4. Organic Compounds:
Cycloalkanes and their
Stereochemistry
Based on
McMurry’s Organic Chemistry, 7th edition,
Chapter 4
4.1 Cycloalkanes
Cycloalkanes are alkanes that have
carbon atoms that form a ring (called
alicyclic compounds)
 Simple cycloalkanes are rings of
CH2 units, (CH2)n, or CnH2n
 Structure is shown as a regular polygon
with the number of vertices equal to the
number of C’s (a projection of the
actual structure)

2
Cycloalkanes
3
Complex Cycloalkanes
 Naturally
occurring materials
contain cycloalkane structures
 Examples:
chrysanthemic acid
(cyclopropane; from
chrysanthemum pyrethrins),
 prostaglandins (cyclopentane),
 steroids (cyclohexanes and
cyclopentane)

4
Complex Cycloalkanes
5
Properties of Cycloalkanes

Melting points
are affected by
the shapes and
the way that
crystals pack so
they do not
change
uniformly
6
4.1 Naming Cycloalkanes



Count the number of carbon atoms in the ring
and the number in the largest substituent
chain. If the number of carbon atoms in the
ring is equal to or greater than the number in
the substituent, the compound is named as an
alkyl-substituted cycloalkane
For an alkyl- or halo-substituted cycloalkane,
start at a point of attachment as C1 and
number the substituents on the ring so that the
second substituent has as low a number as
possible.
Number the substituents and write the name
7
1. Find the parent:
or butylcyclopropane
8
Number the
substituents &
write the
name:
9
Examples:
10
Problem 3.15: IUPAC names?
11
4.2 Cis-Trans Isomerism in Cycloalkanes
Rotation about C-C bonds in cycloalkanes
is limited by the ring structure
 Rings have two “faces” and substituents
are labeled as to their relative facial
positions
 There are two different 1,2-dimethylcyclopropane isomers, one with the two
methyls on the same side (cis) of the ring
and one with the methyls on opposite
sides (trans)

12
Cis-Trans Isomerism in Cycloalkanes
13
Stereoisomers
Compounds with atoms connected in the
same order but which differ in threedimensional orientation, are
stereoisomers
 The terms “cis” and “trans” should be
used to specify stereoisomeric ring
structures
 Recall that constitutional isomers have
atoms connected in different order

14
Stereoisomers
15
Practice Prob. 4.4: Name?
16
Problem 4.18: IUPAC Name?
17
Problem 4.49: IUPAC names?
18
Problem 4.55: Four cis-trans stereoisomers
of menthol? This is the natural one:
19
4.3 Stability of Cycloalkanes: The Baeyer
Strain Theory

Baeyer (1885): since
(sp3) carbon prefers to
have bond angles of
approximately 109°,
ring sizes other than
five and six may be
too strained to exist

Rings from 3 to 30 C’s
do exist but are
strained due to bond
bending distortions
and steric interactions
20
Baeyer’s hypothesis: angle strain
21
Heats of Combustion
22
Stability of Cycloalkanes
23
The Nature of Ring Strain
Rings larger than 3 atoms are not flat
(planar).
 Cyclic molecules can assume nonplanar
conformations to minimize angle strain
and torsional strain by ring-puckering
 Larger rings have many more possible
conformations than smaller rings and are
more difficult to analyze

24
Types of Strain



25
Angle strain - expansion or compression of bond
angles away from most stable
Torsional strain - eclipsing of bonds on
neighboring atoms
Steric strain - repulsive interactions between
nonbonded atoms in close proximity
Angle Strain
26
Torsional Strain
27
Steric Strain
28
Strain Energies
29
Summary: Types of Strain
 Angle
strain - expansion or
compression of bond angles away
from most stable
 Torsional strain - eclipsing of
bonds on neighboring atoms
 Steric strain - repulsive interactions
between nonbonded atoms in close
proximity
30
4.4 Cyclopropane: An Orbital View




3-membered ring must have planar structure
Symmetrical with C–C–C bond angles of 60°
Requires that sp3 based bonds are bent (and
weakened)
All C-H bonds are eclipsed
31
Bent Bonds of Cyclopropane

Structural
analysis of
cyclopropane
shows that
electron
density of C-C
bond is
displaced
outward from
the internuclear
axis
32
“Bent” bonds in cyclopropane: less than
maximum orbital overlap
33
Conformations of Cyclobutane and
Cyclopentane


Cyclobutane has less angle strain than
cyclopropane but more torsional strain because
of its larger number of ring hydrogens
Cyclobutane is slightly bent out of plane - one
carbon atom is about 25° above

The bend increases angle strain but decreases
torsional strain
34
Cyclobutane
35
Cyclopentane



Planar cyclopentane would have no angle strain but very
high torsional strain
Actual conformations of cyclopentane are nonplanar,
reducing torsional strain
Four carbon atoms are in a plane

The fifth carbon atom is above or below the plane – looks like
an envelope
36
Cyclopentane
37
4.5 Conformations of Cyclohexane




Substituted cyclohexanes occur widely in nature
The cyclohexane ring is free of angle strain and torsional
strain
The conformation has alternating atoms roughly in a
common plane, and tetrahedral angles between all carbons
This is called a chair conformation
38
Chair Conformations
39
Cholesterol: three chair conformations
40
How to Draw Cyclohexane
41
4.6 Axial and Equatorial Bonds in
Cyclohexane


