How To Draw A Fischer Projection

What are the Fischer projections

Fischer projections are just another way of drawing compounds contacting chirality centers. They were initially proposed by Emil Fischer for making it easier to draw the structures of compounds containing multiple chirality centers with the main thought of not having to draw the wedge and dash lines for every unmarried chiral eye. This is peculiarly applicable and used more often than not for drawing sugars.

How to describe Fischer projections

Suppose you have this chemical compound with one chirality heart:

Before getting to cartoon its Fischer projection, lets number the carbons in whatever order (no IUPAC rules needed). Remember, numbering carbons volition ever be helpful no matter what you need to do with an organic structure.

Here is what Fischer suggested:

If you await at the molecule from the top, y'all will see the following representation where the two groups on the side are pointing towards and the ones on the top and on the lesser are pointing away from you lot. We will evidence the ones on the sides with wedge lines and the others with dashed lines:

There are two wedge and ii dash lines which may look strange to you since nosotros ever take one of each and then the two solid lines merely it is okay-it all depends on the direction we are a looking at the molecule.

This, nevertheless, is not the Fischer projection still, since, remember, we said the primary idea was to avoid showing wedge and dash lines withal beingness able to convey accented configuration of the chirality centers (R, Southward).

For this, nosotros are going to draw the molecule and simply show all the bonds with airplane solid lines, keeping in mind that the horizontal groups are pointing towards y'all and the ones on the vertical line are pointing away from you:

So, how do you remember which ones are pointing towards you?

Well, you can remember that Fischer projections like you and they are coming to give yous a hug with open up arms:

Or, you lot expect at the Fischer projection similar y'all are in the gym and need to catch the molecule. In this instance, likewise, the horizontal groups have to be pointing towards you.

Determining the absolute configuration (R, S) of Fischer projections

To determine the absolute configuration of chirality centers in a Fischer projection, we demand to follow the same steps every bit we do for any other representation such as Bond-line or Newman, according to the Cahn-Ingold-Prelog rules.

In this molecule, for case, we need to assign the priorities of the groups on the chiral center based on the atomic numbers:

Next, depict the arrow going from priority 1-two-3:

The arrow goes clockwise which indicates R configuration. However, this is where you need to recollect that the horizontal groups (Cl and H) are pointing towards you, therefore, the configuration must be switched from R to S . This is because ane of the rules of Cahn-Ingold-Prelog arrangement is that the lowest priority must betoken abroad from the viewer.

How to draw the enantiomer and a diastereomer of a Fischer projection

Fischer projections go far easy to describe dissimilar stereoisomers. As an example, if you are asked to draw the enantiomer of the following molecule with three chiral centers, you tin can draw an imaginary mirror plane and draw the reflection of the molecule, which is achieved simply by swapping the two grouping of a chiral heart:

To ostend that these two are enantiomers, assign the accented configuration of the chirality centers. They are all inverted from R to Due south and South to R.

If you demand to depict a diastereomer of the molecule, you lot need to switch merely one chilaity middle or, alternatively, switch all except ane. In other words, they two representation should be non-mirror image stereoisomers:

Fischer projections are especially useful in cartoon carbohydrates since they contain multiple chiral centers which are more than time-consuming to draw.

For example,glucose ane of the most common and important carbohydrates also used extensively for the initial studies, was establish to benaturally as one enantiomer designated as D isomer. The enantiomer, 50-glucose tin can still be prepared synthetically:

What can be done with a Fischer Project and what is disallowed?

When redrawing a Fischer projection shown from a different management, you are allowed to rotate the molecule by 180^o but not by 90^o.

The reason for this is the fact that the accented configuration of the chirality centers must be retained since information technology is the aforementioned molecule. This is but possible if the horizontal groups stay as horizontal and vertical groups stay as vertical as well:

If you rotate the molecule by 90^o, the horizontal groups get in the vertical positions, which in Fischer projections mean that they are now pointing abroad from you lot. This changes the absolute configuration from S toR and therefore you are showing the enantiomer if the molecule:

In the following practice problems, y'all can practise converting Fischer projections to bail-line representation and assigning the R and Southward absolute configuration of chirality centers on Fischer projections.