Cycloalkanes
• Alkanes with closed ring(s) of C atoms
• General formula: CnH2n (C3H6, C4H8, C5H10, etc.)
• Naming: use cyclo- prefix before alkane name
cyclopropane
n=3
C3H6
cyclobutane
n=4
C4H8
cyclopentane
n=5
C5H10
cyclohexane
n=6
C6H12
Naming substituted cycloalkanes:
• 1 substituent: no numbering necessary
• 2 or more substituents: highest alpha priority on C #1
CH3
CH3
methylcyclopentane
1-chloro-5-methylcyclohexane
Cl
1-chloro-3-methylcyclohexane
Fun Common Names
windowpane
basketane
broken
windowpane
boxane
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
Going into the page away from you
H3C
H
Coming out at you (like 3-D movie)
H
H
H
CH3
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
H
C
C
Cl
H
H
C
trans isomer – substituents on
opposite faces of the ring
trans-1,2-dichlorocyclobutane
H
C
H
C
H
H
Cl
H
Cl
Cl
Cl
H
C
H
H
C
Cl
C
H
Conformational Analysis
Dash-wedge diagram
H3C
H
H
H
CH3
180° rotation
H
H 3C
CH3
H
H
H
H
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
CH3
H3C
H
H
H
CH3
H

H
H
H
H
CH3
H 3C
CH3
H
H
H
H
CH3 CH3

H
H
H
H
staggered is more stable than eclipsed
Steric strain: atoms/substituents “bumping into” each other
• More steric strain  higher in energy, less stable
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
Cyclopropane and Cyclobutane
Cyclopropane
Cyclobutane
Bond angles 60°
Puckers to stagger C-H bonds
H
All C-H bonds eclipsed
No way to relieve strain
UNSTABLE
H
H
H
H
H
H
H
Puckering decreases bond
angles to less than 90°
UNSTABLE
Cyclopentane
H
H
H
H
H
H
H
H H
H
“envelope”
conformation
Bond angles close to tetrahedral (108°)
Puckers to stagger C-H bonds
STABLE
Cyclohexane
H
H
“chair”
conformation
H
H
H
H
H
H
H
H
H
H
Puckers to relieve bond angle strain and torsional strain
Result: All bond angles ~109°, all C-H bonds staggered
STABLE
H
H
H
H
H
H
H
H
H
axial bonds: straight up or down
equatorial bonds: ~ in plane of ring
H
H
H
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
(Ea ~ 50 kJ/mol)
cis-1-bromo-4-chlorocyclohexane
Br H
H
H
H
H
Br
H
H
H
H
H
Cl
H
Br axial
Cl equatorial
Cl
H
H
H
H
H
H
H
HH
H
“boat”
H
Br
H
H
H
H
Cl
H
H
H
H
Br equatorial
Cl axial
• Intermediate is a boat conformation, higher energy than chair
Drawing chairs from flat pictures
CH3
CH3
H
CH3
CH3
H
cis-1,2-dimethylcyclohexane
Br
H
Br
CH3
CH3
trans-1-bromo-2-methylcyclohexane
H
Optical Isomers
• Optical isomers are non-superimposable mirror images
– interact differently with polarized light
– type of stereoisomer
Br
*
Cl
H
H
F
F
I
I
Br
*
Cl
bromochloroiodomethane
Cl
H3C
*
H
H
Cl
*
CH3
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
No optical
isomers
Br
 plane of symmetry:
molecule halves are mirror images
Chirality: Chiral vs. Achiral
right shoe
left mit
Chiral
right-handed
scissors
glass
plate
Achiral
tennis
racket
Chirality
H
mirror
H
H
H
Cl
Cl
Br
Cl
I
Timberlake, Chemistry 7th Edition, page 484
Br
I
Br
Cl
Br
I
I