Newman Projections and Conformational Isomers

Newman Projections Is a way to draw chemical conformations and views a carbon - carbon chemical bond from front to back, front carbon as a dot and back carbon as a circle. This type of representation makes it easy to assess the torsional angle between two bonds one at each carbon atom.

Torsional Strain Is caused by the electrons in the two bonds that are eclipsed. These electrons are closer together in the eclipsed form than in the staggered form, therefore the repulsions are greater. Recall that repulsion between similarly charged particles represent an increase in potential energy.

Conformational Analysis of Ethane H H H H H H

Energy Differences of Ethane Conformations

Conformations of Butane

Gauche vs. Anti Conformations

Cyclohexane Conformations Chair Conformation

Cyclohexane Conformations (cont.) Boat Conformation

Interconversion of Conformations

Axial Hydrogens in Cyclohexane Axial Positions

Equatorial Hydrogens in Cyclohexane Equatorial Positions

Types of Strain in Molecules Eclipsing strain- overlap Steric Strain – crowding of atoms in the molecule Angle Strain > or < than 109.5º

Strain in Cyclohexane

Cyclohexane is free of ring strain? Why? 1. All Bonds are staggered and therefore no torsional strain. 2. All bond angles are near so no angle strain. 3. It does possess steric strain (internal gauche interaction) but no more than a noncyclic alkane. Cyclohexane – the most stable conformation of cyclohexane is the chair. Chair conformation showing only carbon atoms Chair conformations showing positions of hydrogens Newman projection shows that all bonds are staggered. Bond angles are about o