Cycloalkanes• Alkanes with closed ring(s) of C atoms• General formula: CnH2n (C3H6, C4H8, C5H10, etc.)• Naming: use cyclo- prefix before alkane name

cyclopropanen = 3

cyclopentanen = 5

cyclobutanen = 4

cyclohexanen = 6

Naming substituted cycloalkanes:• 1 substituent: no numbering necessary• 2 or more substituents: highest alpha priority on C #1

CH3

methylcyclopentane

CH3

Cl1-chloro-3-methylcyclohexane

C3H6 C4H8 C5H10 C6H12

1-chloro-5-methylcyclohexane

Fun Common Names

boxanewindowpane

basketane

brokenwindowpane

Teepee-ane

Dashes and Wedges Bonds

H3C

CH3

H

HHH

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

Cl

Cl

Cl

Cl

cis isomer – substituents on same face of the ringcis-1,2-dichlorocyclobutane

trans isomer – substituents on opposite faces of the ringtrans-1,2-dichlorocyclobutane

CC

C CH

ClH

Cl

H

H

H

H

CC

C CH

ClH

H

Cl

H

H

H

Cl

ClCl

Cl

Cl

Cl

Cl

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

staggered eclipsed

Conformational isomers: differ only by rotation about single bonds• Also called conformers or rotamers• Interconvert easily at room temperature

H3C

H

CH3

HHH

H3C

CH3

H

HHHDash-wedge diagram 180° rotation

H3C

CH3

H

HHH

CH3

HH

CH3

HH

CH3

H H

H

CH3

H

H3C

H

CH3

HHH

is more stable than

Strain in CycloalkanesWhat 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

CyclopropaneBond angles 60°

All C-H bonds eclipsed

No way to relieve strain

UNSTABLE

CyclobutanePuckers to stagger C-H bonds

Puckering decreases bond angles to less than 90°

UNSTABLE

H H

HH

H

H

HH

Cyclopentane

Bond angles close to tetrahedral (108°)

Puckers to stagger C-H bonds

“envelope”conformation

STABLE

H

H

HH

H

H

H

H

HH

CyclohexaneHH

HH

H H

H

H

HH

HH

Puckers to relieve bond angle strain and torsional strain

Result: All bond angles ~109°, all C-H bonds staggered

STABLE

“chair”conformation

HH

HH

H H

H

H

HH

HH

axial bonds: straight up or downequatorial bonds: ~ in plane of ring

axial positions: crowded, only H can fitequatorial positions: plenty of room

Cyclohexane “ring flip”• Axial and equatorial positions can be swapped by ring flipping

(Ea ~ 50 kJ/mol)

• Intermediate is a boat conformation, higher energy than chair

Br

H

H

H H

H

H

ClH

HH

H

H Cl

HH

HH

H

Br

H H

HH

HBr

HH

H H

H

Cl

HH

HH

cis-1-bromo-4-chlorocyclohexane

Br axialCl equatorial

Br equatorialCl axial

“boat”

Drawing chairs from flat pictures

CH3

CH3

CH3

H

CH3

H

H

H

CH3

BrBr

CH3

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

I

HCl

BrI

HCl

Br

bromochloroiodomethane

* *

F

HCH3

ClF

HH3C

Cl

1-chloro-1-fluoroethane

* *

C* = stereocenter: C with 4 different substituentshint that molecule may have optical isomers

mirror plane in molecule no optical isomers

Br 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

tennisracket

H

Br

I

Cl

Chirality

Timberlake, Chemistry 7th Edition, page 484

H

Cl

I

Br

H

Br

I

Cl

mirrorH

Cl

I

Br