Fischer Projections

Fischer Projections

Molecules containing several chiral carbons (i.e., carbohydrates) are represented by Fischer projections. In Fischer projections the stereoisomer is represented as a flat surface and the chiral carbon is represented by the center of two crossed lines.

If there are more than one chiral center, then the chain is drawn vertically with the C-1 drawn at the top.

Drawing Fischer Projection:

Fischer projections are drawn as a cross. The chiral carbon is the point at which the lines cross. The horizontal lines are the wedges which projects out of the plane towards the viewer and the vertical lines are the dashed lines extending back from the plane away from the observer.

To convert a perspective drawing into Fischer projection first of all draw the perspective 3-D structure. After that rotate the molecule in such a way that four bonds represent two cross lines, two on horizontal axis projecting out of the plane of the paper towards the viewer and two lines on vertical axis extending back out of the plane of the paper away from the observer.

Interconverting Fischer projections, Bond-line structures, and Newman Projection:

To convert Fischer projection to bond line structure, first assign the wedge (x-axis) and dash (y-axis) bonds to Fischer projection. The wedge line represents bonds towards the viewer while, dash bond represent bond away from the viewer.

After that rotate the 3-D molecule in such a way that two bonds align on the plane of the paper, one bond extending backwards from the plane of the paper and the last one projecting upwards from the plane of the paper towards the viewer.

If there are more than one chiral carbon atoms present in the molecule, then follow given step.

• A) Draw the Fischer projection and assign carbon atom numbers.

• B) Rotate the entire molecule at 90° counterclockwise and draw it in zig-zag fashion.

• C) The above zig-zag structure is in eclipsed form. Rotate the bond between C2 and C3 to form more stable staggered conformation.

To convert Fischer projections to bond-line structure the above steps are followed in opposite direction.