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Cycloalkanes

Cycloalkanes. Alkanes with closed ring(s) of C atoms General formula: C n H 2n (C 3 H 6 , C 4 H 8 , C 5 H 10 , etc.) Naming: use cyclo- prefix before alkane name. cyclo butane n = 4. cyclo pentane n = 5. cyclo hexane n = 6. cyclo propane n = 3. C 6 H 12. C 3 H 6. C 5 H 10.

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Cycloalkanes

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  1. Cycloalkanes • Alkanes with closed ring(s) of C atoms • General formula: CnH2n (C3H6, C4H8, C5H10, etc.) • Naming: use cyclo- prefix before alkane name cyclobutane n = 4 cyclopentane n = 5 cyclohexane n = 6 cyclopropane n = 3 C6H12 C3H6 C5H10 C4H8 • Naming substituted cycloalkanes: • 1 substituent: no numbering necessary • 2 or more substituents: highest alpha priority on C #1 1-chloro-5-methylcyclohexane 1-chloro-3-methylcyclohexane methylcyclopentane

  2. Fun Common Names broken windowpane boxane windowpane basketane

  3. Teepee-ane

  4. Dashes and Wedges Bonds Wedge Indicates bond coming out of the page toward you Dashes Indicates bond going into the page away from you Going into the page away from you Coming out at you (like 3-D movie)

  5. Stereoisomers • Cycloalkane rings have two faces • Stereoisomers: same connectivity, but different arrangement of atoms in space • Geometric Isomers: type of stereoisomer involving rings or multiple bonds with substituents on two different carbons • Designated as either cis or trans and must be included in the name Cl Cl cis isomer – substituents on same face of the ring cis-1,2-dichlorocyclobutane Cl Cl Cl Cl trans isomer – substituents on opposite faces of the ring trans-1,2-dichlorocyclobutane Cl Cl

  6. Conformational Analysis 180° rotation Dash-wedge diagram • Conformational isomers: differ only by rotation about single bonds • Also called conformers or rotamers • Interconvert easily at room temperature Newman projections: looking down a C-C bond   is more stable than staggered eclipsed • Steric strain: atoms/substituents “bumping into” each other • More steric strain  higher in energy, less stable

  7. Strain in Cycloalkanes What if cycloalkane rings were flat? Bond angles: 60° 90° 108° 120° Tetrahedral bond angle = 109.5° Bond angle strain Flat rings would require all C-H bonds to be eclipsed Torsional (twisting) strain Result: When possible, rings will compensate to relieve both kinds of strain

  8. Cyclopropane and Cyclobutane Cyclopropane Bond angles 60° All C-H bonds eclipsed No way to relieve strain UNSTABLE Cyclobutane Puckers to stagger C-H bonds Puckering decreases bond angles to less than 90° UNSTABLE

  9. Cyclopentane “envelope” conformation Bond angles close to tetrahedral (108°) Puckers to stagger C-H bonds STABLE

  10. Cyclohexane “chair” conformation Puckers to relieve bond angle strain and torsional strain Result: All bond angles ~109°, all C-H bonds staggered STABLE axial bonds: straight up or down equatorial bonds: ~ in plane of ring axial positions: crowded, only H can fit equatorial positions: plenty of room

  11. Cyclohexane “ring flip” • Axial and equatorial positions can be swapped by ring flipping (Ea ~ 50 kJ/mol) cis-1-bromo-4-chlorocyclohexane Br axial Cl equatorial Br equatorial Cl axial “boat” • Intermediate is a boat conformation, higher energy than chair

  12. Drawing chairs from flat pictures cis-1,2-dimethylcyclohexane trans-1-bromo-2-methylcyclohexane

  13. Optical Isomers • Optical isomers are non-superimposable mirror images • interact differently with polarized light • type of stereoisomer * * * * 1-chloro-1-fluoroethane bromochloroiodomethane C* = stereocenter: C with 4 different substituents hint that molecule may have optical isomers mirror plane in molecule  no optical isomers No optical isomers  plane of symmetry: molecule halves are mirror images

  14. Chirality: Chiral vs. Achiral right shoe left mit right-handed glass plate scissors tennis racket Chiral Achiral

  15. Chirality H H H Cl Br Br Cl Cl Br H I I I Cl Br I mirror Timberlake, Chemistry 7th Edition, page 484

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