Conformational Isomers
- Conformational isomers are seen in molecules that have single bonds and can therefore rotate around them
- Because of steric hindrance, some conformations of a molecule are more stable than others
- Conformational isomers are observed in identical molecules
Example:
Newman Projections
- A Newman projection is drawn by looking down a single bond (typically carbon carbon single bonds) and seeing how substituents on neighbouring carbon atoms are related.
- Once the axis we are looking down is defined, the front carbon is a dot with the three substituents coming out from it. The back carbon is a sphere with its three substituents coming off the sphere.
- Rotomers: Isomers which are created by rotating a carbon carbon single bond.
Eclipsed vs Staggered
Eclipsed vs Staggered & Steric interactions in Eclipsed Conformation
In a staggered geometry we can have our largest groups be anti (like the CH3 above) or gouche (where the CH3 groups are beside on another)
Example: Convert Between Line Bond Diagrams and Newman Projections
Convert the following line bond diagrams to Newman projections and vice-versa.
a)
b)
c)
** Make the Carbon attached to the chlorine the back carbon **
d)
Anti vs Gauche Interactions
In the staggered rotomer, adjacent substituents can be gauche or anti to one another.
Anti vs Gauche
Steps to Ranking the Energies of Different Rotomers:
- Eclipsed conformations will always be higher in energy than staggered.
- Within each category (eclipsed or staggered) look at how many, H-R, and R-R steric interactions are taking place. For eclipsed it is only between groups which are eclipsed, for staggered we are looking only at gauche interactions.
- Compounds with more steric R-R interactions are higher in energy than those with fewer. R-R interactions have a greater effect than R-H
Example: Ranking Rotomers from Lowest to Highest Energy
Rank the following rotomers of 2-methylbutane from lowest to highest energy.
1. Label as Staggered or Eclipsed
2. Count the R-R and R-H interactions (where R is a random C chain)
3. The Highest energy will be the eclipsed species with the worst steric interactions and the lowest energy will be the staggered with the fewest steric interactions. Remember, R-R is much worse than R-H.
4. Our final ranking is
(6 and 4) > 2 > 5 > (1 and 3)