Stereochemistry of Alkanes and Cycloalkanes

1. Stereochemistry of Alkanes and Cycloalkanes Chapter 2. Continue

2. The Shapes of Molecules • The systematic study of the shapes molecules and properties from these shapes is stereochemistry • A molecule may assume different shapes, called conformations صورتندی, that result from rotation about a carbon-carbon single bond,that are in equilibrium at room temperature (the conformational isomers are called conformers)

3. Representing Conformations • Representing three dimensional conformers in two dimensions is done with standard types of drawings • Sawhorse representations: these view the C-C bond from an oblique angle and indicate spatial orientations by showing all the C-H bonds. • Newman projections: these site along a particular C-C bond and represent the two carbon atoms by a single circle. Substituents on the front carbon are represented by lines going to the center of the circle, and substituents on the rear carbon are indicated by lines going to the edge of the circle.

4. Although there are many possible conformations available to a particular molecule; each molecule will spend most of its time in its most stable conformation. The most stable conformation for any molecule is the one that minimizes the mutual repulsion of bonded electron clouds on adjacent carbons.

5. Conformers interconvert rapidly and a structure is an average of conformers Molecular models are three dimensional objects that enable us to visualize conformers 2.1 Conformations of Ethane

6. Conformations of Ethane staggered conformation eclipsed conformation

7. Ethane’s Conformations • The most stable conformation of ethane has all six C–H bonds away from each other (staggered) • The least stable conformation has all six C–H bonds as close as possible (eclipsed) in a Newman projection – energy due to torsional strain

8. This increased energy due to eclipsing interactions is called torsional strain and is one kind of strain energy. Torsional strain is due to mutual repulsion between electron clouds as they pass by each other in eclipsed conformers

9. DG~ 3 kcal/mol torsional strain (eclipsing strain) lower energy higher energy Ea~ 3 kcal/mol = barrier to free rotation (but at RT most molecules have KE > Ea so rotation is essentially “free”) krot ~ 106 s-1 3 DG 0

11. Ethane’s Conformations

12. 2.2 Conformations of Propane • Propane (C3H8) torsional barrier around the carbon–carbon bonds 14 kJ/mol • Eclipsed conformer of propane has two ethane-type H–H interactions and an interaction between C–H and C–C bond

13. Propane conformations

14. 2.3 Conformations of Butane • anti conformation has two methyl groups 180° away from each other • Rotation around the C2–C3 gives eclipsed conformation • Staggered conformation with methyl groups 60° apart is gauche conformation

15. Conformations of Butane

16. CH3–CH2–CH2–CH3 I anti (180º) II III gauche (60º) IV V gauche (60º) VI gauche~ 0.8 kcal higher energy than anti - van der Waals repulsions = steric strain eclipsed: 3 kcal torsional strain + 0.3 kcal each CH3-H eclipse + ~ 3 kcal each CH3-CH3 eclipse

17. Gauche conformation: steric strain

18. Eclipsed Conformations of Butane

19. Conformations of Butane

20. جمعیت صورت بندی های گوناگون طبق معادله زیر به دست می آید. DG° = -RT ln Keq percentage

21. DS° = -R ln w , T = 298 K , R = 1.987 cal/mol.K w: Number of states DS° = -R ln 2 = -1.38 cal

22. DG° = DH° - TDS° DG° = -0.8 x 1000 – 298(-1.38) DG° = -388.8 cal= -0.39 kcal DG° = -RT ln Keq Keq = 1.9 at 298 K

23. 1-chloropropane Some experiments show that G conformer is more stable than A in both 1-chloropropane. 1-Durig, J. R.; Godbey, S. E.; Sullivan, J. F. J Chem Phys 1984, 80, 5983. 2- Barnes, A. J.; Evans, M. L.; Hallam, H. E. J Mol Struct 1983, 99, 235.

24. vdw radii inc. bond length inc

25. Stability of Cycloalkanes: The Baeyer Strain Theory • In 1885, Baeyer proposed a theory to explain the apparent lack of cyclic alkanes having certain ring sizes. • More specifically only 5 and 6 membered cycloalkane rings were known but smaller and larger rings could not be prepared. • Baeyer theorized that these could not be prepared because their bond angles would necessarily deviate from the preferred sp3 bond angle of 109.5 degrees. This deviation would cause such angle strain that the rings would be too unstable to exist.

