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Conformational Analysis of Cycloalkanes

Conformational Analysis of Cycloalkanes. Conformational Analysis of Ethane, Propane and Butane. Representation of Conformer. Conformers interconvert rapidly and a structure is an average of conformers. Molecular models are three dimensional objects that enable us to visualize conformers.

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Conformational Analysis of Cycloalkanes

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  1. Conformational Analysis of Cycloalkanes Conformational Analysis of Ethane, Propane and Butane

  2. Representation of Conformer Conformers interconvert rapidly and a structure is an average of conformers Molecular models are three dimensional objects that enable us to visualize conformers Representing three dimensional conformers in two dimensions is done with standard types of drawings

  3. Representing Conformations • Sawhorse representations C-C bonds are at an angle to the edge of the page and all C-H bonds are shown

  4. Newman Projections • C-C bond would project end-on onto the paper • Bonds to front carbon are lines going to the center • Bonds to rear carbon are lines going to the edge of the circle

  5. End-on representations for conformations are commonly drawn using a convention called a Newman projection. How to Draw a Newman Projection: Look directly down the C—C bond (end-on), and draw a circle with a dot in the center to represent the carbons of the C—C bond.

  6. Draw in the bonds. Draw the bonds on the front C as three lines meeting at the center of the circle. Draw the bonds on the back C as three lines coming out of the edge of the circle.

  7. Add the atoms on each bond.

  8. Conformations of Ethane staggered conformation eclipsed conformation

  9. Conformations of EthaneSpace filled models staggered conformation eclipsed conformation

  10. Ethane’s Conformations

  11. Rotational Conformations of Ethane

  12. The potential energy diagram of the conformations of ethane shows that the staggered conformation is more stable than eclipsed by 12 kJ mol-1

  13. Conformations of Propane Propane (C3H8) torsional barrier around the carbon–carbon bonds 14 kJ/mol Eclipsed conformer of propane has two ethane-type H–H interactions and an interaction between C–H and C–C bond

  14. Propane conformations

  15. Conformations of Butane Anti conformation has two methyl groups 180° away from each other Rotation around the C2–C3 gives eclipsed conformation Staggered conformation with methyl groups 60° apart is gauche conformation

  16. Conformations of Butane

  17. Eclipsed Conformations of Butane

  18. Gauche conformation: steric strain Space fill model

  19. Conformational Analysis of Butane Rotation around C2-C3 of butane gives six important conformations The gauche conformation is less stable than the anti conformation by 3.8 kJ mol-1 because of repulsive van der Waals forces between the two methyls

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