1. 1 Fall, 2009 Organic Chemistry I Cycloalkanes Organic Chemistry I Cycloalkanes Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University

2. Fall, 20092 Objectives Nomenclature Nomenclature Ring Strain Ring Strain Conformational Analysis Conformational Analysis

3. Fall, 20093 Cycloalkanes Hydrocarbons where carbons join in a ring Hydrocarbons where carbons join in a ring General formula is C n H 2n General formula is C n H 2n Referred to as alicyclic compounds Referred to as alicyclic compounds Chemistry is the same as straight-chain alkanes Chemistry is the same as straight-chain alkanes They burn They burn They react with halogen in light They react with halogen in light

4. Fall, 20094 Examples

5. 5 Nomenclature Determine number of carbons in ring Determine number of carbons in ring Name the alkane with cyclo in front Name the alkane with cyclo in front 12 carbons 12 carbons Dodecane Dodecane cyclododecane cyclododecane

6. Fall, 20096 Nomenclature Sustituted cycloalkanes Sustituted cycloalkanes If substituent has less than or equal to the number of carbons in the ring If substituent has less than or equal to the number of carbons in the ring name as an alkyl substituted ring name as an alkyl substituted ring

7. Fall, 20097 Nomenclature If the substituent has more carbons than the ring If the substituent has more carbons than the ring Name as a cycloalkyl substituted alkane Name as a cycloalkyl substituted alkane

8. Fall, 20098 Nomenclature Number substituents Number substituents If two groups can receive the same number, prioritize using alphabet If two groups can receive the same number, prioritize using alphabet

9. Fall, 20099 Nomenclature Halogens are treated like alkyl groups Halogens are treated like alkyl groups Fluorine becomes fluoro Fluorine becomes fluoro Chlorine becomes chloro Chlorine becomes chloro Bromine becomes bromo Bromine becomes bromo Iodine becomes iodo Iodine becomes iodo

10. Fall, 200910 Nomenclature Three substituents Three substituents Number substituents such that sum of the numbers chosen is as low as possible Number substituents such that sum of the numbers chosen is as low as possible

11. Fall, 200911 Nomenclature

12. Fall, 200912 Nomenclature

13. Fall, 200913 Isomerism in Cycloalkanes Rotation about C-C bonds in cycloalkanes is limited Rotation about C-C bonds in cycloalkanes is limited Rings have two “faces” and substituents are labeled as to their relative facial positions Rings have two “faces” and substituents are labeled as to their relative facial positions There are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyls on the same side (cis) of the ring and one with the methyls on opposite sides (trans) There are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyls on the same side (cis) of the ring and one with the methyls on opposite sides (trans)

14. Fall, 200914 Isomerism Atoms are connected the same Atoms are connected the same Differ only in spatial arrangement Differ only in spatial arrangement Stereoisomerism Stereoisomerism Constitutional isomerism Constitutional isomerism Differ in arrangement of atoms Differ in arrangement of atoms

15. Fall, 200915 Constitutional and Stereoisomers

16. Fall, 200916 Stability of Cycloalkanes: Baeyer Ring Strain Baeyer (1885): since carbon prefers to have bond angles of approximately 109°, ring sizes other than five and six may be too strained to exist Baeyer (1885): since carbon prefers to have bond angles of approximately 109°, ring sizes other than five and six may be too strained to exist Angle Strain Angle Strain Based upon geometry Based upon geometry

17. Fall, 200917 Angle Strain

18. Fall, 200918 Angle Strain Experimental Data for Strain in Rings Experimental Data for Strain in Rings

19. Fall, 200919 Angle Strain Cyclopropane and cyclobutane – strained Cyclopropane and cyclobutane – strained Cyclopentane more strain than predicted Cyclopentane more strain than predicted Cyclohexane – no strain Cyclohexane – no strain Baeyer assumed rings are flat Baeyer assumed rings are flat Rings adopt 3-dimensional shape Rings adopt 3-dimensional shape Reduces angle strain Reduces angle strain Angle strain present in small rings which have little flexibility Angle strain present in small rings which have little flexibility

20. Fall, 200920 Strain in Cycloalkanes Torsional Strain Torsional Strain due to eclipsing H’s on adjacent carbon atoms due to eclipsing H’s on adjacent carbon atoms Steric Strain Steric Strain due to repulsion between nonbonded atoms that get too close due to repulsion between nonbonded atoms that get too close Angle Strain Angle Strain present in small nonflexible rings present in small nonflexible rings

21. Fall, 200921 Cyclopropane 3-membered ring must have planar structure 3-membered ring must have planar structure C–C–C bond angles of 60° C–C–C bond angles of 60° Requires that sp 3 based bonds are bent (and weakened) Requires that sp 3 based bonds are bent (and weakened) All C-H bonds are eclipsed All C-H bonds are eclipsed

22. Fall, 200922 Bonding in Cyclopropane Structural analysis of cyclopropane shows that electron density of C-C bond is displaced outward from inter-nuclear axis Structural analysis of cyclopropane shows that electron density of C-C bond is displaced outward from inter-nuclear axis

