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Structure, continued

Structure, continued. Dr. Clower CHEM 2411 Spring 2014 McMurry (8 th ed.) sections 3.5-3.7 , 4.3-4.9 , 7.2, 7.6. Topics. Conformations of Alkanes and Cycloalkanes Unsaturation Alkene Stability. Molecular Model Kits. How to use Make a model for ethane Make a model for butane

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Structure, continued

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  1. Structure, continued Dr. Clower CHEM 2411 Spring 2014 McMurry (8th ed.) sections 3.5-3.7, 4.3-4.9, 7.2, 7.6

  2. Topics • Conformations of Alkanes and Cycloalkanes • Unsaturation • Alkene Stability

  3. Molecular Model Kits • How to use • Make a model for ethane • Make a model for butane • Make a model for cyclohexane • Use 6 white hydrogens and 6 green hydrogens • Put 1 green and 1 white hydrogen on each carbon atom • The green and white hydrogen atoms should alternate (so as you look at the molecule from the top the H’s should alternate green-white-green-white-green-white around the ring)

  4. Alkane Three-dimensional Structure • Methane: • With 2 or more carbons, 3D arrangement can change due to C─C bond rotation • Conformations • Same molecular formula • Same atom connectivity • Different 3D arrangement due to rotation around single bond • Ethane:

  5. Newman Projections • Used to better visualize conformations • View the C─C from the end (look down the C─C bond) • Represent the C atoms as a dot (front carbon) and circle (back carbon) • Show bonds coming out of the circle and dot • Example:

  6. Ethane Conformations • Staggered vs. eclipsed • Staggered is more stable (lower E) due to maximum separation of electron pairs in covalent bonds • Eclipsed is less stable (higher E) due to electron repulsions

  7. Dihedral Angle • The degree of rotation between C-H bonds on the front and back carbons • Torsional strain • Accounts for energy difference between eclipsed and staggered • Barrier to rotation • Caused by electron repulsion • Overcome by collisions of molecules

  8. Butane Conformations • Look down C2─C3 bond to draw Newman projections • Each C has 2 H atoms and 1 CH3 group • Dihedral angle is angle between CH3 groups • There are six conformations of butane: • How many staggered conformations? How many eclipsed?

  9. Strain in Butane Conformations • Torsional strain • Barrier to rotation • Example: eclipsed vs. staggered conformations • Steric strain • Repulsive interaction when atoms are forced close together (occupy the same space) • Example: CH3-H eclipsed vs. CH3-CH3 eclipsed conformations • Example: Anti vs. gauche conformations • So, which conformation is lowest in E? Highest in E?

  10. Butane Conformations

  11. Cycloalkane Three-dimensional Structure • C atoms in cycloalkanes are sp3 • Bond angles are not always 109.5º • Bond angles are dictated by the number of atoms in the ring • Angle strain = Forcing angles smaller or larger than 109.5º • Cycloalkanes can also have torsional strain (eclipsed H’s)

  12. Strain in Cycloalkanes

  13. Cycloalkane Conformations • Cycloalkanes adopt more stable conformations to relieve strain • Cyclopropane • “Bent” bonds

  14. Cycloalkane Conformations • Cyclobutane • Puckered conformation • Cyclopentane • Envelope conformation

  15. Cyclohexane • Most stable cycloalkane • Most abundant in nature • No angle strain (109.5º) • No torsional strain (all H’s staggered) • Conformation = chair

  16. Cyclohexane • Axial and equatorial hydrogens • Axial = parallel to axis through ring • Equatorial = perpendicular to axis • Each C has one axial H and one equatorial H • Look at molecular model

  17. Cyclohexane

  18. Ring Flip • Interconversion of two chair conformations • Try this with your molecular model • If no substituents, these conformations are equal in energy

  19. MonosubstitutedCyclohexanes • Two conformations • Substituent in axial position • Substituent in equatorial position • These conformations are not equal in energy • Example: methylcyclohexane Steric strain = 1,3-diaxial interactions Larger groups have more steric strain

  20. DisubstitutedCyclohexanes • The most stable conformation has the most substituents in the equatorial position • Conformational analysis • Look at all chair conformations (cis and trans) and analyze stability • Example: 1,4-dimethylcyclohexane

  21. Additional Cyclohexane Conformations • Boat • No angle strain • High torsional strain • High steric strain • Very unstable • Twist-boat • Relieves some torsional and steric strain • No angle strain • Lower E than boat • Higher E than chair

  22. Conformations of Polycyclic Molecules • Fused rings • Typically adopt chair conformations • Norbornane and derivatives locked in boat conformation

  23. Degree of Unsaturation • Unsaturated compounds • Have less than (2n+2) H atoms for (n) C atoms • Contain elements of unsaturation • p bonds • Rings • Calculating degree of unsaturation • Index of Hydrogen Deficiency (IHD) • IHD = C - ½ (H + X) + ½ (N) + 1 • Ex: C6H14 IHD = 6 - ½(14) + 1 = 0 Alkane • Ex: C6H12 IHD = 6 - ½(12) + 1 = 1 1 p bond or 1 ring • Ex: C6H10 IHD = 6 - ½(10) + 1 = 2 2 pbonds, 2 rings, or 1 of each C6H14 C6H12 C6H10

  24. Alkene Stability • Which alkene is more stable, cis or trans? • Cis has steric strain between R groups

  25. Alkene Stability • Stability determined by heats of hydrogenation • Heat of reaction for addition of H2 (with metal catalyst) to alkene • Heat of reaction is proportional to energy of alkene • Smaller magnitude DH = more stable alkene

  26. Alkene Stability • Trends in alkene stability • Trans is more stable than cis • More substituted C=C is more stable • Why? • Hyperconjugation • Stabilizing effect of adjacent orbital overlap • Bond strengths • sp2-sp3 bond more stable than sp3-sp3

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