How To Determine R and S Configurations On A Fischer Projection (2024)

Determining R and S Configurations On Fischer Projections

From time to time you may be asked to determine R/S on a Fischer projection.

How To Determine R and S Configurations On A Fischer Projection (1)

So how do you do that?

The Fischer projection is an early solution to the problem of drawing 3-dimensional molecules on a 2-dimensional page. Think of a light source illuminating a 3-dimensional object that casts a shadow on a flat surface. If these objects are always arranged in a consistent way, we can infer the 3-D structure of the object from the shadow. In the days before dashes, wedges, and the CIP rules, that’s what Fischer was going for.

[Note: Thispost was co-authored with Matthew Pierce ofOrganic Chemistry Solutions. Ask Matt about scheduling an online tutoring sessionhere. ]

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Table of Contents

  1. Interpreting A Fischer Projection
  2. Step 1: Identify The Chiral Centers
  3. Assign Cahn-Ingold-Prelog (CIP) Priorities On The Fischer Projection
  4. Trace A Path From The 1, 2, and 3 Priority Groups. Clockwise or Counterclockwise?
  5. How Do We Assign R/S When The Priority #4 Group Is In The Front?
  6. With Fischer Projections, “Reverse Rules” Will Apply 99% Of The Time
  7. Converting A Fischer Projection To A Line Diagram
  8. Summary: Determining R/S On A Fischer Projection
  9. Notes

1. Interpreting A Fischer Projection

Recall how the Fischer projection works. The longest carbon chain is arranged vertically and the substituents are drawn out to the side. Although the “projection” is drawn flat, it’s important to remember that the carbons are still tetrahedral. The convention is that the horizontal groups on the side are “wedges” – that is, as my teacher Professor Kazlauskas used to say, “the arms come out to hug you”. Or strangle you, as one of my students once suggested.

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How do you determine R/S on a Fischer? Let’s use the first molecule (a) as an example.

2. Identify The Chiral Centers On The Fischer Projection

  • The first thing to do, as always, is to identify the chiral centers. Look for carbons attached to four different groups.
  • Next, it will likely help to draw out the side groups as wedges.

How To Determine R and S Configurations On A Fischer Projection (4)

Don’t worry about putting in dashes for the groups that point up and down. Their stereochemistry isimpliedby drawing out the side groups as wedges.

3. Assign Cahn-Ingold-Prelog (CIP) Priorities

The next step is to assign priorities. [see post: Introduction to Assigning R and S – The Cahn Ingold Prelog Rules]

It’s very straightforward when the four groups are different atoms. However, for cases when the atoms directly attached to the chiral center are identical, we’ve seen that it can be helpful to apply the “method of dots” to break ties. [see post: Assigning Cahn-Ingold-Prelog (CIP) Priorities – The Method of Dots

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Here, in Round 2, (Br, H, H ) beats (O, O, O) since Br has a higher atomic number than O. (Where’d we get that “third oxygen” ? The C-O pi bond makes O a “phantom atom” for nomenclature purposes.)

4. Now Trace A Path From Priorities 1-2-3 To Determine R orS.

Recall that the Cahn-Ingold-Prelog (CIP) rules for determining R and S configurations outline a set of rules for assigning priorities (1, 2, 3, and 4) to each of the groups assigned to a chiral center.

With the #4 ranked substituent pointing “in the back”, we trace the path from the groups ranked #1, #2, and #3. If that path is clockwise, the chiral center is R.If the path is counter-clockwise, the chiral center is S.

However, many Fischer projections are not drawn so helpfully with the #4 group in the back.

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5. What do you do when #4 is in the front?

When #4 is in the front, one option is to rotate the molecule so as to put #4 in the back. This is a bad option IMO, because it’s too easy to make a mistake.

A better option, as we’ve discussed in the Single Swap Rule, is to remember that determining R/S when #4 is in the front is equivalent to determining R/S on the mirror image of the chiral center.

[When you look at your left hand in the mirror, it looks like your right hand. You use a “reverse rule” to remind yourself that your left hand is flipped in the mirror and is actually your right hand. Here, we do the same thing!]

  • Therefore if 1,2 and 3 go “clockwise” (CW) when #4 is in the front, then the true configuration is actually S, notR!
  • And if 1, 2 and 3 go “counterclockwise” (CCW) when #4 is in the front, then the true configuration is actuallyR, notS!

So when #4 is in the front, determine the direction in which priorities #1, #2, and #3 go, and then “flip” the result.

6. With Fischer Projections, “Reverse Rules” Apply Most Of The Time

Let’s go through the next two examples, a little more quickly. Assigning priorities to each of the two chiral centers in questions 2 and 3, we get2S, 3R for question #2 and2S, 3S for question #3.

How did we get these priorities? See Note 1for a walkthrough of Question #2.

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There is a useful shortcut here that comes up particularly with sugars.

For a sugar drawn in a Fischer projection with the most oxidized group at the top (i.e. a carboxylic acid or aldehyde), a chiral center with OH on the right will be R, and a chiral center with OH on the left will be S.

This makes assigningR andS to sugars in the Fischer projection a very quick process.

Be more careful with non-sugar molecules, however – one bromine atom on the bottom of the molecule is enough to throw it off.

7. Converting A Fischer Projection To A Line Diagram

Matt sends us this helpful hint on converting Fischer projections to line diagrams which works best for Fischer projections with 2, 3, or 4 carbons.

First, draw in the real shape with wedges and dashes:

How To Determine R and S Configurations On A Fischer Projection (8)

Next, we’re going to pretend to “rotate” this Fischer projection either to the left or to the right.

Imagine looking down from the ceiling on someone sleeping on their back with the arms out to hug you. If they roll over to their “right side”, we’d get the following:

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The groups on the right side on the Fischer end up as wedges, and the groups on the left side of the Fischer end up as dashes.

