Chapter 5: Stereochemistry

# 5.5 Fisher Projection

For the discussions so far, the perspective formula with solid and dashed wedges has been used to represent the 3D arrangement of groups bonded to a chirality center. In addition, there is another broadly applied formula for that purpose: the Fisher projection. A Fisher projection is a shortcut for showing the spatial group arrangement of a chirality center; it is easier to draw and recognize and is particularly useful for showing structures with more than one chirality center.

In a Fisher projection, the chirality center is shown as the intersection of two perpendicular lines. **The horizontal lines represent the bonds that point out of the plane, and the vertical lines represent the bonds that point behind the plane. **

**It is very important to keep in mind that the lines in Fisher projection are not just bonds, they represent the bonds with specific spatial arrangements and stereochemistry.**

**Assigning an R/S Configuration in a Fisher projection**

Taking the following compound as an example:

- Assign group priority as we usually do.

- 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 isand counterclockwise is**R**.**S**

So, the example here is an * R*-isomer, and the complete name of the compound is (

**R**)-2-chlorobutane.

- If the lowest priority group is on a
**horizontal**bond (as is the case in the following structure), determine the priority decrease direction as in step 2, then**reverse the answer to the opposite**way to get the final configuration.

So, the example here is a * S*-isomer, and the complete name of the compound is (

**S**)-2-chlorobutane.

Exercises 5.6

Explain why in step 3 of the above procedure, the answer should be **reversed** to get the final (actual) configuration.

Answers to Chapter 5 Practice Questions

Exercises 5.7: Indicate the configuration of the following structures.

**Properties of a Fisher projection**:

1. One switch (interchange) of two groups in a Fisher projection inverts the configuration, and two switches bring the original isomer back.

For the above structures:

- one switch of
**A**leads to**B**, and**A**and**B**are enantiomers. - one switch of
**B**leads to**C**, and**B**and**C**are enantiomers. - two switches of
**C**lead to**A,**and**A**and**C**are identical.

2. Rotate the Fisher projection 180º to get the same structure, with the configuration retained.

- A 180º rotation of
**A**leads to**B**, and**A**and**B**are identical.

3. Rotate the Fisher projection 90º to get the configuration inverted.

- 90º rotation of
**A**leads to**B**, and**A**and**B**are enantiomers.

**Do NOT rotate the Fisher projection 90º, unless you have to. Keep in mind that the configuration gets inverted by a 90º rotation.**