Stereochemistry of Alkanes - PowerPoint PPT Presentation

1 / 98
About This Presentation
Title:

Stereochemistry of Alkanes

Description:

Chapter 4 Introduction Stereochemistry It is the systematic study of the three-dimensional shapes of molecules and properties that arise from these shapes The three ... – PowerPoint PPT presentation

Number of Views:537
Avg rating:3.0/5.0
Slides: 99
Provided by: mSeHccsE
Category:

less

Transcript and Presenter's Notes

Title: Stereochemistry of Alkanes


1
Chapter 4
Stereochemistry of Alkanes and Cycloalkanes
2
Introduction
  • 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
  • Ethane
  • Propane
  • Butane

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

8
Representing Conformations
  • There are two representations
  • Sawhorse representation
  • Newman projection

9
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
  • 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

10
Ethanes 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.

11
Ethanes 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

12
Ethanes 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))

15
Practice Problem Make a graph of potential
energy versus angle of bond rotation
for propane, and assign values to the
energy maxima
16
Practice 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

18
Butanes Conformations
  • anti conformation is the most stable conformation
    of butane
  • It has two methyl groups 180 away from each
    other

19
Butanes Conformations
  • Rotation around the C2C3 gives eclipsed
    conformation

20
Butanes 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.

21
Butanes 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.

22
Butanes 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.

23
(No Transcript)
24
For any alkane, the most favorable conformation
is the staggered arrangement on C-C bonds and
large substituents arranged anti to one another.
25
One 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.
26
Practice 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

27
Practice Problem Sight along the C2-C3 bond of
2,3-dimethyl- -butane, and draw a
Newman projection of the most stable
conformation.
28
Practice 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

33
Strain 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
34
Baeyers theory is not fully correct
  • Cyclopropane and cyclobutane are strained as
    predicted.
  • Cyclopentane is more strained than predicted.
  • Cyclohexane is strain-free.

35
Practice 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.

38
Summary 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

39
Practice 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?
40
Practice 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

42
Cyclopropane
  • 3-membered ring must have planar structure
  • It is symmetrical with CCC bond angles of 60
  • All C-H bonds are eclipsed

43
Bent 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

44
Bent 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
  • Cyclobutane
  • Cyclopentane

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)
47
Cyclobutane
  • 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.

49
Cyclopentane
  • 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

50
Practice 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?
51
Practice 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

55
Cyclohexane
  • 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

56
How to Draw Cyclohexane
57
(No Transcript)
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

59
Chair 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

60
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

61
Drawing 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

63
Ring-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.

64
Bromocyclohexane
  • 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

65
Practice Problem Draw two different chair
conformations of cyclohexanol
(hydroxycyclohexane), showing all
hydrogen atoms. Identify each position as
axial or equatorial.
66
Practice Problem Draw two different chair
conformations of trans-1,4-dimethylcyc
lohexane, and label all positions as
axial or equatorial.
67
Practice 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.

70
Energy 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

71
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

72
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

(2 x 3.8 kJ/mol)
73
In 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
75
Practice 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.
76
Practice 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.
77
Practice 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

79
Conformational Analysis of 1,2-dimethylcyclohexane
80
cis-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

81
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 (gt99) in the diequatorial
    conformation

82
Conformational 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.

83
Axial and Equatorial Relationships among
substituents
84
Practice 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

85
Practice 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

89
Practice 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
  • Overview

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

92
Decalin 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.

97
Practice Problem Which isomer is more stable,
cis-decalin or trans-decalin?
Explain.
98
Chapter 4
The End
Write a Comment
User Comments (0)
About PowerShow.com