Fischer Projection

How to read one

Take any chiral carbon. Real geometry: four bonds pointing into the corners of a tetrahedron. To squash it onto paper, Fischer chose a fixed viewing angle: hold the molecule so the central carbon's two horizontal bonds wedge out toward you and the two vertical bonds wedge away from you. Then erase the wedges and just draw a cross.

Real 3D                      Fischer projection
       OH                            CHO
        \                             |
   H — C — CHO       ───────►    H — C — OH
        /                             |
     CH₂OH                          CH₂OH
                              (D-glyceraldehyde)

Once you adopt the convention, you can stack stereocenters by connecting them through the vertical line. Read top-to-bottom along the chain; horizontal arms tell you the configuration at each carbon.

Worked example: D-glucose vs L-glucose

D-glucose has four stereocenters (C2, C3, C4, C5). Its Fischer projection:

D-glucose            L-glucose
   CHO                  CHO
   |                    |
H— C —OH           HO— C —H        (C2)
   |                    |
HO— C —H            H— C —OH       (C3)
   |                    |
H— C —OH           HO— C —H        (C4)
   |                    |
H— C —OH           HO— C —H        (C5  ← determines D vs L)
   |                    |
  CH₂OH               CH₂OH

Read the C5 row: in D-glucose the OH is on the right, so the molecule is D. L-glucose is the mirror image — every horizontal pair is reflected, including C5, where the OH is now on the left. L-glucose is real, syntheable, has the same melting point and same solubility as D-glucose, and the same calorie content if you could digest it — but no animal enzyme will touch it. L-glucose was briefly developed as a non-caloric sweetener (it tastes the same as D-glucose to D-receptors on the tongue, but is not metabolized) before the cost of preparing it killed the program.

D and L, glance assignment

Find the chiral carbon farthest from the carbonyl group (the lowest carbon when the carbonyl is drawn at the top). Look at the horizontal substituent that is the OH (or NH₂, in amino acids).

• OH on the right → D
• OH on the left → L

That is the entire rule. It comes from Fischer's original choice: he assigned D to the dextrorotatory enantiomer of glyceraldehyde and L to its mirror image, then defined every other sugar by relating the bottom-most chiral carbon back to glyceraldehyde. (His arbitrary 1891 guess about the absolute configuration turned out, by lucky X-ray crystallography in 1951, to be correct.)

Allowed and forbidden moves

You will redraw Fischer projections constantly. The moves that change nothing about the molecule:

1. Rotate 180° in the plane of the page. Top swaps with bottom, left with right; net effect is two simultaneous swaps at every stereocenter — even, no inversion.
2. Hold one group fixed and cycle the other three clockwise (or counter-clockwise). Equivalent to two swaps; allowed.

The moves that secretly invert your molecule:

1. Rotate 90° in the plane. Horizontal bonds become vertical and vice versa — every stereocenter inverts.
2. Single substituent swap. Inverts that one stereocenter.
3. Lift off the page and flip. Mirror image; every stereocenter inverts.

Mnemonic: swaps that come in pairs are free; an odd swap is a stereoinversion.

Fischer vs other projections

	Fischer	Wedge-dash (Natta)	Haworth	Newman
Best for	Acyclic sugars, amino acids	General 3D structure	Cyclic sugars	Bond rotation, conformations
Carbon shown as	Implicit cross	Explicit C with bonds	Ring corner	Front/back circle
Viewing convention	Horizontal out, vertical in	Wedges/dashes	Ring oxygen at upper right; bottom edge toward viewer	Looking down one bond
D/L visible?	Yes, instant	Requires CIP analysis	Yes, by C5 CH₂OH up/down	No
Conformation visible?	No (eclipsed lie)	Sometimes	Approximate (chair version: yes)	Yes (this is its job)
Compact?	Most compact for chains	Medium	Most compact for rings	One bond at a time
Mistake-prone moves	Forbidden 90° rotation	Wedge/dash assignment	Up vs down on ring	Front vs back atoms

Each notation has a job. Fischer is for "is this D-glucose or galactose?" Wedge-dash is for "what's the transition state geometry?" Haworth is for "is this α or β?" Newman is for "is this anti or gauche?". Mixing them up is the first sin of an organic chemistry course.

Fischer projections of amino acids

Fischer applied the same trick to amino acids. Draw the COOH at the top, the side chain at the bottom, the H and NH₂ on the horizontal arms.

L-alanine               D-alanine
  COOH                   COOH
   |                      |
H₂N — C — H          H — C — NH₂
   |                      |
  CH₃                    CH₃

NH₂ on the left = L. NH₂ on the right = D. Almost all amino acids in proteins are L. Bacteria use D-amino acids in cell-wall peptidoglycan (D-Ala-D-Ala is the cross-link that vancomycin and penicillin block); D-serine is a real signaling molecule in the brain. But the L convention is so dominant that "L-" is usually omitted from biochemistry papers.

Converting Fischer to wedge-dash and back

Step-by-step for D-glyceraldehyde:

1. Draw the Fischer projection: CHO top, CH₂OH bottom, H on the left, OH on the right.
2. Imagine bending the horizontal arms toward you (out of the page) and the vertical arms away from you — a tetrahedron with you looking at the front face.
3. Translate to wedge-dash: bold-wedge bonds for H and OH (out toward viewer), hashed-wedge bonds for CHO and CH₂OH (into page).
4. Apply Cahn-Ingold-Prelog: priorities OH (1) > CHO (2) > CH₂OH (3) > H (4). Looking with the lowest priority (H) pointing away — but H is on a wedge, so we're looking at the back of the molecule and must reverse the apparent rotation. Visible 1→2→3 = clockwise from front; reversing gives R. D-glyceraldehyde is (R)-glyceraldehyde.

This Fischer-to-CIP exercise is the standard exam question. The trap: forgetting that wedges/dashes in the Fischer mean the lowest-priority group may already be pointing toward you, requiring the rotation to be reversed.

Variants and modern uses

• Stick-Fischer — Fischer projections drawn with implicit hydrogen, useful for ribose and deoxyribose where the H is obvious.
• Truncated Fischer — only the bottom-most chiral carbon shown when context fixes the rest, common in textbooks discussing D/L only.
• Mills projection — horizontal Fischer, used occasionally for natural products with very long chains.
• Software input — SMILES strings and 2D editors have largely replaced hand-drawn Fischer in research papers, but Fischer remains the dominant teaching notation for carbohydrates.

Pitfalls

• The 90° rotation trap. A Fischer projection on a sticky note that someone rotates a quarter turn is now an invisible mirror image. Always re-anchor the carbonyl at top.
• Confusing D with R. D refers to the configuration at the lowest stereocenter relative to glyceraldehyde; R is an absolute Cahn-Ingold-Prelog descriptor. D-glucose is a (2R,3S,4R,5R)-pentahydroxyhexanal — D and R are not interchangeable.
• Eclipsed reality. Fischer projections imply every C-C bond is eclipsed, which is energetically impossible for a real molecule. Do not draw a Newman projection from a Fischer without first staggering it.
• Cyclic ambiguity. A linear Fischer of glucose hides the equilibrium with α- and β-D-glucopyranose. For pyranose chemistry, switch to Haworth or chair.
• Lowest chiral carbon ≠ lowest carbon. In ketoses (e.g. fructose) the lowest carbon is CH₂OH, not chiral. The D/L assignment uses the lowest chiral carbon (C5 in fructose).