Question

Write Fischer projections for L-ribose and L-arabinose.

Short answer

Answer: The Fischer projection for L-ribose is: CHO | C(OH)(H) | C(OH)(H) | C(OH)(H) | CH_2OH with an S, S, S configuration at chiral centers C2, C3, and C4. The Fischer projection for L-arabinose is: CHO | C(OH)(H) | C(OH)(H) | C(H)(OH) | CH_2OH with an S, S, R configuration at chiral centers C2, C3, and C4.

Step-by-step solution

Understand Fischer Projections

Fischer projection is a simplified 2D representation of 3D molecular structures, where horizontal lines represent bonds coming out of the plane (toward the viewer) and vertical lines represent bonds going into the plane (away from the viewer). The main functional groups are usually placed on the top of the projection. In this exercise, the main functional group for both L-ribose and L-arabinose are aldehyde (CHO) groups.

Determine the Stereochemistry of L-Ribose

L-Ribose is an aldopentose, meaning it is a five-carbon sugar with an aldehyde functional group. Its stereochemistry is such that there are 3 chiral centers (C2, C3, and C4). Because d-ribose has R, R, R configuration at these chiral centers, L-Ribose will have the opposite configuration, which is S, S, S.

Create Fischer Projection for L-Ribose

Now that we know the stereochemistry of L-ribose, we can create its Fischer projection. Place the aldehyde group on top, followed by hydroxyl groups at carbon atoms 2, 3, and 4 (OH) and then carbon atom 5 (CH_2OH) at the bottom. Based on the S, S, S configuration, the OH groups on C2, C3, and C4 will be on the left side. L-Ribose Fischer projection: CHO | C(OH)(H) | C(OH)(H) | C(OH)(H) | CH_2OH

Determine the Stereochemistry of L-Arabinose

L-Arabinose is also an aldopentose with an aldehyde functional group. It has three chiral centers (C2, C3, and C4) like L-ribose, but its stereochemistry differs. For d-arabinose, the configuration is R, R, S, so for L-arabinose, it will be S, S, R.

Create Fischer Projection for L-Arabinose

Now that we know the stereochemistry of L-arabinose, we can create its Fischer projection. Place the aldehyde group on top, followed by hydroxyl groups at carbon atoms 2, 3, and 4 (OH) and then carbon atom 5 (CH_2OH) at the bottom. Based on the S, S, R configuration, the OH groups on C2 and C3 will be on the left side, and the OH group on C4 will be on the right side. L-Arabinose Fischer projection: CHO | C(OH)(H) | C(OH)(H) | C(H)(OH) | CH_2OH