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How can Fischer projection formulas distinguish the two isomers of glyceraldehyde?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — isomerism Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)

The two isomers of glyceraldehyde—a three-carbon sugar—are enantiomers, meaning they are non-superimposable mirror images of each other. These isomers are distinguished using Fischer projection formulas, which represent the three-dimensional spatial arrangement of atoms on a two-dimensional surface.12

In a Fischer projection, the carbon atom at position 2 serves as the chiral center. The isomers are differentiated by the specific spatial orientation of the hydrogen (—H) and hydroxyl (—OH) groups attached to this carbon. By convention, horizontal lines in the projection represent bonds projecting toward the viewer, while vertical lines represent bonds projecting away.12

Because these structures cannot be made identical without breaking and reforming chemical bonds, the Fischer projection provides a clear visual method to identify the two forms: d-glyceraldehyde and l-glyceraldehyde. These two forms rotate the plane of polarized light in opposite directions, a property known as optical rotation.1