Chapter 4 With Video Links

John E.
McMurry
www.cengage.com/chemistry/mcmurry
Chapter 4
Organic Compounds:
Cycloalkanes and their
Stereochemistry
Paul D. Adams • University of Arkansas
Organic Compounds can be
Open-Chained or Cyclic
 We have discussed open-chained compounds up to this point
 Many organic compounds contain rings of carbon atoms
e.g.
- Prostaglandins
- Steroids
Why this Chapter?
 Because cyclic molecules are commonly encountered in all
classes of biomolecules:
- Proteins
- Lipids
- Carbohydrates
- Nucleic acids
It is important to understand the nature of these molecules as you

will encounter them in the next phases of your academic training
4.1 Naming Cycloalkanes
 Cycloalkanes are saturated cyclic hydrocarbons
 Have the general formula (CnH2n)
Naming Cycloalkanes
1) Find the parent. # of carbons in the ring.
2) Number the substituents
Naming Cycloalkanes - Videos
1) Organic Chemistry Naming Examples 2
http://www.khanacademy.org/science/organic-
chemistry/v/organic-chemistry-naming-examples-2


chemistry/v/organic-chemistry-naming-examples--4

4.2 Cis-Trans Isomerism in
Cycloalkanes
 Cycloalkanes are less flexible than open-chain
alkanes
 Much less conformational freedom in
cycloalkanes
Cis-Trans Isomerism in
Cycloalkanes (Continued)
 Because of their cyclic structure, cycloalkanes have 2 faces as
viewed edge-on
“top” face “bottom” face
- Therefore, isomerism is possible in substituted cycloalkanes
- There are two different 1,2-dimethylcyclopropane isomers
Cycloalkanes (Continued)
 Stereoisomerism
 Compounds which have their atoms connected
in the same order but differ in 3-D orientation
Cycloalkanes - Video
1) Cis-Trans and E-Z Naming Scheme for Alkenes
chemistry/v/cis-trans-and-e-z-naming-scheme-for-
alkenes
4.3 Stability of Cycloalkanes:
Ring Strain
 Rings larger than 3 atoms are not flat
 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
Stability of Cycloalkanes: The
Baeyer Strain Theory
 Baeyer (1885): since 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
Types of Strain That Contribute to
Overall Energy of a Cycloalkane
 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
4.4 Conformations of
Cycloalkanes
Cyclopropane
 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
Bent Bonds of Cyclopropane
 In cyclopropane, the C-C bond is displaced
outward from internuclear axis
Cyclobutane
 Cyclobutane has less angle strain than cyclopropane but more
torsional strain because of its larger number of ring hydrogens, and
their proximity to each other
 Cyclobutane is slightly bent out of plane - one carbon atom is about
25° above
 The bend increases angle strain but decreases torsional strain
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
Cyclohexane
 Substituted cyclohexanes occur widely in nature
 The cyclohexane ring is free of angle strain and
torsional strain
 The conformation has alternating atoms in a common
plane and tetrahedral angles between all carbons
 This is called a chair conformation
How to Draw Cyclohexane
Step 1 Draw two parallel lines, slanted downward

and slightly offset from each other. This
means that four of the cyclohexane carbons lie in
a plane.
Step 2 Place the topmost carbon atom above and to

the right of the plane of the other four, and
connect bonds.
Step 3 Place the bottommost carbon atom below

and to the left of the plane of the middle four,
and connect the bonds. Note that the bonds
to the bottommost carbon atom a parallel to
the bonds to the topmost carbon.
Steric Strain
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
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
Drawing the Axial and Equatorial
Hydrogens
Conformational Mobility of
Cyclohexane
 Chair conformations readily interconvert,
resulting in the exchange of axial and equatorial
positions by a ring-flip
Chair and Boat Shapes for
Cyclohexane - Video
1) Chair and Boat Shapes for Cyclohexane

chemistry/v/chair-and-boat-shapes-for-
cyclohexane
Monosubstituted Cyclohexanes
 Cyclohexane ring rapidly flips between chair conformations at room temp.
 Two conformations of monosubstituted cyclohexane aren’t equally stable.
 The equatorial conformer of methyl cyclohexane is more stable than the
axial by 7.6 kJ/mol
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
Steric Strain in Monosubstituted
Cyclohexanes
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 the more stable isomer
Double Newman Diagram for
Methcyclohexane - Video
Double Newman Diagram for Methcyclohexane
chemistry/v/double-newman-diagram-for-
methcyclohexane
Disubstituted Cylcohexanes
 In disubstituted cyclohexanes the steric effects of both substituents must be
taken into account in both conformations
 There are two isomers of 1,2-dimethylcyclohexane. cis and trans
 In the cis isomer, both methyl groups are on the same face of the ring, and
the compound can exist in two chair conformations
 Consider the sum of all interactions
 In cis-1,2, both conformations are equal in energy
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
 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-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in the
diequatorial conformation
Axial and Equatorial Relationships
in Disubstituted Cyclohexanes
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
 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
Conformations of Polycyclic
Molecules (Continued)
Let’s Work a Problem
Draw two constitutional isomers of cis-1,2-
dibromocyclopentane?
Answer
First, we need to understand what constitutional isomer
means…the #’s of atoms, and types of atoms are the
same, just the arrangement may be different. We have
a 5 Carbon cyclic alkane, so we can only have a case
when we have a 1,2- or a 1,3- dibromo linkage, as
these links will be symmetrical with respect to middle
carbon.

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John E.

It is important to understand the nature of these molecules as you

2) Organic Chemistry Naming Examples 3

3) Organic Chemistry Naming Examples 4

4) Organic Chemistry Naming Examples 5

Step 1 Draw two parallel lines, slanted downward

Step 2 Place the topmost carbon atom above and to

Step 3 Place the bottommost carbon atom below

 Chair cyclohexane has

1) Chair and Boat Shapes for Cyclohexane

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