Stereocheistry

STEREOCHEMISTRY

 

Fischer  projection

 This is a method of representing three dimensional structures in two dimension. In this method, the chiral atom(s) lies in the plane of paper. The horizontal substituents are pointing towards the observer and the vertical substituents are away from the observer. Fisher projection formula for tartaric acid is given below.

Newman projection

In this method the molecules are viewed from the front along the carboncarbon bond axis. The two carbon atom forming the σ bond is represented by two circles. One behind the other so that only the front carbon is seen. The front carbon atom is shown by a point where as the carbon lying further from the eye is represented by the origin of the circle. Therefore, the C-H bonds of the front carbon are depicted from the circle while C-H bonds of the back carbon are drawn from the circumference of the circle with an angle of 1200 to each other.

Sawhorse projection

Here the bond between two carbon atoms is drawn diagonally and slightly elongated. The lower left hand carbon is considered lying towards the front and the upper right hand carbon towards the back. The Fischer projection inadequately portrays the spatial relationship between ligands attached to adjacent atoms. The sawhorse projection attempts to clarify the relative location of the groups

Conversion of Newman projection to fisher

A good strategy here is to convert the Newman projection to a bond-line structure and from there get to the Fischer projection as we did above.

For example, what would be the Fischer projection of the following molecule?

 

Again, if it the direction is not specified, you can choose one and draw the bond-line structure based on that.

Let’s look at the molecule from the right side. This will put the CN and Br groups as wedge lines, the OH and Cl as dash lines. The zig-zag will have the methyl on the bottom-left side and the ethyl group on the right side respectively:

 

Now, to transform the bond-line into a Fischer projection, we can look at from the top such that the methyl group goes on the top of the Fischer projection:

 

Remember, to flip the bond between the carbons in order to have the horizontal groups as wedge lines before drawing out the Fischer projection: