PROJECTION FORMULA OR MOLECULAR REPRESENTATION OF STEREOISOMERS

Configurational and conformational isomers have a three-dimensional structures, and is not easy to depict on a paper having only two dimensions. To overcome this problem, the following four two-dimensional structures known as projections have been evolved.

    ·       Fischer Projection

    ·       Newman Projection

    ·       Sawhorse Projection

    ·       Flying Wedge Projection

     A. Fischer Projection:

Fischer projections are always generated by orienting the molecules in such a manner that vertical bonds connected to the chirality centre are directed away from the viewer, and the horizontal bonds point towards the viewer. On paper, the projection of bonds is in shape of a cross with the chiral centre at the centre of the cross.

Fischer projection of one stereoisomer of glyceric acid ( HOOC-CHOH-CH₂OH) is shown below.

Characteristics Features of Fischer Projection:

(i) Fischer projection of a chiral molecule can be changed into another identical Fischer projection by interchanging the position of any two pairs of substituents generates the identical structure. 

(ii)  Rotation of a Fischer projection by an angle of 180o about the axis which is perpendicular to the plane of the paper gives identical structure.

     B. Newman Projection:

In Newman projection, the molecule is looked from down the length of a particular carbon-carbon bond. The carbon atom away from the viewer is called rear carbon, and is represented by a circle. The carbon atom away from the viewer is called rear carbon, and is represented by a circle. The carbon atom facing the viewer is called front carbon, and is represented as the centre of the above circle, which is shown by a dot. The remaining bonds on each carbon are shown by small straight lines at angles of 120o as follows:

a.    Bonds joined to front carbon intersect at the central dot.

b. Bonds joined to rear carbon are shown as emanating from the circumference of the circle.

Staggered and eclipsed conformations arise due to free rotation about the carbon-carbon single bond ( front and rear carbon atoms).

    C. Sawhorse Projection:

In this projection, the bond between two carbon atoms is shown by a longer diagonal line, because we look at this bond from an oblique angle. The bonds linking other substituents to these carbons are shown projecting above and below this line. Thus, Sawhorse projection of staggered and eclipsed conformations of n-butane can be drawn as follows:

Due to free rotation along the central bond, two extreme conformations are possible, staggered and eclipsed.

     D. Flying Wedge Projection:

It is a three-dimensional representation, and is particularly useful for chiral molecules. The procedure for drawing this type of projection formula is as follows:

Out of four bonds connecting substituents to a chiral carbon, two bonds are drawn in the plane of the paper, which is shown by normal lines, third bond lies above this plane, which is shown by a solid cone and the fourth bond lies below the plane of the paper which is shown by broken lines. The flying wedge formulae of two enantiomeric lactic acid are shown below.

Both these structures are mirror images of each other. In such representation, the main functional group is, generally, held on the upper side in the vertical plane.