Newman Projections

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Newman Projections

• and
• Conformational Isomers

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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.

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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.

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Conformational Analysis of Ethane
H
H
H
H
H
H
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Energy Differences of Ethane Conformations
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Conformations of Butane
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Gauche vs. Anti Conformations
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Cyclohexane Conformations

• Chair Conformation

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Cyclohexane Conformations (cont.)

• Boat Conformation

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Interconversion of Conformations
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Axial Hydrogens in Cyclohexane

• Axial Positions

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Equatorial Hydrogens in Cyclohexane

• Equatorial Positions

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Types of Strain in Molecules

• Eclipsing strain- overlap
• Steric Strain crowding of atoms in the molecule
• Angle Strain gt or lt than 109.5º

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Strain in Cyclohexane
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Cyclohexane is free of ring strain? Why?
1. All Bonds are staggered and therefore no
torsional strain. 2. All bond angles are near
109.50 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.
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Chair conformations showing positions of
hydrogens
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1
Newman projection shows that all bonds are
staggered. Bond angles are about 109.5o
Chair conformation showing only carbon atoms
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