What are the forces in a molecular structure?

1. Torsional strain: Strain between groups on adjacent atoms. A-B-C-D. Worst when eclipsed; best when staggered. Bond angle strain: when a bond angle, A-B-C, diverges from the ideal (180, 120, 109) What are the forces in a molecular structure?

2. Conformations of cycloalkanes: cyclopropane Planar ring (three points define a plane); sp3 hybrization: 109o. Hydrogens eclipsing. Torsional angle strain. Bond angle strain. Should be 109 but angle is 60o. Cyclopropane exhibits unusual reactivity for an alkane.

3. Folded, bent: less torsional strain but increased bond angle strain Conformation of cyclobutane Fold on diagonal Planar: eclipsing, torsional strain and bond angles of 90o

4. Cyclobutane molecular dynamics

5. Cyclopentane

6. Boat conformation Chair conformation Cyclohexane Ideal solution: Everything staggered and all angles tetrahedral.

7. Axial: Equatorial: Chair Conformation

8. Axial and Equatorial Axial Up/Equatorial Down: (A/E) Equatorial Up/Axial Down: (E/A) A/E E/A E/A A/E A/E E/A

9. Ring Flips Chair Boat or Twisted Boat A becomes E E becomes A Up stays Up Down stays Down Chair

10. Substituents: Axial vs Equatorial

11. Each repulsion is still about 3.6 kJ. Note that the gauche interaction in butane is about 3.8. Substituent Interactions Destabilizes axial substituent. Each repulsion is about 7.28/2 kJ = 3.6 kJ 1,3 diaxial repulsions Alternative description: gauche interactions

12. Newman Projection of methylcyclohexane Axial methyl group Equatorial methyl group gauche anti

13. 7.3 kJ (axial) 7.3 kJ (axial) Disubstituted cyclohexanes 1,2 dimethylcyclohexane 3.6 kJ (gauche) 3.6 kJ (gauche) interactions 0.0 kJ equatorial 0.0 kJ equatorial 7.3 + 3.6 = 10.9 kJ 7.3 + 3.6 = 10.9 kJ

14. 7.3 kJ (axial) 0.0 kJ equatorial 3.6 kJ (gauche) 0.0 kJ equatorial 7.3 kJ (axial) 0.0 kJ + 3.6 kJ = 3.6 kJ 14.6 kJ + 0.0 kJ = 14.6 kJ diequatorial diaxial

15. When does the gauche interaction occur?

16. Translate ring planar structure into 3D E/A A/E A/E Assume the tert-butyl group is equatorial. E/A A/E E/A Energy accounting No axial substituents One 1,2 gauche interaction between methyl groups, 3.6 kJ/mol Total: 3.6 kJ

17. Problem: Which has a higher heat of combustion per mole, A or B? 7.3 3.6 3.6 3.6 7.3 18.2 7.2 More repulsion, higher heat of combustion by 11.0 kJ/mol

18. Trans and Cis Decalin Now build cis decalin, both same side. Build trans decalin starting from cyclohexane, one linkage up, one down Trans sites used on the left ring Cis sites used on left ring. Trans sites used on the right ring Cis sites used on right ring. Trans decalin Locked, no ring flipping Cis decalin, can ring flip

19. Trans fusions determine geometry What is the geometry of the OH and CH3? E/A A/E A/E E/A E/A A/E Trans fusions, rings must use equatorial position for fusion. Rings are locked. The H’s must both be axial Work out axial / equatorial for the OH and CH3. OH is equatorial and CH3 is axial

20. Inversion Point

21. Cycloalkanes Vertical reflection plane. Horizontal reflection plane. Look for reflection planes! There are other reflection planes as well. Do you see them? Based on these planar ring diagrams we observe reflection plane and expect optical inactivity…. But the actual molecule is not planar!! Examine cyclohexane. This plane of symmetry (and two similar ones) are still present. Achiral. Optically inactive. The planar diagrams predicted correctly.

22. Substituted cyclohexanes The planar diagram predicts achiral and optically inactive. But again we know the structure is not planar. cis Mirror objects!! This is a chiral structure and would be expected to be optically active!! But recall the chair interconversion…. Earlier we showed that the two structures have the same energy. Rapid interconversion. 50:50 mixture. Racemic mixture. Optically Inactive. Planar structure predicted correctly

23. More… trans No mirror planes. Predicted to be chiral, optically active. Enantiomer. Ring Flips?????? R,R R,R trans 1,2 dimethylcyclohexane Each structure is chiral. Not mirror images! Not the same! Present in different amounts. Optically active! Other isomers for you… 1,3 cis and trans, 1,4 cis and trans.

24. Resolution of mixture into separate enantiomers. Mixtures of enantiomers are difficult to separate because the enantiomers have the same boiling point, etc. The technique is to convert the pair of enantiomers into a pair of diastereomers and to utilize the different physical characteristics of diastereomers. Formation of diastereomeric salts. Racemic mixture of anions allowed to form salts with pure cation enantiomer. Racemic mixture reacted with chiral enzyme. One enantiomer is selectively reacted. Racemic mixture is put through column packed with chiral material. One enantiomer passes through more quickly.

25. Chirality in the Biological World A schematic diagram of an enzyme surface capable of binding with (R)-glyceraldehyde but not with (S)-glyceraldehyde. All three substituents match up with sites on the enzyme. If two are matched up then the third will fai!

26. Sumber • academic.brooklyn.cuny.edu/.../chem%2051%20Lecture%20Slides/Lecture%2004.ppt • 31 Januari 2010 jam 22.10 • (Alkane filetype:ppt)