That looks a lot more like a typical line diagram!

Now imagine the same person rolling over onto their “left side”. We’d get this:

How To Determine R and S Configurations On A Fischer Projection (10)

Here, the groups on the right side of the Fischer end up as dashes, and the groups on the left side of the Fischer end up as wedges.

Note that in both cases, groups A and B end up being in the plane of the page.

There are limits to this method; once you get past 4 stereocenters it gets unwieldy. But it’s very useful for small Fischer projections. {Thanks to Matt Pierce for his work on this post. [Ask Matt about scheduling an online tutoring session here. ]

8. Summary: Converting A Fischer Projection To A Line Diagram

DeterminingR/S on a Fischer projection isn’t too hard once you remember that “the arms come out to hug you”, and that the #4 priority group is almost always “in the front”. Just remember to apply the “reverse rules”.

Notes

Related Articles

  • Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules
  • The Meso Trap
  • Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) – The Method of Dots
  • Assigning R/S To Newman Projections (And Converting Newman To Line Diagrams)
  • How To Draw The Enantiomer Of A Chiral Molecule
  • Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems
  • Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers
  • Stereochemistry Practice Problems and Quizzes (MOC Membership)
  • How To Draw A Bond Rotation

Note 1. Here’s a walkthrough of how priorities were determined for question #2.

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How To Determine R and S Configurations On A Fischer Projection (2024)

FAQs

How do you determine if it is R or S configuration? ›

After all 4 substituents are labeled 1 through 4, the atom is rotated so substituent 4 is pointing away. An arrow is drawn from substituent 1 toward substituent 2. If the arrow is travelling clockwise, that center is labelled R-, and if it is traveling counterclockwise, the center is labelled S-.

How are R and S designators applied to Fischer projections? ›

For a sugar drawn in a Fischer projection with the most oxidized group at the top (i.e. a carboxylic acid or aldehyde), a chiral center with OH on the right will be R, and a chiral center with OH on the left will be S. This makes assigning R and S to sugars in the Fischer projection a very quick process.

How to assign configuration in Fischer projection? ›

A chiral molecule's branches are given priority numbers, and in decreasing order, molecules are either referred to as R or S configured. When the priority groups are in clockwise order, then the configuration is R, whereas when the priority groups are in anticlockwise order, then the configuration is S.

How to determine Fischer projection? ›

In Fischer projections, the convention is that the lines going up and down on a page are going away from you in space (into the desk below the page), and the lines going left and right are coming out toward you (as if to hug you). Remember each chirality center is determined (R) or (S) individually.

How to find R and S configuration in cyclic compounds? ›

Draw an arrow starting from priority one and going to priority two and then to priority 3: If the arrow goes clockwise, like in this case, the absolute configuration is R. As opposed to this, if the arrow goes counterclockwise then the absolute configuration is S.

What is the trick for R and S configuration? ›

It is important to note that if the lowest priority is on the dash line meaning if it goes backwards (behind the plane) then in clockwise rotation and configuration is R and for anticlockwise the configuration is S and if the lowest priority group is on the wedge line meaning if it comes forward (in front of the plane) ...

What is the thumb rule for R and S configuration? ›

There are two general methods for determining the R- or S-configuration: The hand method and the clock method. The hand method points the thumb in the direction of the atom of lowest priority. The curled fingers point in the direction of descending priority of the remaining atoms.

How to determine the priority of functional groups? ›

The higher the oxidation number of the group, the higher it tends to rank. In simple terms, a functional group with MORE bonds to oxygen will outrank one with fewer bonds.

What are the rules for correct Fischer projection? ›

All the horizontal lines of a Fischer projection are projected towards the viewer and all the vertical lines are projected away from the viewers.

Is clockwise R or S? ›

When using a model, make sure the lowest priority is pointing away from you. Then determine the direction from the highest priority substituent to the lowest: clockwise (R) or counterclockwise (S).

How do you know if a Fischer projection is D or L? ›

D and L designations of sugars are based on the position of the hydroxyl on the chiral carbon farthest from the carbonyl group in the Fischer projection of the molecule. All D-sugars have the –OH on the right side and L-sugars have the –OH on the left side.

How to find R and S Fischer projection? ›

Assigning R/S Configuration in Fisher projection

If the lowest priority group (#4 group) is on a vertical bond, determine the priority decrease direction from #1→#2→#3 as usual to get the configuration, clockwise is R and counterclockwise is S.

How to determine R or S configuration? ›

When using a model, make sure the lowest priority is pointing away from you. Then determine the direction from the highest priority substituent to the lowest: clockwise (R) or counterclockwise (S).

How are Fischer projections arranged? ›

Fisher projections show sugars in their open chain form. In a Fischer projection, the carbon atoms of a sugar molecule are connected vertically by solid lines, while carbon-oxygen and carbon-hydrogen bonds are shown horizontally.

How to determine priority for R and S? ›

When using a model, make sure the lowest priority is pointing away from you. Then determine the direction from the highest priority substituent to the lowest: clockwise (R) or counterclockwise (S).

How to tell if Fischer projections are enantiomers? ›

A Fischer projection may not be rotated by 90 degrees. Such a rotation typically changes the configuration to the enantiomer. To find the enantiomer of a molecule drawn as a Fischer projection, simply exchange the right and left horizontal bonds.

How do you identify the Fischer projection as the D or L isomer? ›

One method for determining whether a molecule is D- or L- by looking at the Fischer projection of a molecule. If the -OH (-NH2 for amino acids) on the bottom-most chiral center is on the right-hand side of the Fischer projection, the molecule is “D“. If it is on the left-hand side, the molecule is “L”.

References

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