Title: Stereochemistry of Alkanes
1Chapter 4
Stereochemistry of Alkanes and Cycloalkanes
2Introduction
- Stereochemistry
- It is the systematic study of the
three-dimensional shapes of molecules and
properties that arise from these shapes - The three-dimensional shapes of molecules result
from many forces
3- Conformations are different shapes that a
molecule may assume. - Conformers are conformational isomers.
- They are in equilibrium at room temperature.
- They cant usually be isolated because they
interconvert too rapidly
4- Alkanes
- have C-C single bonds formed by s overlap of sp3
hybrid orbitals - Rotation is possible around s bonds because of
their cylindrical symmetry gt Many Conformers
5 I. Conformations
6 A. Ethane
- Conformers interconvert rapidly and a structure
is an average of conformers - Representing three dimensional conformers in two
dimensions is done with standard types of
drawings
7- Molecular models are three dimensional objects
that enable us to visualize conformers
8Representing Conformations
- There are two representations
- Sawhorse representation
- Newman projection
9Representing 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 - 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
10Ethanes Conformations
- The most stable conformation of ethane has all
six CH bonds away from each other (staggered). - The least stable conformation has all six CH
bonds as close as possible (eclipsed) in a Newman
projection.
11Ethanes Conformations
- The barrier to rotation between conformations is
small (12 kJ/mol 2.9 kcal/mol) - The eclipsed conformers are 12 kJ/mol higher in
energy than the staggered conformers energy due
to torsional strain
12Ethanes Conformations
- The torsional strain (12 kJ/mol) of the eclipsed
conformers are due to 3 H-H eclipsing
interactions. - Each H-H interaction contributes 4.0 kJ/mol
13 B. Propane
- Propane (C3H8) has torsional barrier around the
carboncarbon bonds (14 kJ/mol). - Eclipsed conformer of propane has two ethane-type
HH interactions and an interaction between CH
and CC bond
14- The torsional strain (14 kJ/mol) of the eclipsed
conformers are due to 2 ethane-type H-H
interactions and an interaction between CH and
CC bond. - The CH and CC bond interaction contributes 6.0
kJ/mol ( 14 (2 x 4.0))
15Practice Problem Make a graph of potential
energy versus angle of bond rotation
for propane, and assign values to the
energy maxima
16Practice Problem Draw Newman projections of the
most stable and least stable
conformations of bromoethane
17 C. Butane
- As the alkane becomes larger, the conformations
become more complex. - Butane has eclipsed and staggered conformers with
different energy level around C2-C3
18Butanes Conformations
- anti conformation is the most stable conformation
of butane - It has two methyl groups 180 away from each
other
19Butanes Conformations
- Rotation around the C2C3 gives eclipsed
conformation
20Butanes Conformations
- gauche conformation is the staggered conformation
with methyl groups 60 apart. - Although it has no eclipsing interactions, it is
3.8 kJ/mol higher in energy than the anti
conformation. - This is due to steric strain.
21Butanes Conformations
- The steric strain (3.8 kJ/mol) of the gauche
conformation is due to the repulsive interaction
that occurs when atoms are forced together than
their atomic radii allow.
22Butanes Conformations
- The least stable eclipsed conformation is one in
which the methyl groups are too close. - 19 kJ/mol is due to steric and torsional strain.
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24For any alkane, the most favorable conformation
is the staggered arrangement on C-C bonds and
large substituents arranged anti to one another.
25One particular conformer is more stable than
another means a large percentage of molecules
will be found a in more stable conformation than
in a less stable one.
26Practice Problem Consider 2-methylpropane
(isobutane). Sighting along the C2-C1
bond
- Draw a Newman projection of the most stable
- conformation
- Draw a Newman projection of the least stable
- conformation
- Make a graph of energy versus angle of rotation
around the C2-C1 bond - Since a hydrogen-hydrogen eclipsing interaction
costs 4.0 kJ/mol and a hydrogen-methyl eclipsing
interaction costs 6.0 kJ/mol, assign relative
values to the maxima and minima in your graph
27Practice Problem Sight along the C2-C3 bond of
2,3-dimethyl- -butane, and draw a
Newman projection of the most stable
conformation.
28Practice Problem Draw a Newman projection along
the C2-C3 bond of the following
conformation of 2,3- dimethylbutane,
and calculate a total strain energy
29 II. Stability of Cycloalkanes
-
- The Baeyer Strain Theory
- Heat of Combustion
- The Nature of Ring Strain
- Cyclopropane An Orbital View
-
30 A. 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.
31- Angle strain is the strain introduced in a
molecule when a bond angle deviates from the
ideal tetrahedral value, 109. - Rings from 3 to 30 Cs do exist, despite Baeyers
theory.
32 B. Heat of Combustion
- Heat of Combustion (DH) is the amount of heat
released when the compound burns completely with
O2.
