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Stereochemistry of Alkanes
and Cycloalkane
This week: Chapter 3.6, 3.7, 4.2-4.8
McMurry 9th Edition
CHM136: Intro to Organic Chemistry
Thanks to: A. Dicks. N. Morra, S. Browning
Stereochemistry: the 3D arrangement of atoms in a molecule of
interest
Models are very useful for visualizing this material!
Alkane Conformations (Use Models!!)
• bonds are cylindrically symmetrical
• Rotation is possible around C-C bonds in open-chain
molecules, e.g. ethane…
2
Representing Conformations
Conformation: any three-dimensional arrangement of atoms in a molecule that
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results from rotation about a single bond
Newman projection: a way to view a molecule by looking along a carbon-carbon
bond
rotate
rotate
180o
dihedral (torsion) angle
60o
Conformations
Staggered conformation: a conformation where atoms on one carbon are as far
apart as possible from the atoms on the adjacent carbon
Eclipsed conformation: a conformation where atoms on one carbon are as close as
possible to the atoms on the adjacent carbon
Energy of Conformations
Torsional strain: a destabilization that arises during rotation as bonds eclipse
each other. Torsional strain drives molecules away from eclipsed
conformations.
+12 kJ/mol
eclipsed staggered
Conformations of Ethane
dihedral angle (degrees)
torsional strain
Steric Strain
Steric strain: the strain that arises when atoms not bonded to each other are forced
abnormally close to one another
gauche conformation: substituents 60o
from one another.
Consider butane:
anti conformation: substituents 180o
from one another.
no torsional strain
steric strain is approximately 3.8 kJ/mol
no steric strain
no torsional strain
Strain
Conformations of Butane
10
Butane
11
Zig-zag Drawings
• Why do we represent alkanes with a zig-zag line?
12
Cycloalkane Stereoisomerism
• Cycloalkanes less flexible than open-chain alkanes
• Much less “conformational freedom”
Stereoisomerism
Stereoisomers: compounds having the same atom
connectivity but different 3D atomic arrangements in
space
13
Stereoisomerism
These two isomers (diastereomers) cannot interconvert
through bond rotation
Isomerism common in
multi-substituted cycloalkanes
14
Problem
15
Draw a skeletal structure to represent
trans-1-methyl-2-(2-methylpropyl)cyclobutane
16
Cycloalkane Stability
• Rings larger than three atoms are not flat
• Cyclic molecules adopt non-planar conformations (the
ring “puckers”) to minimize angle strain and torsional
strain
• Larger rings are harder to analyze: have many possible
conformations
17
Cycloalkane Stability
Rings from 3 to 30 C’s exist but many are strained due to
steric interactions
18
Strain Summary
• Angle strain: expansion or (often) compression of
bond angles away from most stable
• Torsional strain: eclipsing or alignment of bonds on
neighbouring atoms
• Steric strain: repulsive interactions between non-
bonded atoms in close proximity
Cycloalkane Conformations: Cyclopropane
• 3-membered (3-sided) ring must be planar!
• Symmetrical with C–C–C bond angles of 60°
• Requires that sp3-sp3 bonds are bent (weakened)
• All C-H bonds eclipsed
19
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Cycloalkane Conformations: Cyclobutane
• Less angle strain than cyclopropane but more torsional
strain because of more ring H’s
• One C atom about 25° above plane of other three
• Ring bend increases angle strain BUT decreases
torsional strain: paper airplane conformation
Cycloalkane Conformations: Cyclopentane
• Planar cyclopentane would have NO angle strain BUT very
high torsional strain
• Non-planarity reduces torsional strain
• Four carbon atoms are in a plane, fifth is above the plane:
envelope conformation
21
22
Cyclohexane Conformations
• Substituted cyclohexane rings occur widely in nature
(cholesterol has two): free of angle strain and
torsional strain
• Tetrahedral angles between all carbons: chair
conformation
Drawing Cyclohexane
23
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Drawing Cyclohexane
25
Axial & Equatorial Bonds in Cyclohexane
• Chair conformation has two kinds of positions for the 12 H
atoms: axial and equatorial
