It is important to have a clear idea of the inter-relationship between various projections of a given organic molecule to understand the stereochemical implications of the reactions. For this, we should be well conversant with the methods of translating one projection into another. The methods used for converting one projection into another without changing the configuration are given in the following description.
Conversion of Fischer Projection into Sawhorse Projection and vice-versa:
(i) Fischer Projection to Sawhorse Projection:
Fischer projection of a compound can be converted into Sawhorse projection; first in the eclipsed form (in Fischer projection the groups on neighbouring carbons are considered to be eclipsing each other), by holding the model in horizontal plane in such a way that the groups on the vertical line point above, and the last numbered chiral carbon faces the viewer. Then, one of the two carbons is rotated by an angle of 1800 to get the staggered form (more stable or relaxed form).
For example, Fischer projection of an optically active tartaric acid is converted into staggered Sawhorse projection as shown.
(ii) Sawhorse Projection to Fischer Projection:
First, the staggered Sawhorse projection is converted to an eclipsed projection. It is then held in the vertical plane in such a manner that the two groups pointing upwards are away from the viewer, i.e. both these groups are shown on the vertical line. Such a conversion for 2,3-dibromobutane is shown.
Conversion of Sawhorse Projection to Fischer Projection via Newman Projection and vice-versa:
(i) Sawhorse Projection to Newman Projection And then Fischer Projection:
Conversion of Sawhorse projection to Newman projection is quite easy. The molecule is viewed from front carbon (the central C-C bond being invisible) to get the staggered Newman projection. The rear carbon is rotated by 180o to get eclipsed Newman projection. Then, the molecule is held in the vertical plane, i.e. central bond is visible in the vertical plane in such a manner that front carbon is the lowest carbon.
(ii) Fischer Projection to Newman Projection and then Sawhorse Projection:
The molecule is viewed through the lowest chiral carbon, which becomes the front carbon, and thus eclipsed Newman projection is drawn. It is then converted into staggered conformation. Finally, the molecule is viewed through the bond connecting the front carbon with rear carbon. Such a conversion of D-erythrose is illustrated in the following scheme.
Conversion of Fischer Projection into Flying Wedge Projection and vice-versa:
(i) Fischer Projection to Flying Wedge Projection:
The vertical bonds in the Fischer projection are drawn in the plane of the paper using simple lines(—). Consequently, horizontal bonds will project above and below the plane (‘a’ and ‘b’ in the fig.). Conversion of Fischer projection of one of the enantiomers of α-bromopropanoic acid into five flying wedge formulae (without changing the configuration) is illustrated in the fig.
(ii) Flying Wedge Projection to Fischer Projection:
The molecule is rotated (in the vertical plane) in such a way that the bonds shown in the plane of the paper go away from the viewer, and are vertical.
0 comments:
Post a Comment