1 / 46

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

faunus
Download Presentation

Organic Chemistry I Cycloalkanes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related