The chair conformation
has two kinds of positions
for substituents on the
ring: axial positions and
equatorial positions
Chair cyclohexane has six
axial hydrogens
perpendicular to the ring
(parallel to the ring axis)
and six equatorial
hydrogens near the plane
of the ring
42
Axial and Equatorial Bonds
43
Axial and Equatorial Positions


Each carbon atom in cyclohexane has one axial
and one equatorial hydrogen
Each face of the ring has three axial and three
equatorial hydrogens in an alternating
arrangement
44
Drawing the Axial and Equatorial
Hydrogens
45
Axial and Equatorial Hydrogens
46
Conformational Mobility of
Cyclohexane

Chair conformations readily interconvert, resulting in the
exchange of axial and equatorial positions by a ring-flip
47
Conformational Mobility
48
Bromocyclohexane


When bromocyclohexane
ring-flips the bromine’s
position goes from
equatorial to axial and
so on
At room temperature the
ring-flip is very fast and
the structure is seen as
the weighted average
49
Bromocyclohexane
50
4.7 Conformations of Monosubstituted
Cyclohexanes


The two conformers of a monosubstituted
cyclohexane are not equal in energy
The equatorial conformer of methyl cyclohexane
is more stable than the axial by 7.6 kJ/mol
51
Methylcyclohexane
52
Energy and Equilibrium


The relative
amounts of the
two conformers
depend on their
difference in
energy DE = RT
ln K
R is the gas
constant [8.315
J/(K•mol)], T is
the Kelvin
temperature,
and K is the
equilibrium
constant
between isomers
53
1,3-Diaxial Interactions


Difference between axial and equatorial conformers is
due to steric strain caused by 1,3-diaxial interactions
Hydrogen atoms of the axial methyl group on C1 are
too close to the axial hydrogens three carbons away on
C3 and C5, resulting in 7.6 kJ/mol of steric strain
54
1,3-Diaxial Interactions
55
Relationship to Gauche Butane
Interactions



Gauche butane is less
stable than anti butane
by 3.8 kJ/mol because of
steric interference
between hydrogen atoms
on the two methyl
groups
The four-carbon
fragment of axial
methylcyclohexane and
gauche butane have the
same steric interaction
In general, equatorial
positions give more
stable isomer
56
Gauche Butane Interactions
57
Monosubstituted Cyclohexanes
58
4.8 Conformational Analysis of
Disubstituted Cyclohexanes
 In
disubstituted cyclohexanes the
steric effects of both substituents
must be taken into account in both
conformations
 There are two isomers of 1,2dimethylcyclohexane. cis and trans
59
4.12 Conformational Analysis of
Disubstituted Cyclohexanes



In the cis isomer, both
methyl groups same face
of the ring, and compound
can exist in two chair
conformations
Consider the sum of all
interactions
In cis-1,2, both
conformations are equal in
energy
60
Cis-1,2-dimethylcyclohexane
61
Cis-1,2-dimethylcyclohexane
62
Trans-1,2-Dimethylcyclohexane



Methyl groups are on
opposite faces of the ring
One trans conformation
has both methyl groups
equatorial and only a
gauche butane interaction
between methyls (3.8
kJ/mol) and no 1,3-diaxial
interactions
The ring-flipped
conformation has both
methyl groups axial with
four 1,3-diaxial interactions
63
Trans-1,2-Dimethylcyclohexane


Steric strain of 4  3.8
kJ/mol = 15.2 kJ/mol
makes the diaxial
conformation 11.4 kJ/mol
less favorable than the
diequatorial conformation
trans-1,2dimethylcyclohexane will
exist almost exclusively
(>99%) in the diequatorial
conformation
64
Trans-1,2-Dimethylcyclohexane
65
Trans-1,2-Dimethylcyclohexane
66
Axial/Equatorial Relationships
67
t-Butyl Groups
68
t-Butyl Groups
69
t-Butyl Groups
70
Prob. 4.37: Most stable
conformation of Menthol?
71
Solution:
CH3
OH
H3C
CH
HO
CH3
H3C
more stable
CH
H3C
CH3
72
Problem 4.36: Galactose has an axial
OH group at C4. Draw the chair:
73
Solution:
OH
CH2OH
O
HO
OH
HO
Galactose
74
Boat Cyclohexane






Cyclohexane flips through a
boat conformation
Less stable than chair
cyclohexane due to steric
and torsional strain
C-2, 3, 5, 6 are in a plane
H on C-1 and C-4 approach
each other closely enough
to produce considerable
steric strain
Four eclipsed H-pairs on C2, 3, 5, 6 produce torsional
strain
~29 kJ/mol (7.0 kcal/mol)
less stable than chair
75
76
Boat & Twist-boat conformations:
77
4.9 Conformations of Polycyclic
Molecules

Decalin consists of two cyclohexane rings joined to share
two carbon atoms (the bridgehead carbons, C1 and C6)
and a common bond
78
Decalin
79
4.9 Conformations of Polycyclic
Molecules





Two isomeric forms of decalin: trans fused or cis
fused
In cis-decalin hydrogen atoms at the bridgehead
carbons are on the same face of the rings
In trans-decalin, the bridgehead hydrogens are
on opposite faces
Both compounds can be represented using chair
cyclohexane conformations
Flips and rotations do not interconvert cis and
trans
80
Cis- and trans- decalins
81
Steroids
82
Cholesterol
83
Testosterone
84
Bicyclic Compounds
85
Camphor
86
Morphine: and Opium Alkaloid
87
(Demerol)
88