Conformational Analysis Newman Projections Ring Strain
Cyclohexane Conformations Views of Ethane
The Newman Projection
Rotational Conformations of Ethane Definitions
Conformations - Different spatial arrangments that a molecule can adopt due to rotation about sigma bonds.
Staggered - A low energy conformation where the bonds on adjacent atoms bisect each other (60o dihedral angle),
maximizing the separation. Eclipsed - A high energy conformation where the bonds on adjacent atoms are
aligned with each other (0o dihedral angle). 60o Rotation Causes Torsional
or Eclipsing Strain Types of Strain
Steric - Destabilization due to the repulsion between the electron clouds of atoms or groups. Groups try to occupy some common space.
Torsional - Destabilization due to the repulsion between pairs of bonds caused by the electrostatic repulsion of the electrons in the
bonds. Groups are eclipsed. Angle - Destabilisation due to distortion of a bond angle from it's optimum value caused by
the electrostatic repulsion of the electrons in the bonds. e.g. cyclopropane
Definitions Anti - Description given to two substitutents attached to adjacent atoms when their bonds are
at 180o with respect to each other. Syn - Description given to two substitutents attached to adjacent atoms when their bonds are
at 0o with respect to each other. Gauche - Description given to two substitutents attached to adjacent atoms when their bonds are
at 60o with respect to each other. Potential Energy Diagram
Propane Conformations: Larger Barrier to Rotation (link)
Butane Conformations (C2-C3)
Gauche Interaction in Butane 2 Different Eclipsed
Conformations Strain Energy can be Quantified
Butane has Steric and Torsional Strain When Eclipsed
PE Diagram for Butane (link)
1-Chloropropane Saturated Cyclic Compounds
Cyclopropane Angle and Torsional Strain
Electron Density Map
All Dihedral Angles = 0 o
Cyclobutane is not Planar
Cyclopentane Cyclohexane
Chair Conformation link to active site
Boat Conformation
How to Draw a Chair Conformation all opposite bonds
are parrallel Axial bonds and
Equatorial bonds Rings can Flip from one Chair
Conformation to Another Flipping Chair Conformations
All axial bonds become equatorial
All equatorial bonds become axial All up bonds stay up All down bonds stay down
axial up
eq. up Axial-up becomes Equatorial-up
Equatorial Conformation is Preferred (link)
Axial Methyl group is Gauche to C3 in the ring
Gauche Interactions are Flagged by Parallel Hs
1,3-Diaxial Interactions Equatorial Methyl Group is Anti
to C3 in the ring cis 1,3-Dimethylcyclohexane
trans 1,3-Dimethylcyclohexane
cis 1-Chloro-4-t-butylcyclohexane