23. Fall, 200923 Bent Bonds in Cyclopropane Bond reacts with bromine leading to ring opening and addition products

24. Fall, 200924Cyclobutane Cyclobutane - less angle strain than cyclopropane Greater torsional strain Because of its larger number of ring hydrogens Cyclobutane is slightly bent out of plane The bending increases angle strain But decreases torsional strain

25. Fall, 200925 Cyclopentane Planar cyclopentane would have no angle strain but very high torsional strain Planar cyclopentane would have no angle strain but very high torsional strain Actual conformations of cyclopentane are nonplanar Actual conformations of cyclopentane are nonplanar Reducing torsional strain Reducing torsional strain Four carbon atoms are in a plane Four carbon atoms are in a plane The fifth carbon atom is above or below the plane The fifth carbon atom is above or below the plane Envelope Envelope

26. Fall, 200926 Cyclohexane Strain free molecule? Strain free molecule? Why? Why?

27. Fall, 200927 Cyclohexane No eclipsing H’s – no torsional strain No eclipsing H’s – no torsional strain No angle strain No angle strain No steric strain No steric strain No strain energy No strain energy

28. Fall, 200928 Cyclohexane Strain free molecule Strain free molecule Adopts a chair conformation Adopts a chair conformation Important in carbohydrate chemistry Important in carbohydrate chemistry

29. Fall, 200929 Axial and Equatorial Positions 3 hydrogens up and down (red) – axial 3 hydrogens up and down (red) – axial 6 hydrogens in plane – equatorial 6 hydrogens in plane – equatorial Each C atom has one axial, one equatorial Each C atom has one axial, one equatorial

30. Fall, 200930 Axial and Equatorial Positions

31. Fall, 200931 Conformational Mobility Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip

32. Fall, 200932 Ring Flip Conformational mobility is fast Conformational mobility is fast One methylcyclohexane One methylcyclohexane One cyclohexanol One cyclohexanol One bromocyclohexane One bromocyclohexane

33. Fall, 200933 Ring Flip

34. Fall, 200934 Ring Flip Equatorial methyl becomes axial methyl Equatorial methyl becomes axial methyl Equatorial bromo becomes axial bromo Equatorial bromo becomes axial bromo Equatorial hydroxy becomes axial hydroxy Equatorial hydroxy becomes axial hydroxy Barrier to ring flip is 45kJ/mole Barrier to ring flip is 45kJ/mole Rapid process at room temperature Rapid process at room temperature See only a single structure See only a single structure

35. Fall, 200935 Methylcyclohexane C1 to C4 = butane C1 to C4 = butane When equatorial – no interaction with ring When equatorial – no interaction with ring When axial – gauche interaction (3.8kJ/mole) When axial – gauche interaction (3.8kJ/mole)

36. Fall, 200936 Axial Methyl Interactions

37. Fall, 200937 Equilibrium Ring flip occurs Ring flip occurs Equilibrium process Equilibrium process Calculate K Calculate K

38. Fall, 200938 Determination of K for Ring Flip

39. Fall, 200939 Determination of K for Methylcyclohexane

40. Fall, 200940 Determination of % of Isomers

41. Fall, 200941 Disubstituted Cyclohexanes Steric effects of both substituents must be taken into account in both conformations Steric effects of both substituents must be taken into account in both conformations There are two isomers of 1,2- dimethylcyclohexane. cis and trans There are two isomers of 1,2- dimethylcyclohexane. cis and trans Consider the sum of all interactions Consider the sum of all interactions

42. Fall, 200942 cis-1,2-dimethylcyclohexane

43. Fall, 200943 cis-1,2-dimethylcyclohexane In the cis isomer, both methyl groups same face of the ring, and compound can exist in two chair conformations In the cis isomer, both methyl groups same face of the ring, and compound can exist in two chair conformations Interactions for both ring-flip conformations are the same Interactions for both ring-flip conformations are the same Energy is the same Energy is the same K = 1 K = 1 50% of each conformer is present 50% of each conformer is present

44. Fall, 200944 trans-1,2-dimethylcyclohexane

45. Fall, 200945 trans-1,2-dimethylcyclohexane Methyl groups are on opposite faces of the ring Methyl groups are on opposite faces of the ring One trans conformation has both methyl groups equatorial with only a gauche butane interaction between methyls (3.8 kJ/mol) and no 1,3-diaxial interactions One trans conformation has both methyl groups equatorial with only a gauche butane interaction between methyls (3.8 kJ/mol) and no 1,3-diaxial interactions The ring-flipped conformation has both methyl groups axial with four 1,3-diaxial interactions The ring-flipped conformation has both methyl groups axial with four 1,3-diaxial interactions Steric strain of 4  3.8 kJ/mol = 15.2 kJ/mol makes the diaxial conformation 11.4 kJ/mol less favorable than the diequatorial conformation Steric strain of 4  3.8 kJ/mol = 15.2 kJ/mol makes the diaxial conformation 11.4 kJ/mol less favorable than the diequatorial conformation trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in the diequatorial conformation trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in the diequatorial conformation

46. Fall, 200946 Summary All organic molecules face the same strains All organic molecules face the same strains –Angle –Torsional –Steric Molecule will adopt the structure that reduces the total strain in molecule Molecule will adopt the structure that reduces the total strain in molecule –A minimum energy structure –Trade-offs among the strains is necessary