26. 2.4 Stability of Cycloalkanes: The Baeyer Strain Theory • Baeyer (1885): since (sp3) carbon prefers to have bond angles of approximately 109°, ring sizes other than five and six may be too strained to exist • Rings from 3 to 30 C’s do exist but are strained due to bond bending distortions and steric interactions

28. Angle Strain

29. Stability of Cycloalkanes • Heats of combustion can be used to measure the total amount of energy strain in a compound. • The notion here is that the more strained a compound, the higher is its energy content and the more heat delivered per CH2 unit upon combustion to CO2 + H2O. Alkane + O2 CO2 + H2O + Heat

30. Bayer’s Theory Busted • Heats of combustion data indicate that Baeyer's theory was not fully correct. • Cyclopropane and cyclobutane are quite strained but cyclopentane is more strained then first predicted while cyclohexane is less strained. • For larger rings, there is no regular increase in strain and rings of more than 14 carbons are strain free. • Why was Baeyer's theory incorrect? • Baeyer assumed that all cycloalkanes are flat when in fact most adopt a puckered 3-D conformation. Furthermore, he did not consider the contribution of torsional strain to the overall strain energy of a molecule.

31. 2.5 The Nature of Ring Strain • Rings larger than 3 atoms are not flat (planar) • Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering • Larger rings have many more possible conformations than smaller rings and are more difficult to analyze

32. No "head-on" overlap of atomic orbitals poor overlap = bond angle strain (i.e., 109.5º sp3 in 60º triangle) ° 109.5 22_499 C ° 6 0 C C plus (a) (b) all H’s eclipsed = torsional strain

33. Summary: Types of Strain • Angle strain- expansion or compression of bond angles away from the preferred 109.5 • Torsional strain - eclipsing of bonds on neighboring atoms • Steric strain - repulsive interactions when two atoms or groups bump in to one another

34. Torsional Strain

35. Steric Strain

36. Strain Energies

37. 2.6 Cyclopropane: An Orbital View • 3-membered ring must have planar structure • Symmetrical with C–C–C bond angles of 60° • Requires that sp3based bonds are bent (and weakened) • All C-H bonds are eclipsed

38. 2.7 Conformations of Cyclobutane and Cyclopentane • Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens • Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above • The bend increases angle strain but decreases torsional strain

39. planar, 90º but all eclipsed “puckered”, 88º slightly more angle strain, but less eclipsing strain

40. “envelope” relieves eclipsing planar, 108º but all eclipsed Cyclopentane • Planar cyclopentane would have no angle strain but very high torsional strain • Actual conformations of cyclopentane are nonplanar, reducing torsional strain • Four carbon atoms are in a plane • The fifth carbon atom is above or below the plane – looks like an envelope

41. سیکلو هگزان • اطلاعات حاصل از سوختن نشان دهنده تحت فشار نبودن حلقه است. • اگر مسطح بود زاویه حلقه 120 می شود. • در کنفورمر صندلی زاویه 109.5 وهمه هیدروژن ها نامتقابل هستند. • فشار زاویه ای وفشار پیچشی وجود ندارد.

42. سیکلو هگزان محوری Move this carbon down استوایی Axial Move this carbon up Equatorial

45. پیکر بندی های سیکلو هگزان محوری Move this carbon down استوایی Axial Move this carbon up Equatorial The equatorial conformer of methyl cyclohexane is more stable than the axial by 7.6 kJ/mol

46. Bromocyclohexane • When bromocyclohexane ring-flips the bromine’s position goes from equatorial to axial and so on • At room temperature the ring-flip is very fast and the structure is seen as the weighted average

47. Difference between axial and equatorial conformers is due to steric strain caused by 1,3-diaxial interactions Hydrogen atoms of the axial methyl group on C1 are too close to the axial hydrogens three carbons away on C3 and C5, resulting in 7.6 kJ/mol of steric strain 1,3-Diaxial Interactions

48. سیکلو هگزان

49. 2.12 Boat Cyclohexane • Cyclohexane flips through a boat conformation • ~29 kJ/mol (7.0 kcal/mol) less stable than chair

50. سیکلو هگزان boat conformation Less stable than chair cyclohexane due to steric and torsional strain C-2, 3, 5, 6 are in a plane H on C-1 and C-4 approach each other closely enough to produce considerable steric strain Four eclipsed H-pairs on C- 2, 3, 5, 6 produce torsional strain