Key concepts: Dihedral angle (also known as torsional angle).
Newman Projections
Basic Concepts
Newman Projection: A way to visualize molecule conformation by looking down the bond axis.
Dihedral angle: Angle between atoms or groups on front and rear carbons in Newman projection.
Example: In ethane, angle between two hydrogens on different carbons can be 60° (called staggered conformation) or 0° (called eclipsed conformation).
Staggered vs. Eclipsed Conformations
Staggered Conformation: All dihedral angles are 60°.
Lower in energy due to minimized electron repulsion.
Eclipsed Conformation: All dihedral angles are 0°.
Higher in energy due to electron repulsion.
Energy Differences
Energy difference between staggered and eclipsed is about 12 kJ/mol.
Staggered: Lower energy
Eclipsed: Higher energy
Butane Conformations
Newman Projection of Butane
Analyzes dihedral angles between groups on carbons 2 and 3.
Important conformations: Anti, Gauche, and Eclipsed.
Conformation Names
Anti-Conformation: Two largest groups 180° apart (lowest energy, staggered).
Gauche Conformation: Two largest groups 60° apart.
Eclipsed Conformation: Groups directly behind each other (highest energy).
Energy Diagram
Four distinct energy levels identified.
Lowest energy: Anti (staggered, all dihedral angles 60°).
Highest energy: Eclipsed (two largest groups 0°, all dihedral angles 0°).
Types of Strain
Torsional Strain: Electron cloud repulsion in bonds.
Steric Strain: Atoms physically bumping into each other.
Notable in butane due to larger methyl groups.
Cyclohexane and Chair Conformations
Drawing Chairs
Chair conformation: Easier to represent 3D structure of cyclohexane.
Requires practice for accuracy.
Types of Bonds in Chair Conformation
Axial Bonds: Bonds pointing straight up/down.
Equatorial Bonds: Bonds pointing slightly outward from the ring.
Ring Flip
Converts axial bonds to equatorial and vice versa.
Helps in understanding stability based on the position of substituents.
Stability Factors
1,3-Diaxial Interactions: Bulkier groups prefer equatorial positions to avoid these destabilizing interactions.
Example: Methyl group more stable in equatorial.
Ring flip leads to axial/equatorial interchanging.
Practice and Application
Draw chair conformations with substituents and identify most stable ones.
Important for organic chemistry exams and practical work.
Conclusion
Understand Newman projections and chair conformations to analyze molecular stability.
Use these principles to solve problems in organic chemistry.