2. Molecules in Disguise Draw 5-propylheptane. What is its real name? 4-ethyloctane

3. 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 cyclopropane n = 3 cyclopentane n = 5 cyclobutane n = 4 cyclohexane n = 6 Naming substituted cycloalkanes: 1 substituent: no numbering necessary 2 or more substituents: highest alpha priority on C #1 methylcyclopentane 1-chloro-3-methylcyclohexane C3H6C3H6 C4H8C4H8 C 5 H 10 C 6 H 12 1-chloro-5-methylcyclohexane

4. Fun Common Names boxane windowpane basketane broken windowpane

5. Teepee-ane

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

7. 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 cis isomer – substituents on same face of the ring cis-1,2-dichlorocyclobutane trans isomer – substituents on opposite faces of the ring trans-1,2-dichlorocyclobutane Cl

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

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

10. 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

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

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

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

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

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

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

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