Stereochemistry Stereochemistry

1. Flying-Wedge or Wedge-Dash projection The Flying-Wedge projection is the most common three-dimensional representation of a three dimensional molecule on a two dimensional surface (paper). This kind of representation is usually done for molecules containing chiral centre. In this representation, the ordinary lines represent bonds in the plane of the paper. A solid Wedge ( ) represents a bond above the plane of the paper and a dashed wedge ( ) or a broken line ( ) represents a bond below the plane of the paper.

Methane

The Flying-Wedge projection formula of (R)- Lactic acid , for example, can be shown as follows…..

2. FISCHER PROJECTION FORMULAE The carbon chain is projected vertically, the horizontal bonds attached to a carbon are considered to be above the plane of the paper and towards the viewer and the vertical bonds are considered to be below the plane of the paper and at the back of viewer. In Fischer Formula, if two like groups are on the same side, the molecule is called ‘Erythro” and if two like groups are on opposite side it is called ‘threo’ .

3. SAWHORSE FORMULAE In this representation, the molecule is viewed slightly from above and form the right and then projected on the paper. The bond between the two carbon atoms is drawn diagonally and of a relatively greater length for the sake of clarity. The lower left hand carbon is taken as the front carbon and the upper right hand carbon as the back carbon . All parallel bonds in sawhorse formula are Eclipsed and all anti parallel bonds are opposite or trans/anti to each other. The sawhorse presentation of Eclipsed and staggered conformations of Ethane are as follow.

4. NEWMAN PROJETION FORMULAE Newman devised a very simple method of projecting three dimensional formula on paper which are known as Newman projections. In these Formulae the molecule is viewed from the front. The carbon atom nearer to the eye is represented by a point and the three atoms or groups are shown attached to it by three lines at an angle of 1200 to each other. In Newman’s formula all parallel bonds are Eclipsed and all anti- parallel (or) opposite bonds are

Newman projections for Eclipsed and staggered conformation of Ethane are

I S O M E R I S M Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. Stereoisomers- In stereoisomerism, the atoms making up the isomers are joined up in the same order, but still manage to have a different spatial arrangement.

Conformations of Acyclic Alkanes

Names are given to two different conformations. In the eclipsed conformation, the C—H bonds on one carbon are directly aligned with the C—H bonds on the adjacent carbon. In the staggered conformation, the C—H bonds on one carbon bisect the H—C—H bond angle on the adjacent carbon.

Rotating the atoms on one carbon by 60° converts an eclipsed conformation into a staggered conformation, and vice versa. The angle that separates a bond on one atom from a bond on an adjacent atom is called a dihedral angle. For ethane in the staggered conformation, the dihedral angle for the C—H bonds is 60°. For eclipsed ethane, it is 0°.

Anti Relationships H 180° Two bonds are anti when the angle between them is 180°. 6

Syn – eclipsed Relationships when the angle between them is 0°

Gauche Relationships H 60° H H H H H H H H H H H Two bonds are gauche when the angle between them is 60°. 6

Cnformational Analysis of Ethane Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds. 3

eclipsed conformation Ethane eclipsed conformation 4

staggered conformation Ethane staggered conformation 5

12 kJ/mol Energy 0° 60° 120° 180° 240° 300° 360° Dihedral angle 11

Torsional strain The eclipsed conformation of ethane is 12 kJ/mol less stable than the staggered. The eclipsed conformation is destabilized by torsional strain. Torsional strain is the destabilization that results from eclipsed bonds. 6

3.2 Conformational Analysis of Butane 13

14 kJ/mol 3 kJ/mol 0° 60° 120° 180° 240° 300° 360° 14

van der Waals strain The gauche conformation of butane is 3 kJ/mol less stable than the anti. The gauche conformation is destabilized by van der Waals strain (also called steric strain). van der Waals strain is the destabilization that results from atoms being too close together. 6

van der Waals strain The conformation of butane in which the two methyl groups are eclipsed with each other is the least stable of all the conformations. It is destabilized by both torsional strain (eclipsed bonds) and van der Waals strain. 6

An energy minimum and maximum occur every 60° as the conformation changes from staggered to eclipsed. Conformations that are neither staggered nor eclipsed are intermediate in energy. Butane and higher molecular weight alkanes have several C—C bonds, all capable of rotation. It takes six 60° rotations to return to the original conformation. Figure 4.9 Six different conformations of butane

Figure 4.10 Graph: Energy versus dihedral angle for butane