oxalate, the methyl carbons are directly bonded to oxygen. They would instead be expected in the 50-90 ppm region.

The proton NMR evidence is equally important:

• Succinic acid has four methylene hydrogens, producing the 4H signal at 2.6 ppm.
• Its two carboxylic acid groups provide two exchangeable O-H hydrogens, consistent with the broad signal at 11.3 ppm.
• Dimethyl oxalate would have six methoxy hydrogens and no carboxylic acid O-H hydrogens.

The observed proton data therefore strongly supports succinic acid.

The tempting mistake is to assume that one carbonyl signal means one carbonyl group, or that a taller signal must represent more carbon atoms. Neither assumption is justified.

The correct interpretation comes from combining symmetry, chemical shifts, molecular formula, and proton NMR evidence.

07Where to go next

Once you’ve matched a carbonyl signal to its likely functional group, the next step is to test whether the proposed structure is consistent with the compound’s chemical behaviour.

This is where spectroscopy becomes particularly useful alongside qualitative analysis.

You can connect your structural assignment to the reactions and observations used to identify organic compounds in Qualitative Tests for Organic Functional Groups in HSC Chemistry.

For carbon-13 NMR questions, remember the central distinction:

Signal count tells you how many different carbon environments exist. Chemical shift helps identify those environments. Neither peak height nor signal count alone tells you exactly how many carbonyl groups are present or which functional group must be responsible.