Aldehyde vs Ketone: Tell Them Apart by the Position of the Carbonyl Group

Aldehydes and ketones are functional groups that appear very frequently in organic chemistry. Both of them contain a carbonyl group.

A carbonyl group is the part in which a carbon atom and an oxygen atom are connected by a double bond.

C=O

The difference between aldehydes and ketones is not the carbonyl group itself. The difference lies in what is attached to the carbonyl carbon.

First, summarize the difference between aldehydes and ketones

Type	General formula	Key identification point	Representative example
Aldehyde	R-CHO	The carbonyl carbon is bonded to hydrogen	Acetaldehyde
Ketone	R-CO-R'	Carbon groups are attached on both sides of the carbonyl carbon	Acetone

When reading a structural formula, first look for C=O, and then check what is bonded to that carbon.

What is an aldehyde?

An aldehyde is a compound in which the carbonyl carbon is bonded to at least one hydrogen atom.

Its general formula is written as follows.

R-CHO

This -CHO is the aldehyde group. A representative example is acetaldehyde.

CH3CHO

In this structure, the carbonyl carbon is double-bonded to oxygen and also bonded to hydrogen and a methyl group. Therefore, acetaldehyde is an aldehyde.

What is a ketone?

A ketone is a compound in which the carbonyl carbon is bonded to two carbon groups.

Its general formula is written as follows.

R-CO-R'

A representative example is acetone.

CH3COCH3

In acetone, the central carbonyl carbon is bonded to methyl groups on both sides. No hydrogen is directly bonded to the carbonyl carbon. Therefore, acetone is a ketone.

How to distinguish them in structural formulas

The procedure for distinguishing aldehydes from ketones is simple.

1. Find C=O.
2. Check whether the carbonyl carbon has a hydrogen attached.
3. If it has a hydrogen, it is an aldehyde.
4. If both sides are carbon groups, it is a ketone.

This can be summarized as follows.

What is attached to the carbonyl carbon	Classification
Hydrogen and a carbon group	Aldehyde
Carbon group and carbon group	Ketone

For beginners, the following rule of thumb is useful.

• A C=O at the end of a chain is often an aldehyde.
• A C=O in the middle of a carbon chain is often a ketone.

However, in more complex structures, the safest method is always to check what is bonded to the carbonyl carbon.

Difference in reactivity

Both aldehydes and ketones contain a carbonyl group, so they show similar kinds of reactivity. In a carbonyl group, oxygen pulls electron density toward itself, so the carbonyl carbon tends to carry a partial positive charge.

As a result, the carbonyl carbon is a site that is susceptible to nucleophilic attack.

However, aldehydes are generally more reactive than ketones. There are two main reasons for this.

They are less sterically crowded

In an aldehyde, the carbonyl carbon is bonded to a small hydrogen atom. In a ketone, carbon groups are attached on both sides. Therefore, in a ketone, the carbon groups create more steric hindrance when a nucleophile approaches the carbonyl carbon.

Ketones are often more electronically stabilized

Carbon groups can have electron-donating character. In ketones, two carbon groups are attached to the carbonyl carbon, which somewhat reduces the positive character on that carbon. As a result, aldehydes are often more easily attacked by nucleophiles.

Difference in ease of oxidation

Aldehydes and ketones also differ in how easily they are oxidized.

Type	Ease of oxidation	Product after oxidation
Aldehyde	Easily oxidized	Often becomes a carboxylic acid
Ketone	Usually resistant to oxidation	Requires strong conditions

Aldehydes have a hydrogen on the carbonyl carbon, so they are easily oxidized to carboxylic acids. Ketones, on the other hand, are not oxidized as easily as aldehydes under ordinary conditions.

This difference is also related to distinguishing tests such as the silver mirror reaction and Fehling's reaction.

Difference in nomenclature

In IUPAC names, aldehydes and ketones use different suffixes.

Type	Suffix	Example
Aldehyde	-al	Ethanal
Ketone	-one	Propanone

For example, CH3CHO is ethanal, and CH3COCH3 is propanone. In common names, ethanal is called acetaldehyde and propanone is called acetone.

Familiar examples

Aldehydes and ketones are found in many familiar substances.

Type	Example	Use or characteristic
Aldehyde	Formaldehyde	Used as a resin raw material, etc.
Aldehyde	Acetaldehyde	Related to ethanol metabolism
Aldehyde	Vanillin	Aroma component of vanilla
Ketone	Acetone	Solvent, nail polish remover, etc.
Ketone	Cyclohexanone	Industrial raw material

Summary

Aldehydes and ketones are both compounds containing a carbonyl group C=O. The difference lies in what is attached to the carbonyl carbon.

• Aldehydes are R-CHO
• Ketones are R-CO-R'
• If the carbonyl carbon has a hydrogen, it is an aldehyde
• If both sides of the carbonyl carbon are carbon groups, it is a ketone
• Aldehydes tend to be more reactive than ketones
• Aldehydes are easily oxidized to carboxylic acids
• Ketones are usually difficult to oxidize under normal conditions

When looking at a structural formula, it is useful to make a habit of first finding C=O and then checking what is attached to the carbonyl carbon.

References

1. LibreTexts, Aldehydes and Ketones.
2. OpenStax, Chemistry: Atoms First 2e, Aldehydes, Ketones, Carboxylic Acids, and Esters.