Newman Projections
Newman Projections:
Conformational isomerism:
The rotation around double bond is impossible because it needs breaking of π bond and required high energy which is not available at room temperature. While sigma bonds are free to rotate, sigma bond is formed by head on overlap of orbitals which enable it to rotate in 3-dimensional space. Different rotations at different angles cause change in shape of molecule, and many different 3- dimensional arrangements of molecule are possible. These arrangements are called conformations, resulting via rotations about single bond and one particular conformation is called as conformational isomer or conformer.
Conformational isomers or Conformations/Conformers:
The structures having same molecular formula but differ in arrangements of groups of atoms in 3-dimensional space associated to different energy, and can be interconverted by rotation around single bond.
Drawing conventions:
Different drawing conventions are used to visualize different conformations of molecule i-e Sawhorse, Natta, Newman projection. One convenient drawing convention which is often use to explain the stereochemistry of alkanes is Newman projection.
Newman Projection:
Melvin Spencer Newman introduced Newman projection in 1952 as replacement for Fischer Projection. C-C bond when taken perpendicular to plane of paper an end on view results which is known as Newman projection. Stereochemistry of Alkanes can be better explained by using Newman projection, which visualizes conformation of a chemical bond from front to back. In Newman projection front atom is represented by dot called as proximal and back atom is represented by circle called as distal. A specific angle formed between the proximal and distal atoms known as dihedral angle.
- Front Carbon atom nearer to viewer is present at the junction of three bonds .
- Rare care carbon atom which is further away from viewer and can’t be seen in end on view is represented by a circle
- C-H bonds of rare carbon atom join the edge of circle.
- C-H bonds in eclipsed form are drawn slightly displaced for clarity.
Dihedral Angle:
The angle between C-H bond of front and back carbon atom is called dihedral angle.
Steps to draw Newman Projection:
- 1. Selection of bond
While looking at the alkane, you have to select a bond to be taken perpendicular to plane of paper , and have to decide which ’C’ you want as front(Proximal) carbon atom and which as rare(Distal) carbon atom.
- 2. Draw Newman Skeleton
Draw a circle and a dot in center of circle, this will represent your back and front carbon atom respectively.
- 3. Draw Newman Skeleton
Draw a circle and a dot in center of circle, this will represent your back and front carbon atom respectively.
- 4. Draw Newman Skeleton
Draw a circle and a dot in center of circle, this will represent your back and front carbon atom respectively.
- 5. Other Conformations
Draw other conformations simply by rotating back C atom.
Energy associated with different Conformations:
Different energy associated with different conformations. Stability of conformer is directly linked with its energy. Staggered and eclipsed conformations have different energy due to different distance between groups on adjacent atoms. In eclipsed conformations, distance between adjacent groups is minimum, which causes steric hindrance, raises its energy and decrease its stability. On other hand staggered conformation have maximum distance between adjacent groups , which leads to high stability and low energy.
Newman projections of cycloalkanes:
Newman projections also use to draw conformations of cycloalkanes.
