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Conformational Analysis of Alkanes: Ethane to Higher Alkanes

Explore the spatial arrangements of alkanes like ethane, highlighting eclipsed vs. staggered conformations and the impact of torsional and van der Waals strain on stability. Learn about anti and gauche relationships in various conformations.

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Conformational Analysis of Alkanes: Ethane to Higher Alkanes

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  1. Chapter 3Conformations of Alkanes and Cycloalkanes

  2. 3.1 Conformational Analysis of Ethane Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds.

  3. Ethane Eclipsed conformation

  4. Ethane Eclipsed conformation

  5. Ethane Staggered conformation

  6. Ethane Staggered conformation

  7. H H H H H H H H H H H H Projection formulas of the staggeredconformation of ethane Newman Sawhorse

  8. H H H H H H H H H H H H Anti relationships 180° Two bonds are anti when the angle between them is 180°.

  9. Gauche relationships H 60° H H H H H H H H H H H Two bonds are gauche when the angle between them is 60°.

  10. An important point: The terms anti and gauche applyonly to bonds (or groups) on adjacentcarbons, and only to staggeredconformations.

  11. 12 kJ/mol 0° 60° 120° 180° 240° 300° 360°

  12. Torsional strain • The eclipsed conformation of ethane is 12 kJ/molless stable than the staggered. • The eclipsed conformation is destabilized bytorsional strain. • Torsional strain is the destabilization that resultsfrom eclipsed bonds.

  13. 3.2 Conformational Analysis of Butane

  14. 14 kJ/mol 3 kJ/mol 0° 60° 120° 180° 240° 300° 360°

  15. van der Waals strain • The gauche conformation of butane is 3 kJ/molless stable than the anti. • The gauche conformation is destabilized byvan der Waals strain (also called steric strain). • van der Waals strain is the destabilization that results from atoms being too close together.

  16. van der Waals strain • The conformation of butane in which the twomethyl groups are eclipsed with each other isis the least stable of all the conformations. • It is destabilized by both torsional strain(eclipsed bonds) and van der Waals strain.

  17. 3.3 Conformational Analysis of Higher Alkanes

  18. The most stable conformation of unbranchedalkanes has anti relationships between carbons Hexane

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