Chapter 04 stereochemistry of alkanes and cycloalkanes

4. Stereochemistry of
Alkanes and Cycloalkanes
CHEM 221 Organic Chemistry Dr. Wong Yau Hsiung

McMurry Organic Chemistry 6th edition Chapter 4
(c) 2003
2
The Shapes of Molecules
 The three-dimensional shapes of molecules result
from many forces
 A molecule may assume different shapes, called
conformations, that are in equilibrium at room
temperature (the conformational isomers are called
conformers, emphasis on the first syllable)
 The systematic study of the shapes molecules and
properties from these shapes is stereochemistry
 The field of stereochemistry is one of the central
parts of organic chemistry and includes many
important topics

(c) 2003
3
4.1 Conformations of Ethane
 Conformers interconvert
rapidly and a structure
is an average of
conformers
 Molecular models are
three dimensional
objects that enable us
to visualize conformers
 Representing three
dimensional conformers
in two dimensions is
done with standard
types of drawings

(c) 2003
4
Representing Conformations
 Sawhorse representations
show molecules at an angle,
showing a molecular model
 C-C bonds are at an
angle to the edge of the
page and all C-H bonds
are shown
 Newman projections show
how the C-C bond would
project end-on onto the
paper
 Bonds to front carbon are
lines going to the center
 Bonds to rear carbon are
lines going to the edge of
the circle

(c) 2003
5
Ethane’s Conformations
 There barrier to rotation between conformations is small (12
kJ/mol; 2.9 kcal/mol) The most stable conformation of ethane
has all six C–H bonds away from each other (staggered)
 The least stable conformation has all six C–H bonds as close as
possible (eclipsed) in a Newman projection – energy due to
torsional strain

(c) 2003
6
4.2 Conformations of Propane
 Propane (C3H8)
torsional barrier
around the carbon–
carbon bonds 14
kJ/mol
 Eclipsed conformer
of propane has two
ethane-type H–H
interactions and an
interaction between
C–H and C–C bond

(c) 2003
7
4.3 Conformations of Butane
 anti conformation has two methyl groups 180° away from each
other
 Rotation around the C2–C3 gives eclipsed conformation
 Staggered conformation with methyl groups 60° apart is gauche
conformation

(c) 2003
8
4.4 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

(c) 2003
9
4.5 The Nature of 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

(c) 2003
10
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

(c) 2003
11
4.6 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

(c) 2003
12
Bent Bonds of Cyclopropane
 Structural analysis of cyclopropane shows
that electron density of C-C bond is displaced
outward from internuclear axis

(c) 2003
13
4.7 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

(c) 2003
14
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

(c) 2003
15
4.8 Conformations of Cyclohexane
 Substituted cyclohexanes occur widely in nature
 The cyclohexane ring is free of angle strain and
torsional strain
 The conformation is has alternating atoms in a
common plane and tetrahedral angles between all
carbons
 This is called a chair conformation

(c) 2003
16
How to Draw Cyclohexane

(c) 2003
17
4.9 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

(c) 2003
18
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

(c) 2003
19
Drawing the Axial and Equatorial
Hydrogens

(c) 2003
20
4.10 Conformational Mobility of
Cyclohexane
 Chair conformations readily interconvert,
resulting in the exchange of axial and
equatorial positions by a ring-flip

(c) 2003
21
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

(c) 2003
22
4.11 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

(c) 2003
23
Energy and Equilibrium
 The relative
amounts of the two
conformers depend
on their difference
in energy ∆E = −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

(c) 2003
24
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

(c) 2003
25
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

(c) 2003
26
4.12 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 1,2-
dimethylcyclohexane. cis and
trans
 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

(c) 2003
27
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

(c) 2003
28
4.13 Boat Cyclohexane
 Cyclohexane can also be in
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 C-
2, 3, 5, 6 produce torsional
strain
 ~29 kJ/mol (7.0 kcal/mol)
less stable than chair

(c) 2003
29
4.14 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

Chapter 04 stereochemistry of alkanes and cycloalkanes

In this document

More Related Content

What's hot

Similar to Chapter 04 stereochemistry of alkanes and cycloalkanes

More from Wong Hsiung

Recently uploaded

Chapter 04 stereochemistry of alkanes and cycloalkanes