Conformations and torsional strain
Conformations are the different shapes a molecule makes by rotating around single bonds. In ethane, the staggered shape is lowest in energy and the eclipsed shape is about 12 kJ/mol higher. In butane, the anti shape beats the gauche shape.
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Analogy
Like a person turning at the waist: the body is the same, but the arms may or may not bump into each other.
Example
Anti (CH₃ groups 180° apart): lowestGauche (60° apart): ≈ 4 kJ/mol higherEclipsed: highest, strain fromcrowding and electron repulsion
In practice
Bulky groups prefer to be far apart, so long chains sit mostly in a zig-zag anti shape.
Practice
Flashcards
- Which ethane conformation is lowest in energy?Staggered.
Interview questions
- Explain why a methyl group on cyclohexane prefers the equatorial position.
- Axial group has 1,3-diaxial interactions
- Steric crowding with axial hydrogens
- Equatorial points away from the ring
- About 7 kJ/mol more stable
Questions people ask
What is a Newman projection?
A Newman projection shows a molecule looking straight down one carbon-carbon bond. The front carbon is a dot with three bonds, and the back carbon is a circle with three bonds behind it. It makes it easy to see whether groups are staggered or eclipsed.
Are conformations the same as isomers?
Conformations of a molecule normally interconvert rapidly at room temperature by rotating about single bonds, so they can't be separated. Isomers have different bonding or fixed 3D arrangements and can be isolated. Rotation is easy for single bonds but not for double bonds or ring bonds.