Basics of Organic Chemistry
 Stereochemistry I
 Conformational analysis of alkanes
 Conformational analysis of cycloalkanes
1
Shapes of Alkanes
 “Straight-chain” alkanes have a zig-zag orientation
when they are in their most straight orientation
Straight chain alkanes are also called unbranched alkanes
Shapes of Alkanes
 Branched alkanes have at least one carbon which is
attached to more than two other carbons
Sigma Bonds and Bond Rotation
 Ethane has relatively free rotation around the carbon



carbon bond
Such rotation leads to the formation of different
conformers
Ethane has two conformers
The staggered conformation has C-H bonds on adjacent
carbons as far apart from each other as possible
Newman projection is a type of drawing that depicts the
conformation of a specific conformer
4
Sigma Bonds and Bond Rotation
The eclipsed conformation has all C-H bonds on adjacent
carbons directly on top of each other
5
Which Conformer Is More Stable?
An analysis of the energy changes associated with a molecule
undergoing rotation about single bonds is called Conformational
Analysis
Also known
as tortional
angle
6
Conformational Analysis of Ethane
 The potential energy diagram of the conformations of
ethane shows that the staggered conformation is more
stable than eclipsed by 12 kJ mol-1
60
rotation
60
rotation
Front
carbon
fixed.
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Conformational Analysis of Butane
 How many conformers does butane have?
 Rotation (clockwise) around C2-C3 of butane
gives six important conformations
Back carbon fixed.
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Conformational Analysis of Butane
 The stability of the different conformers is related to the
repulsive van der Waals forces between the two methyls
 The following is the potential energy diagram for butane
conformers. Assign the six conformers to the appropriate
potential energy
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Show pages in Molecular Conceptor 2
In-Class Practice
1. Draw the Newman Projection formula for propane along
C1-C2 bond.
H
H
H
H H
H
H
H
H
H
H
H
H
CH3
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In-Class Practice
2. How many conformers does propane have?
H
H
H
H
H
H
H
H
H
CH3
H
CH3
H
CH3
H
H
H
CH3
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In-Class Practice
3. Draw the potential energy diagram for all conformers
of propane.
CH3
E
CH3
CH3
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The Relative Stabilities of Cycloalkanes: Ring Strain
 Heats of combustion per CH2 unit reveal cyclohexane has
no ring strain and other cycloalkanes have some ring strain
13
Quick-Review
Which cycloalkane has the lowest heat of combustion per
CH2 group?
a)
b)
c)
d)
e)
14
The Origin of Ring Strain in Cyclopropane: Angle
Strain and Tortional Strain
 Angle strain is caused by bond angles different from
109.5o
 Tortional strain is caused by eclipsing C-H bonds on
adjacent carbons
 Cyclopropane has both high angle and tortional strain
15
The Origin of Ring Strain in Cyclobutane and
Cyclopentane : Angle Strain and Tortional Strain
 Cyclobutane has considerable angle strain
 It bends to relieve some tortional strain
 Cyclopentane has little angle strain in the planar form
but bends to relieve some tortional strain
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Conformations of Cyclohexane
 The chair conformation has no ring strain
All bond angles are 109.5o and all C-H bonds are perfectly
staggered
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The Boat Conformation
 is less stable because of flagpole interactions
and tortional strain along the bottom of the boat
18
The Twist Conformation
 is intermediate in stability between the boat and
the chair conformation
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Substituted Cycloalkanes
 Substituted Cyclohexanes: Axial and Equatorial Hydrogen
Atoms
 Axial hydrogens are perpendicular to the average plane of the
ring
 Equatorial hydrogens lie around the perimeter of the ring
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Drawing Substituted Cyclohexanes
 The C-C bonds and equatorial C-H bonds are all drawn in
sets of parallel lines
 The axial hydrogens are drawn straight up and down
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Conformational Preference of Substituted Cyclohexanes
 Methyl cyclohexane is more stable with the methyl
equatorial
An axial methyl has an unfavorable 1,3-diaxial interaction with
axial C-H bonds 2 carbons away
A 1,3-diaxial interaction is the equivalent of 2 gauche butane
interactions
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Generally
 Substituted cyclohexane is more stable with the
substituent equatorial
 space-filling model of methyl cyclohexane
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Matching Game
Ring strain
Tortional strain
Substituents
in cyclohexane
C-H bond eclipsed
e-position more stable
Bond angle departed
from 109.5º
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Disubstitued Cycloalkanes
 Can exist as pairs of cis-trans stereoisomers
(diastereomers)
Cis: groups on same side of ring
Trans: groups on opposite side of ring
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Conformational Preference of Di-Substituted Cyclohexanes
 Trans-1,4-dimethylcylohexane prefers a trans-
diequatorial conformation
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Conformational Preference of Di-Substituted Cyclohexanes
 Cis-1,4-dimethylcyclohexane exists in an axial-
equatorial conformation
 A very large tert-butyl group is required to be in the
more stable equatorial position
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Multiple Choice Questions
1. The preferred conformation of cis-1-tert-butyl-3methylcyclohexane is the one in which:
a) the tert-butyl group is axial and the methyl group is equatorial
b) the methyl group is axial and the tert-butyl group is equatorial
c) both groups are axial
d) both groups are equatorial
e) the molecule exists in a boat conformation
2. Trans-1,3-Dibromocyclohexane is represented by structure(s):
a) I
b) II
c) III
d) II and III
e) I and III
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Match the molecule with its preferred conformation
cis-1,4bromochlorocyclohexane
Cl
cis-1,3dimethylcyclohexane
Br
Cl
Br
trans-1,2dimethylcyclohexane
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