- The more strain energy, the higher the DH and the
less stable the alkane
33Strain Energy and Heat of Combustion
- The higher the n ( CH2), the higher the DH
- Therefore, one must compare DH/n rather than DH
n DH/n cycloalkane - DH/n reference alkane
Strain Energy of Cycloalkane
34Baeyers theory is not fully correct
- Cyclopropane and cyclobutane are strained as
predicted. - Cyclopentane is more strained than predicted.
- Cyclohexane is strain-free.
35Practice Problem Figure 4.8 shows that
cyclopropane is more strained than
cyclohexane by 115 kJ/mol. Which has
the higher heat of combustion on a
per-gram basis, cyclopropane or
cyclohexane?
36 C. The Nature of Ring Strain
- Rings larger than 3 atoms are not flat.
- They adopt puckered three-dimensional
conformations that allow bond angles to be nearly
tetrahedral - 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
37- Cyclopropane has high torsional strain (in
addition to angle strain). - This is because C-H bonds on neighboring atoms
are eclipsed.
38Summary Types of Strain
These contribute to the overall energy of a
cycloalkane
- Angle strain is caused by expansion or
compression of bond angles away from the normal
109o tetrahedral value - Torsional strain is caused by eclipsing of
bonds on neighboring atoms - Steric strain is caused by repulsive
interactions between nonbonded atoms in close
proximity
39Practice Problem Each H-H eclipsing interaction
in ethane costs about 4.0 kJ/mol. How
many such interactions are present
interactions are present in
cyclopropane? What fraction of the overall
115 kJ/mol (27.5 kcal/mol) strain energy of
cyclopropane is due to torsional
strain?
40Practice Problem cis-1,2-Dimethylcyclopropane
has a larger heat of combustion than
trans-1,2- dimethylcyclopropane. How
can you account for this difference?
Which of the two compounds is more
stable?
41 D. Cyclopropane An Orbital View
- Cyclopropane was first prepared by reaction of Na
with 1,3-dibromopropane
42Cyclopropane
- 3-membered ring must have planar structure
- It is symmetrical with CCC bond angles of 60
- All C-H bonds are eclipsed
43Bent Bonds of Cyclopropane
- Cyclopropane requires that sp3 based bonds are
bent - The orbitals cannot point directly toward each
other they overlap at a slight angle - Cyclopropane bonds are weaker and more reactive
44Bent Bonds of Cyclopropane
- Structural analysis of cyclopropane shows that
electron density of C-C bond is displaced outward
from internuclear axis
45 III. Conformations of Cycloalkanes
46 A. Cyclobutane
- Cyclobutane has less angle strain than
cyclopropane but more torsional strain because of
its larger number of ring hydrogens
Cyclopropane (115 kJ/mol strain)
Cyclobutane (110.4 kJ/mol strain)
47Cyclobutane
- Cyclobutane is slightly bent out of plane - one
carbon atom is about 25 above - The bend increases angle strain but decreases
torsional strain
48 B. Cyclopentane
- Planar cyclopentane would have no angle strain
but very high torsional strain - Actual conformations of cyclopentane are
nonplanar, reducing torsional strain - This increases angle strain.
49Cyclopentane
- Four carbon atoms are in a plane
- The fifth carbon atom is above or below the plane
looks like an envelope - Most of the Hs are nearly staggered
- This increases angle strain but decreases
torsional strain
50Practice Problem How many H-H eclipsing
interactions would be present if
cyclopentane were planar? Assuming
an energy cost of 4.0 kJ/mol or each
eclipsing interaction, how much
torsional strain would planar cyclopentane
have? How much of this strain is relieved by
puckering if the measured total strain
of cyclopentane is 26.0 kJ/mol?
51Practice Problem Two conformations
cis-1,3-Dimethylcyclobu- -tane are
shown. What is the difference
between them, and which do you think is
likely to be more stable?
52 IV. Conformations of Cyclohexanes
-
- Overview
- Axial and Equatorial Bonds in Cyclohexane
- Conformational Mobility of Cyclohexane
53 IV. Conformations of Cyclohexanes
-
- Monosubstituted Cyclohexanes
- Disubstituted Cyclohexanes
- Boat Cyclohexane
54 A. Cyclohexane Overview
- Substituted cyclohexanes occur widely in nature
- The cyclohexane ring is free of angle strain and
torsional strain
55Cyclohexane
- Cyclohexane has a chair conformation
- The conformation has alternating atoms in a
common plane and tetrahedral angles (109o)
between all carbons - All neighboring C-H bonds are staggered
- The ring is strain-free, with neither angle
strain nor torsional strain
56How to Draw Cyclohexane
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58 B. Axial and Equatorial Bonds in Cyclohexane
- The chair conformation has two kinds of positions
for substituents on the ring - axial positions and
- equatorial positions
59Chair Cyclohexane
- 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
60Axial 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
61Drawing the Axial and Equatorial Hydrogens
62 C. Conformational Mobility of Cyclohexane
- Conformational mobility Chair conformations
readily interconvert, resulting in the exchange
of axial and equatorial positions by a ring-flip
63Ring-flip
- A chair cyclohexane can be ring-flipped by
keeping the middle four carbon atoms in place
while folding the two ends in opposite
directions. - An axial substituent in one chair form becomes an
equatorial substituent in the ring-flipped chair
form, and vice-versa.
