Fischer Projections

Fischer Projections: Unveiling Hidden Chirality

Imagine a molecule with a central carbon atom bonded to four different groups. This carbon is a chiral center, meaning its mirror image isn't superimposable on itself, like our left and right hands. But what if the central carbon has two identical groups? Here's where Fischer projections come in.

A Fischer projection is a two-dimensional way to represent a three-dimensional molecule, particularly useful for showcasing chirality. It depicts the molecule as a flat carbon chain with horizontal lines representing bonds projecting towards the observer and vertical lines representing bonds going away. Here's how it works:

• The central carbon: Represented by the intersection of horizontal and vertical lines (forming a cross).
• Substituents: Attached to the ends of the lines.
• The most oxidized carbon is on top and the least oxidized on the bottom.

Examples:

1. D-Glyceraldehyde (a simple sugar):

Glyceraldehyde, the simplest sugar molecule with a chiral center. It has a CHO (aldehyde group), CH₂OH (hydroxyl methyl group), H, and another OH atom attached to the central carbon.

• Following the rules, CHO most oxidized C goes to the top vertical line.
• CH₂OH, the least oxidized C goes to the bottom vertical line.

There are two possible Fischer projections for glyceraldehyde, depending on the placement of the remaining H atom:

• D-Glyceraldehyde: Has the OH on the right.  In general,  you would look at the lowest chiral center, if the heteroatom is on the right it is D.
• L-Glyceraldehyde: Has the OH on the left.

1. Lactic Acid:

Lactic acid has a central chiral carbon with a CH₃, COOH, OH, and H attached. We follow the same procedure, assigning priorities and placing them in the Fischer projection. The resulting projection with the OH to the left represents L-lactic acid.

Benefits of Fischer Projections:

• Visualizing chirality: They clearly show the spatial arrangement around the chiral center, allowing us to distinguish between enantiomers.
• Drawing complex molecules: They simplify the representation of molecules with multiple chiral centers, especially useful for carbohydrates and amino acids.
• Relating to 3D structures: With practice, you can mentally convert a Fischer projection back to the 3D molecule.