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Conformations of Cycloalkanes

Conformations of Cycloalkanes. Cycloalkanes. Most common cycloalkanes. Adolf von Baeyer (19th century). assumed cycloalkanes are planar polygons distortion of bond angles from 109.5° gives angle strain to cycloalkanes with rings either smaller or larger than cyclopentane

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Conformations of Cycloalkanes

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  1. Conformations of Cycloalkanes

  2. Cycloalkanes • Most common cycloalkanes

  3. Adolf von Baeyer (19th century) • assumed cycloalkanes are planar polygons • distortion of bond angles from 109.5° givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane • Baeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor. • But Baeyer was incorrect in his belief that cycloalkanes were planar.

  4. Types of Strain • Torsional strain strain that results from eclipsed bonds • van der Waals strain (steric strain) strain that results from atoms being too closetogether • angle strain strain that results from distortion of bondangles from normal values

  5. Cyclopropane sources of strain: torsional strain angle strain

  6. Cyclobutane nonplanar conformation relieves some torsional strain angle strain present

  7. Nonplanar conformations of cyclopentane Relieve some, but not all, of the torsional strain. Envelope and half-chair are of similar stabilityand interconvert rapidly. Envelope Half-chair

  8. Chair is the most stable conformation of cyclohexane All of the bonds are staggered and the bond angles at carbon are close to tetrahedral.

  9. Boat conformation is less stable than the chair 180 pm All of the bond angles are close to tetrahedralbut close contact between flagpole hydrogenscauses van der Waals strain in boat.

  10. The chair conformation of cyclohexane is themost stable conformation and derivativesof cyclohexane almost always exist in the chair conformation

  11. Conformational Inversion chair-chair interconversion (ring-flipping) rapid process (activation energy = 45 kJ/mol) all axial bonds become equatorial and vice versa

  12. Reactions of Cyclopropane

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