Questions: Fischer Projections and Wedge-Dash Representation

Question 1

In a Fischer projection of an amino acid, the amino group is drawn on the horizontal left and the hydrogen on the horizontal right at the alpha carbon. What is the spatial relationship of these groups relative to the viewer?

• A. Both project away from the viewer into the page
• B. Both project toward the viewer out of the page
• C. The amino group is in the plane of the page; the hydrogen projects forward
• D. The stereochemistry is ambiguous without wedge-dash notation

Answer: (B)

In a Fischer projection, ALL horizontal bonds at a stereocenter project toward the viewer (out of the page), and all vertical bonds project away. This convention is fixed and encodes the 3D structure without any explicit wedge or dash symbols. Both the amino group and the hydrogen (horizontal) therefore project forward. Option D is the core misconception: Fischer projections are not ambiguous — the horizontal = forward rule is exactly what makes them unambiguous.

Question 2

A student has a Fischer projection and rotates it 90° in the plane of the page. What happens to the stereochemical configuration at each stereocenter?

• A. Nothing changes — rotations in the plane preserve configuration
• B. Every stereocenter is inverted — what was horizontal is now vertical and vice versa
• C. The absolute configuration is preserved but the drawing looks different
• D. Only the top and bottom stereocenters are affected; middle ones are unchanged

Answer: (B)

A 90° rotation in the plane swaps what was horizontal (toward viewer) with what was vertical (away from viewer) — inverting the spatial relationship at every stereocenter simultaneously. The rule is: rotating a Fischer projection 90° changes the configuration. Rotating 180° does not, because horizontal bonds rotate to become horizontal again and vertical to vertical again. Students who assume any in-plane rotation is safe will consistently assign incorrect configurations.

Question 3

In a Fischer projection, vertical bonds at a stereocenter point toward the viewer.

• T. True
• F. False

Answer: False

This is false — the opposite is true. In a Fischer projection, HORIZONTAL bonds point toward the viewer (out of the page), and VERTICAL bonds point away from the viewer (into the page). Confusing this convention is the most common error in Fischer projection problems. A helpful memory aid: the horizontal bonds 'reach out' toward you like arms extending forward.

Question 4

Swapping any two groups at a stereocenter in a Fischer projection inverts the configuration at that center.

• T. True
• F. False

Answer: True

True — each two-group swap at a single stereocenter inverts the configuration at that center, producing the mirror image at that position. This is directly analogous to Walden inversion: any permutation that changes the spatial arrangement results in the opposite R/S designation. Two swaps return you to the original. This rule lets you verify Fischer-to-wedge conversions: if you ended up with the wrong configuration, you can diagnose whether you made an odd number of swaps.

Question 5

Why does a 90° rotation of a Fischer projection in the plane of the page change the stereochemical configuration, while a 180° rotation does not?

Model answer: A Fischer projection encodes directionality: horizontal bonds point toward you, vertical bonds point away. A 90° rotation converts what was horizontal (toward viewer) into vertical (away from viewer) at each stereocenter — swapping the spatial direction of all bonds and inverting every stereocenter. A 180° rotation converts horizontal bonds back to horizontal and vertical back to vertical — preserving the toward/away assignments of every bond and leaving all configurations unchanged.

The key is that the Fischer projection's 3D meaning depends on which bonds are horizontal vs. vertical, not on their absolute orientation in the plane. 90° shuffles the horizontal/vertical category; 180° preserves it. This is why flipping the projection off the page (a forbidden operation) also inverts configurations — it reverses which direction 'horizontal' points in 3D space.