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Organic Chemistry I Cycloalkanes

Organic Chemistry I Cycloalkanes. Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University. Objectives. Nomenclature Ring Strain Conformational Analysis. Cycloalkanes. Hydrocarbons where carbons join in a ring General formula is C n H 2n

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Organic Chemistry I Cycloalkanes

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  1. Organic Chemistry ICycloalkanes Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University

  2. Objectives • Nomenclature • Ring Strain • Conformational Analysis

  3. Cycloalkanes • Hydrocarbons where carbons join in a ring • General formula is CnH2n • Referred to as alicyclic compounds • Chemistry is the same as straight-chain alkanes • They burn • They react with halogen in light

  4. Examples

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

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

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

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

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

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

  11. Nomenclature

  12. Nomenclature

  13. Isomerism in Cycloalkanes • Rotation about C-C bonds in cycloalkanes is limited • 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)

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

  15. Constitutional and Stereoisomers

  16. 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 • Angle Strain • Based upon geometry

  17. Angle Strain

  18. Angle Strain • Experimental Data for Strain in Rings

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

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

  21. Cyclopropane • 3-membered ring must have planar structure • C–C–C bond angles of 60° • Requires that sp3based bonds are bent (and weakened) • All C-H bonds are eclipsed

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

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

  24. Cyclobutane • 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. 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 • Envelope

  26. Cyclohexane • Strain free molecule? • Why?

  27. Cyclohexane • No eclipsing H’s – no torsional strain • No angle strain • No steric strain • No strain energy

  28. Cyclohexane • Strain free molecule • Adopts a chair conformation • Important in carbohydrate chemistry

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

  30. Axial and Equatorial Positions

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

  32. Ring Flip • Conformational mobility is fast • One methylcyclohexane • One cyclohexanol • One bromocyclohexane

  33. Ring Flip

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

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

  36. Axial Methyl Interactions

  37. Equilibrium • Ring flip occurs • Equilibrium process • Calculate K

  38. Determination of K for Ring Flip

  39. Determination of K for Methylcyclohexane

  40. Determination of % of Isomers

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

  42. cis-1,2-dimethylcyclohexane

  43. cis-1,2-dimethylcyclohexane • 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 • Energy is the same • K = 1 • 50% of each conformer is present

  44. trans-1,2-dimethylcyclohexane

  45. trans-1,2-dimethylcyclohexane • 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 • 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 • trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in the diequatorial conformation

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

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