Fischer Projections: Compare & Convert Study guide
Because a Fischer projection encodes 3D structure through a fixed convention, manipulating the drawing follows strict rules: a 180° rotation changes nothing, a 90° rotation inverts every center, and one swap at a center inverts just that center. This page mixes two kinds of problems: classify how two depictions are related, and pick the line-angle drawing that matches a Fischer projection.
Molecules:
Streak: 0
Best: 0
Common Questions
Which moves are legal on a Fischer projection?
Rotating the whole drawing 180° in the plane keeps the same molecule. Rotating 90° inverts every stereocenter, which gives the enantiomer (for a meso compound, the same compound). Swapping any two groups on one center inverts that center: one swap changes the molecule, two swaps restore it.
Why does rotating a Fischer projection 90° give the enantiomer?
The projection only encodes 3D information through its fixed convention: horizontal bonds toward you, vertical bonds away. After a quarter turn the groups that pointed toward you now point away, so every stereocenter reads inverted, which is the mirror image of the original molecule.
How do you convert a Fischer projection to a line-angle drawing?
Work one stereocenter at a time. In the Fischer projection the horizontal groups point toward you and the vertical chain bonds point away. Redraw each center that way, then relax the chain into a zigzag and check each center by assigning R/S in both drawings: the labels must match.
Can two different-looking Fischer projections show the same compound?
Yes. Any 180° rotation of the drawing is the same compound, and for a meso compound even the left-right mirrored drawing is the same compound, because a meso molecule is superimposable on its own mirror image.