Wize University Chemistry Textbook > Introduction to Organic Chemistry
Conformational Isomers - Newman Projections
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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:
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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
Wize Concept
The eclipsed geometry is much higher in energy because of the steric repulsion between neighboring substituents.
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.
Wize Concept
Substituents on the top and bottom in the Newman Projection are in the plane of the page (in the line bond diagram they will be drawn with straight lines going up or down).
Substituents on the left and right are either going to be going towards us (dark wedge) or away from us (dashed line)
a)
b)
c)
** Make the Carbon attached to the chlorine the back carbon **
d)
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Anti vs Gauche Interactions
In the staggered rotomer, adjacent substituents can be gauche or anti to one another.
Anti vs Gauche
Wize Concept
A gauche interaction has more steric strain (and has higher energy) than an anti conformation.
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
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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)
Wize Tip
Note: Staggered conformations will always have lower energy than eclipsed conformations.
When deciding which staggered conformations has the lowest energy, look for the ones that have the least interactions between R groups.
When deciding which eclipsed conformations has the highest energy, look for the ones that have the most interactions between R groups.
Practice: Drawing Most Stable Rotomer
Choose the most stable conformer of (R)-3-bromo-3-methylhexane about the C2-C3 bond using Newman projections.
Practice: Ranking the Energy of Rotomers
All the rotomers of butane looking down the C2-C3 bond have been drawn below. Choose the option below that correctly ranks them in order of increasing energy