64Bromocyclohexane
- When bromocyclohexane ring-flips, the bromines
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
65Practice Problem Draw two different chair
conformations of cyclohexanol
(hydroxycyclohexane), showing all
hydrogen atoms. Identify each position as
axial or equatorial.
66Practice Problem Draw two different chair
conformations of trans-1,4-dimethylcyc
lohexane, and label all positions as
axial or equatorial.
67Practice Problem Identify each of the colored
positions red, blue, and green as
axial or equatorial. Then carry out
a ring-flip, and show the new positions
occupied by each color.
68 D. Monosubstituted Cyclohexanes
- The two conformers of a monosubstituted
cyclohexane are not equal in energy - A substituent is always more stable in an
equatorial position than in axial position
69- The equatorial conformer of methyl cyclohexane is
more stable than the axial by 7.6 kJ/mol.
70Energy 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/(Kmol)
- T is the Kelvin temperature
- K is the equilibrium constant between isomers
711,3-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
72Relationship 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
(2 x 3.8 kJ/mol)
73In general, equatorial positions give more stable
isomer
74 The exact amount of 1,3-diaxial steric strain in
a specific compound depends on the nature and
size of the substituent.
Double the value to arrive at the amount of
strain in a monosubstiuted cyclohexane
75Practice Problem How can you account for the
fact (Table 4.2) that an axial
tert-butyl substituent has much
larger 1,3-diaxial interactions than isopropyl,
but isopropyl is fairly similar to
ethyl and methyl? Use molecular
models to help with your answer.
76Practice Problem Why do you suppose an axial
cyano substituent causes practically
no 1,3-diaxial steric strain (0.4
kJ/mol). Use molecular models to
help with your answer.
77Practice Problem Look at Figure 4.18 and
estimate the percentages of axial
and equatorial conformers present at
equilibrium in bromocyclohexane
78 E. Disubstituted Cyclohexanes
- In disubstituted cyclohexanes the steric effects
of both substituents must be taken into account
in both conformations before deciding which
conformation is favored. - There are two isomers of 1,2 dimethylcyclohexane
- cis
- trans
79Conformational Analysis of 1,2-dimethylcyclohexane
80cis-1,2-Dimethylcyclohexane
- In the cis isomer, both methyl groups are on the
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
81trans-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 (gt99) in the diequatorial
conformation
82Conformational Analysis of 1-Bromo-4-t-butylcycloh
exane
- The large amount of steric strain caused by an
axial tert-butyl group holds the cyclohexane ring
in a single conformation. - This allows chemists to study chemical reactivity
of immobile cyclohexane rings.
83Axial and Equatorial Relationships among
substituents
84Practice Problem Draw the most stable chair
conformation of the
following molecules, and estimate the
amount strain in each
- trans-1-Chloro-3-methylcyclohexane
- cis-1-Ethyl-2-methylcyclohexane
- cis-1-Bromo-4-ethylcyclohexane
- cis-1-tert-Butyl-4-ethylcyclohexane
85Practice Problem Name the following compound,
identify each substituent as axial or
equatorial, and tell whether the
conformation shown is the more stable
or less stable chair form (yellow-green
Cl)
86 F. Boat Cyclohexane
- Cyclohexane can also be in a boat conformation
- It is also free of angle strain
- It is less stable than chair cyclohexane due to
steric and torsional strain - 29 kJ/mol (7.0 kcal/mol) less stable than chair
87- 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
88- Boat cyclohexane is 29 kJ/mol less stable than
chair cyclohexane. - This value is reduced to about 23 kJ/mol by
twisting slightly, thereby relieving some
torsional strain Twist boat conformation
89Practice Problem trans-1,3-Di-tert-butylcyclohe
xane is one of the few molecules that
exists largely in a twist-boat
conformation. Draw both a chair
conformation and the likely twist-boat
conformation, and then explain why the twist-
boat form is favored.
90 V. Conformations of Polycyclic Molecules
91 A. Overview
- Decalin consists of two cyclohexane rings joined
to share two carbon atoms (the bridgehead
carbons, C1 and C6) and a common bond
92Decalin has two isomeric forms cis fused or
trans fused
93- 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
94- Polycyclic compounds are common, and many
valuable substances have fused-ring structures.
95- Like decalin, norborane is a bicycloalkane.
- It has a conformationally locked boat cyclohexane
ring in which carbons 1 and 4 are joined by an
additional CH2 group.
96- Substituted norboranes, such as camphor, are
found widely in nature.
97Practice Problem Which isomer is more stable,
cis-decalin or trans-decalin?
Explain.
98Chapter 4
The End