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Conformations of Alkanes and Cycloalkanes

Conformations of Alkanes and Cycloalkanes. Conformational Analysis of Ethane and Butane Conformations of Higher Alkanes Cycloalkanes Cyclopropane, Cyclobutane, Cyclopentane Conformations of Cyclohexane Axial and Equatorial Bonds in Cyclohexane, Ring Flipping

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Conformations of Alkanes and Cycloalkanes

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  1. Conformations of Alkanes and Cycloalkanes • Conformational Analysis of Ethane and Butane • Conformations of Higher Alkanes • Cycloalkanes • Cyclopropane, Cyclobutane, Cyclopentane • Conformations of Cyclohexane • Axial and Equatorial Bonds in Cyclohexane, Ring Flipping • Conformational Analysis of Monosubstituted Cyclohexanes • Disubstituted Cyclohexanes: Stereoisomers • Conformational Analysis of Disubstituted Cyclohexanes

  2. Conformations of alkanes and cycloalkanes contd. • Some New Terms • Conformations, • Conformational Analysis, • Staggered and Eclipse Conformations • Sawhorse Drawing, Newman Projections • Gauche, Dihedral Angle, Torsion Angle • Torsional Strain, Steric Strain, van der Waals Strain, • Ring Strain

  3. Measuring Strain in Cycloalkanes • Heats of combustion can be used to comparestabilities of isomers. • But cyclopropane, cyclobutane, etc. are not isomers. • All heats of combustion increase as the numberof carbon atoms increase.

  4. Heats of Combustion in Cycloalkanes • Cycloalkane kJ/mol Per CH2 • Cyclopropane 2,091 697 • Cyclobutane 2,721 681 • Cyclopentane 3,291 658 • Cyclohexane 3,920 653 • Cycloheptane 4,599 657 • Cyclooctane 5,267 658 • Cyclononane 5,933 659 • Cyclodecane 6,587 659

  5. Fig. 3.12

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

  7. The 12 bonds to the ring can be divided into two sets of 6.

  8. 6 Bonds are equatorial Equatorial bonds lie along the equator

  9. Fig. 3.16

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

  11. Fig. 3.14

  12. Fig. 3.15

  13. CH3 CH3 Methylcyclohexane Chair chair interconversion occurs, but at any instant 95% of the molecules have their methyl group equatorial. Axial methyl group is more crowded than an equatorial one. 5% 95%

  14. Methylcyclohexane Source of crowding is close approach to axial hydrogens on same side of ring. Crowding is called a "1,3-diaxial repulsion" and is a type of van der Waals strain. 5% 95%

  15. Fluorocyclohexane F Crowding is less pronounced with a "small" substituent such as fluorine. Size of substituent is related to its branching. F 40% 60%

  16. C(CH3)3 C(CH3)3 tert-Butylcyclohexane Crowding is more pronounced with a "bulky" substituent such as tert-butyl. tert-Butyl is highly branched. Less than 0.01% Greater than 99.99%

  17. tert-Butylcyclohexane van der Waalsstrain due to1,3-diaxialrepulsions

  18. 3.11Disubstituted Cycloalkanes:Stereoisomers Stereoisomers are isomers that have same constitution but different arrangement of atoms in space

  19. Constitutional isomers Stereoisomers Isomers

  20. 1,2-Dimethylcyclopropane cis-1,2-Dimethylcyclopropane has methyl groupson same side of ring. trans-1,2-Dimethylcyclopropane has methyl groupson opposite sides.

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