Sect 4.3: Conformations of Cycloalkanes

1. Sect 4.3: Conformations of Cycloalkanes

2. H H H H Issues in small rings H Small rings have two problems: C ANGLE STRAIN H A carbon with four bonds requires a tetrahedral hybrid with 109o angles. If that carbon is inside a small ring, the desired 109o angle cannot be achieved. TORSIONAL STRAIN If the rings are planar, there are many eclipsed hydrogen atoms, representing torsional strain. BOTH FACTORS RAISE THE ENERGY OF THE SYSTEM

3. Cyclopropane Cyclopropane has angle strain and torsional strain

4. Planar cyclobutane (actual molecule is not planar) internal angle = 90o = ANGLE STRAIN notice that all hydrogens are eclipsed = TORSIONAL STRAIN

5. H H H H H H H H CYCLOBUTANE - PUCKERED RING Actual molecule is puckered as shown - it folds in the middle about 25o from planar Still has angle strain “PUCKERING” relieves eclipsing which relieves TORSIONAL STRAIN

6. CYCLOPENTANE essentially no ANGLE STRAIN planar pentagon 108o puckering (envelope) relieves eclipsing and, therefore, TORSIONAL STRAIN

7. PUCKERING RELIEVES TORSIONAL STRAIN IN CYCLOPENTANE ~3.0 kcal/mol ENERGY planar 0 kcal/mol envelope conformation planar cyclopentane would have all hydrogens eclipsed = TORSIONAL STRAIN The pucker moves around the ring.

8. METHYLCYCLOPENTANE equatorial methyl axial methyl

9. H H CH3 H H H H H H H EQUATORIAL METHYLCYCLOPENTANE HAS LOWER ENERGY THAN AXIAL PLANAR 3.4 kcal/mol axial ENERGY 0.9 kcal/mol 0 kcal/mol equatorial

10. Molecules viewed with Chime • Click on START, Click on PROGRAMS • Click on Netscape Communicator (4.7), then launch Netscape Navigator • Using Google, type in the address for the Dept. of Chemistry, WWU: http://www.chem.wwu • Select, course materials, select “WWU virtual molecular model set” • You may need the free program, Chime, to run this program. • Note: Internet Explorer and Netscape 7.1 won’t work!

11. Sect 4.4: Evaluation of Angle Strain

12. Sect 4.5: Cyclohexane

13. PLANAR CYCLOHEXANE (doesn’t exist) If cyclohexane were planar all of the hydrogens would be eclipsed, resulting in torsional strain. There would also be angle strain - a hexagon has 120o internal angles. Cyclohexane is not planar ...

14. Cyclohexane is puckered (chair conformation) The perfect molecule! No angle strain and no torsional strain.

15. COMPLETE STAGGERING ABOUT ALL BONDS axial equatorial

16. Cyclohexane(boat conformation) bowsprit-flagpole interaction Torsional strain along side of boat and steric strain at top, but there is no angle strain.

17. THE CHAIR CONFORMATION HAS LOWER ENERGY THAN THE BOAT ENERGY 29 kJ/mol boat 0 kJ/mol chair

18. Cyclohexane(axial bonds)

19. Cyclohexane(equatorial bonds)

20. Cyclohexane(showing both axial andequatorial bonds

21. CHAIR CONFORMATION axial equatorial

22. NITROGEN AND OXYGEN HETEROATOMS CAN REPLACE CARBON All of these atoms have sp3 hybridization.

23. RING INVERSION starting ring axial red equatorial blue axial blue equatorial red axial and equatorial hydrogens are interchanged when the ring is inverted inverted ring

24. THE CHAIR IS A RIGID CONFORMATION IT DOES NOT FLEX THE BOAT IS A FLEXIBLE CONFORMATION IT CAN FLEX TO RELIEVE TORSIONAL AND STERIC STRAIN

25. THE BOAT IS A FLEXIBLE CONFORMATION IT WILL TWIST OR FLEX twisting relieves the eclipsing at the bottom of the ring ... H H … and the bowsprit-flagpole hydrogens move apart TWIST BOAT SKEW BOAT

26. THE TWIST BOAT HAS LOWER ENERGY THAN THE BOAT ENERGY 6 kJ/mol boat 0 kJ/mol twist boat

27. DON’T FORGET VIRTUAL MOLECULAR MODEL SET Click on START at the bottom left of the computer on University computers. Click on PROGRAMS Click on Netscape Communicator (4.7) to launch Netscape Navigator. Using Google, type in the address for the Dept. of Chemistry, WWU: http://www.chem.wwu Select course materials, select “WWU virtual molecular model set” You may need the free program, Chime, to run this program. Note: Internet Explorer and Netscape 7.1 won’t work!