(a) There is only one ketotriose, called dihydroxyacetone. Draw its structure.
(b) There is only one aldotriose, called glyceraldehyde. Draw the two enantiomers of glyceraldehyde.

Verified step by step guidance

1. Step 1: Understand the terms 'ketotriose' and 'aldotriose'. A 'triose' is a monosaccharide with three carbon atoms. A 'ketotriose' contains a ketone functional group, while an 'aldotriose' contains an aldehyde functional group.
2. Step 2: For part (a), dihydroxyacetone is the only ketotriose. Its structure consists of three carbon atoms: the middle carbon is part of a ketone group (C=O), and the other two carbons each have a hydroxyl group (-OH) attached. Draw the structure as follows: CH2OH-C(=O)-CH2OH.
3. Step 3: For part (b), glyceraldehyde is the only aldotriose. Its structure consists of three carbon atoms: the first carbon is part of an aldehyde group (-CHO), the second carbon has a hydroxyl group (-OH) attached, and the third carbon is part of a primary alcohol group (-CH2OH).
4. Step 4: Glyceraldehyde has two enantiomers (non-superimposable mirror images) because the second carbon is a chiral center. To draw the enantiomers, place the -OH group on the second carbon either to the right (D-glyceraldehyde) or to the left (L-glyceraldehyde) in a Fischer projection.
5. Step 5: Verify the structures by ensuring that the ketotriose (dihydroxyacetone) has no chiral centers and that the aldotriose (glyceraldehyde) has one chiral center, leading to two enantiomers. Label the enantiomers clearly as D- and L-glyceraldehyde.

Key Concepts

Ketotriose

A ketotriose is a type of carbohydrate that contains three carbon atoms and a ketone functional group. The only ketotriose is dihydroxyacetone, which has the molecular formula C3H6O3. Its structure features a central carbon atom bonded to a ketone group (C=O) and two hydroxyl groups (–OH), making it a simple sugar that plays a role in metabolism.

Aldotriose

An aldotriose is a carbohydrate with three carbon atoms and an aldehyde functional group. Glyceraldehyde is the only aldotriose, characterized by its molecular formula C3H6O3. It exists in two enantiomeric forms, which are mirror images of each other, due to the presence of a chiral carbon atom, making it an important compound in biochemistry and metabolism.

Enantiomers

Enantiomers are a pair of molecules that are non-superimposable mirror images of each other, often differing in their optical activity. In the case of glyceraldehyde, the two enantiomers are D-glyceraldehyde and L-glyceraldehyde, which have identical physical properties except for their interaction with polarized light. Understanding enantiomers is crucial in organic chemistry, especially in the context of biological systems where chirality can affect the function of molecules.