Glucose has four stereocenters in a row. In a wedge-and-hash drawing, each center has to be worked out on its own, and two such drawings are hard to compare. A Fischer projection packs the same information into a stack of crosses that you can read at a glance. Chemists have drawn carbohydrates this way since Emil Fischer devised it in 1891.
Fischer worked out the shapes of the sugars with chemical reactions and this notation, long before any instrument could show a molecule’s shape. That work was part of what earned him the 1902 Nobel Prize in Chemistry.
The drawing convention
Start with glyceraldehyde, the simplest chiral sugar, which has one stereocenter. Its Fischer projection is the cross on the left of the figure below. The carbon sits where the lines cross and is usually not written. CHO is at the top, CH₂OH at the bottom, and H and OH at the two ends of the horizontal line.
The cross is not a flat drawing. It makes a fixed claim about shape: horizontal lines point toward you, out of the page, and vertical lines point away from you, into the page. So the cross means exactly what the middle drawing shows, with H and OH on wedges (bonds toward you) and CHO and CH₂OH on hashes (bonds away from you). Nothing on the drawing shows this, and it holds for every Fischer projection, so the rule has to be memorized.
For a longer chain, the stereocenters are stacked one above the other and the carbon chain runs down the page. By convention the most oxidized carbon, an aldehyde for example, goes at the top.
Where the convention comes from
The rule comes from the shape of a real carbon chain. Take erythrose, a four-carbon sugar with stereocenters at C2 and C3. Hold a model of it so that the chain arches toward you and both ends curl back into the page, as if the chain were bent round the front of a barrel lying on its side.
In that pose, the bond from C2 up to C1 and the bond from C3 down to C4 curve back into the page, and the H and OH on each carbon stick out toward you. At each carbon, both chain bonds lean away from you. Seen from the front, the model looks like the middle drawing below. Flatten that drawing onto the page and you get the Fischer projection on the right.
In this pose every bond along the chain is eclipsed: look along the C2–C3 bond and the groups on C2 sit directly in front of the groups on C3, as in the Newman projections of Alkanes & Conformations. A Fischer projection always shows this eclipsed conformation, even though the real molecule spends most of its time in others. That does no harm, because turning about a single bond never changes a stereocenter.
Reading R or S from a Fischer projection
Take the glyceraldehyde from the first figure and rank its groups by CIP priority, as in R/S configuration. OH is 1. CHO is 2, because its C=O counts as (O,O,H). CH₂OH is 3, with (O,H,H). H is 4.
Now trace 1 → 2 → 3 on the cross. The path from OH on the right, up to CHO at the top, then round to CH₂OH at the bottom runs counterclockwise, which would read as S. But H is on the horizontal line, so it points toward you, and the answer flips. This molecule is (R)-glyceraldehyde.
This is the same flip you use whenever priority 4 sits on a wedge. You are looking at the center from the wrong side, and that reverses the turn you see. If priority 4 sits on the vertical line instead, it already points away, and you read the turn with no flip.
D and L
Glyceraldehyde is also the reference for an older pair of labels, D and L. Draw it as a Fischer projection with CHO at the top. If the OH is on the right, it is D-glyceraldehyde; if the OH is on the left, it is L-glyceraldehyde. D-glyceraldehyde is the (R) enantiomer from the last section.
Every other sugar is compared with glyceraldehyde at just one stereocenter: the one furthest from the carbonyl. With the carbonyl end at the top, that is the lowest stereocenter, which is C5 in glucose. If that OH points right, as in D-glyceraldehyde, the sugar is D; if it points left, the sugar is L.
Amino acids use the same idea. An amino acid has one carbon, the α carbon, that carries an amino group (NH₂), a carboxyl group (COOH), an H and a side chain. Draw that carbon as a Fischer projection with the COOH at the top and the side chain at the bottom. If the NH₂ is then on the left, the amino acid is L; if it is on the right, it is D.
The rule only works in that orientation. Turn the drawing 180° in the page, so COOH is at the bottom, and the NH₂ of the same molecule moves to the right.
Almost all naturally occurring sugars are D, and almost all of the amino acids that cells build proteins from are L. This one-handedness of living things, called homochirality, is one of the more striking facts about life on Earth, and its origin still has no settled explanation.
D and L are relational labels. They say which enantiomer of glyceraldehyde a compound matches at its reference stereocenter. They do not tell you which way the compound rotates plane-polarized light: D-glucose and D-fructose are both D, yet they rotate light in opposite directions.
Nor do D and L map reliably onto R and S. D-glucose is (2R,3S,4R,5R), a mix of descriptors, and its single D describes C5 alone. L-alanine is (S), but L-cysteine is (R). Cysteine has the same shape as L-alanine; only the ranking changes, because its CH₂SH group outranks COOH under the CIP rules.
