Question

What is the difference between an aldose and a ketose?

Short answer

Aldoses have an aldehyde group at C-1; ketoses have a ketone group at C-2.

Step-by-step solution

Define Aldose

An aldose is a type of monosaccharide (simple sugar) that contains an aldehyde group (-CHO) at the first carbon atom. This means the carbonyl group (C=O) is attached to the first carbon.

Define Ketose

A ketose is another type of monosaccharide that contains a ketone group (C=O) at the second carbon atom. Here, the carbonyl group is attached to the second carbon.

Highlight Key Differences

The primary difference between an aldose and a ketose is the position of the carbonyl group. In aldoses, the carbonyl group is at the end of the molecule (an aldehyde group at C-1), while in ketoses, it is typically at the second carbon (a ketone group at C-2).

Key concepts

Monosaccharide Structure

Monosaccharides are the simplest form of carbohydrates. They are the building blocks of more complex carbohydrates like disaccharides and polysaccharides.
Monosaccharides usually consist of a carbon backbone of 3-7 carbon atoms, with multiple hydroxyl groups (-OH) attached to them.
Each carbon in the chain that isn't part of a carbonyl group (C=O) or bound to another functional group, typically has one -OH group.
Depending on the location of the carbonyl group, monosaccharides are classified either as aldoses or ketoses. Aldoses contain an aldehyde group, while ketoses contain a ketone group.

Aldehyde Group

An aldehyde group (CHO) is a functional group characterized by a carbon atom double-bonded to an oxygen atom, and single-bonded to a hydrogen atom.
In aldoses, the aldehyde group is always at the first carbon atom (carbon-1).
This means the carbonyl group in aldoses is always positioned at the end of the carbon chain.
Examples of aldoses include glucose and galactose.

Ketone Group

A ketone group (C=O) is a functional group where a carbon atom is double-bonded to an oxygen atom, but unlike aldoses, this carbonyl group is located within the carbon chain.
In ketoses, you will find the carbonyl group usually at the second carbon atom (carbon-2).
This central location gives ketones a structural difference compared to aldoses.
Examples of ketoses include fructose and ribulose.

Carbonyl Group Position

The position of the carbonyl group (C=O) plays a key role in determining whether a monosaccharide is an aldose or a ketose.
For aldoses, the carbonyl group is always found at the end of the molecule, on carbon-1.
For ketoses, the carbonyl group is located at the second carbon (carbon-2), within the molecule.
This difference in carbonyl group position not only influences the chemical reactivity but also the biological function of the sugar.

• In aldoses, the carbonyl group is part of an aldehyde.
• In ketoses, the carbonyl group forms a ketone.