Fischer Projection Formula

The Fischer projection is a specific convention for flattening a three-dimensional structure -- particularly one with one or more chiral (stereogenic) centres -- onto two dimensions, drawn as a cross at each chiral carbon. By convention, the chiral carbon itself is understood to sit exactly in the plane of the paper; the two HORIZONTAL bonds at that cross are understood to point TOWARDS the viewer (out of the page), while the two VERTICAL bonds are understood to point AWAY from the viewer (behind the page). Tartaric acid, with its two chiral carbons (each marked with an asterisk), is the worked example: its Fischer projection stacks a COOH group at the top and bottom of the cross-chain, with the OH and H groups of each chiral carbon arran …