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 ring

cis-1,2-dichlorocyclobutane

trans isomer – substituents on

opposite faces of the ring

trans-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 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

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

Cl

H

H

H

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

Br

I

HCl

Br

bromochloroiodomethane

* *

F

HCH3

Cl

F

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