Question

D-(+)-glyceraldehyde was allowed to undergo the Kiliani-Fischer synthesis, and the reaction ran to completion. After separation of any isomers, how many optically active products were formed? A. 0 B. 1 C. 2 D. 4

Short answer

2

Step-by-step solution

Understanding the Kiliani-Fischer Synthesis

The Kiliani-Fischer synthesis lengthens the carbon chain of an aldose by one carbon atom. Starting from D-(+)-glyceraldehyde, the process involves the formation of a new chiral center, resulting in two possible stereoisomers.

Determine the Number of Optically Active Products

Each new chiral center can create two possible stereoisomers, doubling the number of original isomers. Since D-(+)-glyceraldehyde is already chiral and has one stereocenter, after the extension, the number of stereoisomers becomes 2.

Check for Optically Active Compounds

Since we are forming chiral centers, all resulting isomers from this process will be optically active, leading to 2 optically active products.

Key concepts

Optically Active Compounds

An optically active compound is one that can rotate plane-polarized light. This ability is due to the presence of chirality in the molecule.
Chirality arises when a molecule has a chiral center, which is an atom (typically carbon) bonded to four different groups. Such molecules do not have a plane of symmetry, making them non-superimposable on their mirror images.

Here's why optically active compounds are important:

• They help in determining the purity of substances in chemistry.
• They are crucial in fields like pharmacology since different enantiomers (optically active isomers) can have different biological effects.

In the Kiliani-Fischer synthesis example, D-(+)-glyceraldehyde, which is already optically active, produces new products that maintain this property.

Stereoisomers

Stereoisomers are isomers that differ in the spatial arrangement of atoms rather than the order of atomic connectivity. There are several types of stereoisomers:

• Enantiomers: These are mirror images that are non-superimposable. Each enantiomer is optically active, but they rotate plane-polarized light in opposite directions.
• Diastereomers: These are stereoisomers that are not mirror images of each other. They often have different physical properties and can be separated by standard laboratory methods.

In the Kiliani-Fischer synthesis, the introduction of a new chiral center results in the formation of two new stereoisomers. Each new configuration around the chiral center represents a different stereoisomer. Since D-(+)-glyceraldehyde starts as a single stereoisomer, the process results in two distinct products, both of which are optically active.

Chiral Centers

A chiral center, or stereocenter, is an atom that has four different groups attached to it. The presence of a chiral center guarantees that a molecule can exist in two forms: one that is the mirror image of the other. These forms are called enantiomers.
The creation or manipulation of chiral centers is fundamental in organic synthesis, especially in processes like the Kiliani-Fischer synthesis that aim to increase complexity and functionality in molecules.

Several key points about chiral centers include:

• An increase in the number of chiral centers in a molecule increases the number of possible stereoisomers exponentially.
• Each chiral center contributes to the overall chirality of the molecule, thereby affecting its optical activity.
• In biological systems, enzymes and receptors often distinguish between different enantiomers of a chiral molecule, leading to different effects.

The Kiliani-Fischer synthesis involves forming a new chiral center, doubling the number of stereoisomers from one to two. Both resulting isomers have chiral centers, making them optically active.