Aldehydes and ketones are among the most important functional groups in organic chemistry. Their behavior shapes the structure of biological molecules, the synthesis of pharmaceuticals, and many of the reactions chemists rely on in the laboratory. Although both contain the same defining carbonyl group, their structures, properties, and reactivity differ in meaningful ways.
This guide gives a clear, structured explanation of how the carbonyl group works, how aldehydes and ketones are named, and why they react the way they do.
The Carbonyl Functional Group: Structure and Reactivity
The defining feature of both aldehydes and ketones is the carbonyl group (C=O) — a carbon atom double-bonded to an oxygen atom. This bond is strongly polarized: oxygen pulls electron density toward itself, leaving carbon partially positive.
This polarity makes the carbonyl carbon an attractive target for nucleophiles and gives aldehydes and ketones many of their characteristic reactions.
Aldehydes vs. Ketones: Key Structural Differences
Although both contain a carbonyl group, their placement within the molecule differs:
- Aldehydes have at least one hydrogen atom bonded to the carbonyl carbon.
- The carbonyl group appears at the end of the chain.
- Example: ethanal (CH₃CHO)
- Ketones have the carbonyl carbon bonded to two carbon atoms.
- The carbonyl group appears inside the chain.
- Example: propanone (CH₃COCH₃)
This seemingly small difference has biochemical consequences — aldehydes tend to be more reactive because their carbonyl carbon is more exposed.
How Aldehydes and Ketones Are Named (IUPAC Rules)
Naming Aldehydes
Replace the -e ending of the parent alkane with -al.
- Methane → methanal
- Butane → butanal
Because the carbonyl is always on carbon 1, no number is needed.
Naming Ketones
Replace the -e ending of the parent alkane with -one.
- Propane → propanone
- Pentane → pentan-2-one
For ketones, the carbonyl carbon must be numbered to show its position.
Physical and Chemical Properties of Aldehydes and Ketones
Polarity and Solubility
The carbonyl group is highly polar. As a result:
- Aldehydes and ketones can interact with water molecules through dipole-dipole interactions.
- Small carbonyl compounds (1–4 carbons) dissolve readily in water.
- As chain length increases, solubility decreases because the non-polar hydrocarbon portion becomes dominant.
They are more soluble than alkanes but less soluble than alcohols, which can hydrogen-bond directly.
Industrial and Biological Relevance
Because they dissolve a wide variety of organic substances, aldehydes and ketones frequently serve as solvents in manufacturing, extraction, and pharmaceutical processing.
In biology, carbonyl chemistry appears in metabolic pathways, including carbohydrate interconversion, amino acid catabolism, and enzyme-catalyzed oxidation–reduction reactions.
How Aldehydes and Ketones Are Formed: Controlled Oxidation of Alcohols
Oxidation of Alcohols
Unlike combustion (complete oxidation), controlled oxidation introduces oxygen without fully breaking down the molecule.
Common oxidizing agents include:
- potassium permanganate (KMnO₄)
- potassium dichromate (K₂Cr₂O₇)
- hydrogen peroxide (H₂O₂)
Products of Alcohol Oxidation
- Primary alcohol → aldehyde
One hydrogen remains on the carbonyl, placing it at the end of the chain.
- Secondary alcohol → ketone
The carbonyl forms between two carbon groups.
- Tertiary alcohol → no reaction
There is no hydrogen on the carbon bearing the OH group, preventing carbonyl formation.
This reaction is central in laboratory synthesis and mirrors processes in human metabolism (e.g., ethanol oxidation to acetaldehyde).
Hydrogenation: Converting Aldehydes and Ketones Back into Alcohols
Hydrogenation reduces the carbonyl group by adding hydrogen across the C=O double bond.
- One hydrogen attaches to the oxygen
- One attaches to the carbon
A catalyst, high pressure, and elevated temperature are required.
Products
- Aldehyde → primary alcohol
- Ketone → secondary alcohol
This reaction is widely used in industrial chemistry, including the synthesis of fragrances, pharmaceuticals, and polymer precursors.
Aldehydes and ketones form the foundation of carbonyl chemistry, linking basic organic structure to the biochemical reactions that power life. Understanding how the carbonyl group works — from naming rules to oxidation–reduction mechanisms — helps students build a strong conceptual framework for advanced topics in organic chemistry, biochemistry, and molecular biology. With their versatile reactivity and central role in synthesis, aldehydes and ketones remain essential functional groups worth mastering.