Amino acids have a second test for L, called CORN, that works on a wedge-and-hash drawing. It is taught in Amino acids.
Moves that keep the molecule
A Fischer projection often arrives in an awkward orientation, and you may want to turn it to compare it with another. Only some moves are safe. Start from (R)-glyceraldehyde and turn the whole drawing 180° in the plane of the page. CHO ends up at the bottom and OH on the left. The vertical groups are still vertical and the horizontal groups are still horizontal, so every group that pointed at you still points at you. You have spun the real molecule round like a wheel, and it is still (R).
The second safe move is to hold one group in place and cycle the other three one position round the cross. In the figure, H stays on the left while CHO, OH and CH₂OH each move one place clockwise. In 3D this is the same as turning the molecule a third of a turn about its C–H bond, so it is still (R).
Moves that give the enantiomer
Turn the drawing 90° clockwise instead and the result changes. The groups that were vertical are now horizontal, so the drawing says they point toward you. The groups that pointed toward you now sit on the vertical line and point away. Every group has swapped depth, as if reflected in the page, so the new drawing is the mirror image: (S)-glyceraldehyde.
You can check it with the R/S method. H is now at the top, on the vertical line, so it points away and needs no flip. The path from OH at the bottom, to CHO on the right, to CH₂OH on the left runs counterclockwise, which gives (S).
The other unsafe move is to swap any two groups once. Swapping H and OH puts the OH on the left, which gives L-glyceraldehyde, the (S) enantiomer. A single swap always inverts a stereocenter, as R/S configuration showed. That makes it the quick way to draw an enantiomer on purpose, and a common way to draw one by accident.
You can count swaps: a second swap at the same center undoes the inversion caused by the first. So an even number of swaps returns the molecule you started with, and an odd number gives its enantiomer. This lets you rearrange a projection into a handier form: make two swaps and you know nothing has changed. The safe cycle of three groups is two swaps in disguise.
Count the swaps at each stereocenter separately when there is more than one. Start from D-erythrose and swap H and OH at C2 only. C2 is inverted and C3 is not, so the result, D-threose, is a diastereomer of D-erythrose. Swap at both centers instead and every center is inverted, which gives the enantiomer, L-erythrose.
From a wedge-and-hash drawing to a Fischer projection
Going from a Fischer projection to wedges and hashes is direct: draw the horizontal bonds as wedges and the vertical bonds as hashes, as in the first figure. The reverse takes more care, because a wedge-and-hash drawing can show the molecule in any pose. The safest route goes through R and S.
Take (S)-butan-2-ol, drawn with OH at the top, the ethyl group at lower left, the methyl group at lower right and H on a hash. Its priorities are OH, then CH₂CH₃, then CH₃, then H. Put the carbon chain vertical. Neither end is more oxidized than the other, so put C1, the CH₃, at the top and the CH₂CH₃ at the bottom. Then put OH and H on the horizontal line either way round, say OH on the left.
Now read R or S from this draft. With H on the horizontal line, OH → CH₂CH₃ → CH₃ runs counterclockwise and flips to (R). That is the wrong enantiomer, so swap OH and H. The new drawing reads (S) and matches.
So the method has three steps. Assign R or S to the drawing you are given. Draw a cross with the chain vertical and the most oxidized end at the top, and put the other two groups on the horizontal line either way round. Read R or S from the cross, and if it disagrees, swap the two horizontal groups. With several stereocenters, do this at each one in turn.
Why Fischer projections are still used
In a Fischer projection, glucose’s four OH groups form a pattern, right, left, right, right, that you can match against another sugar in a second. Telling glucose from mannose or galactose becomes pattern recognition instead of analysis.
All three sugars in the figure are aldohexoses, six-carbon sugars with an aldehyde at C1. Each has four stereocenters, so 24 = 16 aldohexoses exist. Two sugars that differ at exactly one stereocenter are epimers: glucose and mannose are C2 epimers, and glucose and galactose are C4 epimers. All three are D, because each has its C5 OH on the right, even though each one’s full set of descriptors mixes R and S.
What carries forward
Fischer projections are the standard notation for carbohydrates, and they appear throughout biochemistry. They are also the fastest way to spot an internal mirror plane, as Meso compounds noted. Because the drawing is eclipsed, a meso compound usually shows its mirror plane as a horizontal line across the middle, which is much harder to see in a wedge-and-hash drawing.
Stereochemical notation is a set of tools, each suited to its own job: wedges for mechanisms, chairs for rings, and Fischer projections for chains of stereocenters.