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

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

Fun Common Names boxane windowpane basketane broken windowpane

Teepee-ane

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

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

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

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

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

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

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

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”

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

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

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

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