PART-3, PPT-3, SEM-1, CC-1/GE-1
Dr. Kalyan Kumar Mandal Associate Professor St. Paul’s C. M. College Kolkata STEREOCHEMISTRY PART-3: Projection Formulae CONTENTS
• Molecular Representation • Projection Formula • Fischer Projection Form • Newman Projection Formula Molecular Representation • Stereochemistry refers to molecules in three-dimensions, therefore, appropriate modes of representations of three-dimensional molecules on two-dimensional paper is essential. Since paper and blackboards are two-dimensional, it is very difficult and interconvert to represent the three-dimensional molecules in two- dimensions of paper and blackboard.
• A formal two-dimensional representation of a three-dimensional molecular structure obtained by projection of bonds (symbolized as lines) onto a plane with or without the designation of the positions of relevant atoms by their chemical symbols is called projection formula. A projection formula which indicates the spatial arrangement of bonds is called a stereochemical formula
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Projection Formulae • Several two-dimensional projection formulae have been developed to represent molecules having three-dimensional structures. These projections are:
1. Fischer projection 2. Newman projection 3. Sawhorse projection 4. Flying-wedge projection 5. Zig-zag projection
• In all these cases of depicting a stereoformula in two-dimensions, the stereochemical information about the molecule, i.e., absolute configuration at chiral centre/s is/are preserved.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Fischer Projection Formula • The Fischer projection, devised by Emil Fischer in 1891, is a two- dimensional representation (planar projection formula) of a three- dimensional organic molecule by projection. The atoms pointing sideways must project forward in the model but those pointing up and down in the projection must extend toward the rear.
• A projection formula in which vertically drawn bonds are considered to lie below the projection plane and horizontal bonds to lie above that plane. Thus for the molecule C*abcd: Fischer Projection Formula • Fischer formula is a convention for displaying the three- dimensional configurational relationship of molecules with chiral centres in a planar representation.
• In this representation, the molecule is so oriented that the each chiral carbon is in the plane of the projection (paper or blackboard) and the four bonds are shown by two vertical and two horizontal lines. The point of intersection of these lines is the seat of the chiral centre. The chiral centre is not shown by any atomic symbol.
• For example, Fischer projection of one stereoisomer of lactic acid,
H3C-CH(OH)-CO2H, is shown in Figure 2 in three different ways. Fischer projections are not normally used to represent compounds without chiral centres.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Fischer Projection Formula
• A molecule like C*abcd with one chiral centre can have a pair of enantiomers. Each enantiomer can be represented by twelve different ways in Fischer projections. These are shown in Figure 3.
• For molecules of the types C*abcC*abc (molecules containing alike asymmetric centres) or C*abcC*xyz (molecules containing unlike asymmetric centres), each stereoisomer can be represented by eighteen different Fischer projections.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Fischer Projection Formulae
There are 24 (= 4! this being the number of permutations of 4 ligands among 4 sites) ways of writing the projection formula; 12 correspond to one enantiomer and 12 to the other. Conventions of Fischer Projection • In Fischer formula formulae, certain conventions should be followed, otherwise stereochemical aspects of the structures may be misinterpreted. These are:
1. Although all bonds are represented by plain straight lines, the two vertical bonds are projected below (behind) the plane of the paper or blackboard and the two horizontal bonds are projected above (in front of) the paper or blackboard.
2. For the purpose of comparison between the two Fischer projections, one projection may be rotated 180o within the plane of the paper about an axis perpendicular to the paper. It is not permissible to rotate the projection formula within the plane of the paper either a 90o or a 270o angle.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Conventions of Fischer Projection Formula 3. The projection formula being two-dimensional, may never be lifted out of the projection plane and turned over. If this is done then vertical bonds (which were below the plane of the projection) will come above the plane, horizontal bonds will become projected below the plane, and thus the conventions of Fischer projection will be violated.
• When the number of chiral carbon in a molecule is n, then in the Fischer projection n number of horizontal lines will cut the vertical line and each point of intersection presents a chiral centre in that projection. The optical isomers of D-glyceraldehyde are shown in Figure 4 (A). 3-Bromo-2-butanol, on the other hand, has two chiral centres which are unlike and they are represented by the Fischer projections in Figure 4 (B).
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Fischer Projection Formula Fischer Projection Formula Newman Projection Formula • A projection formula representing the spatial arrangement of bonds on two adjacent atoms in a molecular entity. The structure appears as viewed along the bond between these two atoms, and the bonds from them to other groups are drawn as projections in the plane of the paper. The bonds from the atom nearer to the observer are drawn so as to meet at the centre of a circle representing that atom. Those from the further atom are drawn as if projecting from behind the circle.
• A Newman projection, useful in alkane stereochemistry visualizes the conformation of a chemical bond from front to back, with the front atom represented by a dot and the back carbon as a circle. The front carbon atom is called proximal, while the back atom is called distal. This type of representation clearly illustrates the specific dihedral angle between the proximal and distal atoms.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Newman Projection Formula • In the Newman projection, the molecule bearing the concerned carbon atoms is viewed along the bond joining the carbon atoms (chiral, prochiral or achiral), and these atoms are represented as superimposed circles, only one circle being drawn.
• The centre of the circle represents the front carbon atom and the circumference the back carbon atom. The remaining bonds on each carbon are shown by small straight lines at angles of 120° joined to the centre and to the circumference as shown in Figure 7. Newman Projection Formula • In Newman projection, the line joining the two key carbon atoms represented by circle and its centre is not visible. In eclipsed form of Newman projection, the vertical bonds may be arranged in such a way that they remain below the horizontal plane, as shown in Figure 8 (shown by dotted line), and transformation to Fischer projection is then carried out keeping front chiral atom as the lowest chiral centre in the Fischer projection (counting from the top).
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata