Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
1
CONFORMATIONS
Dr. Mishu Singh
Chemistry Department
Maharana Paratap Govt. P.G College
Hardoi.
Conformations
The infinite number of arrangements of the atoms or groups of a
molecule in three dimentional space which are interconvertible into
each other by rotation about single bond are called Conformations or
Rotational Isomers or simply Rotamers.
These conformers have different internal dimensions (atom-to-atom
distances, dihedral angles, dipole moment etc.)
.
The energy barrier for rotation of carbon-carbon single bonds
(conversion of different spatial arrangements) is normally small, < 0.6
kcal/mol and >16 kcal/mol.
2
Rotation about Carbon–Carbon Bonds
3
Newman & Sawhorse Projections
4
5
Staggered conformation:
A conformation about a carbon-carbon single
bond in which the atoms or groups on one
carbon are as far apart as possible from the
atoms or groups on an adjacent carbon
H
H H
H H
H
6
Eclipsed conformation:
A conformation about a carbon-carbon single bond
in which the atoms or groups of atoms on one
carbon are as close as possible to the atoms or
groups of atoms on an adjacent carbon
H
H H
H
HH
7
•
Eclipsed conformation Staggered conformation
• Each hydrogen on one
carbon as close as
possible to one
hydrogen on the other
carbon
• Hydrogen on one carbon
as far from the hydrogen
from other carbon
A Staggered conformation is more stable than an
eclipsed conformation
8
Types of Strain
Steric - Destabilization due to the repulsion between the electron clouds of atoms or groups. Groups try to occupy some common space.
Torsional - Destabilization due to the repulsion between pairs of bonds caused by the electrostatic repulsion of the electrons in the bonds. Groups are eclipsed.
Angle - Destabilisation due to distortion of a bond angle from it's optimum value caused by the electrostatic repulsion of the electrons in the bonds. e.g. cyclopropane
9
Torsional strain
Also called eclipsed interaction strain.
Strain that results from eclipsed bonds.
Strain that arises when non-bonded atoms/groups,
separated by three bonds are forced from a staggered
conformation to an eclipsed conformation.
The torsional strain between eclipsed and staggered
ethane is approximately 12.6 kJ (3.0 kcal)/mol
+12.6 kJ/mol
10
60o Rotation Causes Torsional or
Eclipsing Strain
11
Dihedral angle (Ɵ)
The angle created by two intersecting planes
12
Conformers of Alkanes
Structures resulting from the free rotation of a C-C
single bond
May differ in energy. The lowest-energy conformer
is most prevalent.
Molecules constantly rotate through all the possible
conformations.
13
Conformations of Ethane
• Staggered conformer has lowest energy.
• Dihedral angle = 600
H
H
H
H
H H
Newman projection Sawhorse Projection
14
Rotational Conformations of Ethane
15
16
Ethane as a function of dihedral angle
17
18
The origin of torsional strain in ethane: Originally thought to be caused by repulsion between eclipsed
hydrogen nuclei
Alternatively, caused by repulsion between electron clouds of eclipsed C-H bonds
Theoretical molecular orbital calculations suggest that the energy difference is not caused by destabilization of the eclipsed conformation but rather by stabilization of the staggered conformation
This stabilization arises from the small donor-acceptor interaction between a C-H bonding MO of one carbon and the C-H antibonding MO on an adjacent carbon; this stabilization is lost when a staggered conformation is converted to an eclipsed conformation
Anti - Description given to two substitutents attached to
adjacent atoms when their bonds are at 180o with respect to
each other.
Syn - Description given to two substitutents attached to
adjacent atoms when their bonds are at 0o with respect to each
other.
Gauche - Description given to two substitutents attached to
adjacent atoms when their bonds are at 60o with respect to
each other.
CH3
CH3
anti
CH3
CH3
gauche
CH3CH3
eclipsed
0o
180o
60o
20
Conformations of Propane
21
Conformations of Butane
25
2 Different Eclipsed Conformations
26
27
Butane has Steric and Torsional
strain when Eclipsed
The totally eclipsed conformation is higher in energy because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion.
2 | 28
Three valleys (staggered forms) 120 apart; Three hills (eclipsed) 120 apart.
Extra slide
30
Draw staggered and eclipsed conformers of
1-Chloropropane?
Draw the Rotational profile of 2-methylbutane about
C2-C3.
Eclipsed Structures:
Me
H
Me
H
Me
H
This was the
high energy
staggered
structure, 180 0
Me
H
Me
H
H
MeMe
H
Me
H
Me
H
1200 2400 1800
Me
H
Me
Me
H
HMe
H
Me
Me
H
H
00 3600
Now relative energies…..
Me
H
Me
H
Me
H
Me
H
Me
Me
H
H
Me
H
Me
H
H
Me
1200 600 3000
Staggered Structures:
33
Conformations
in
Cycloalkane
Stability of Cycloalkanes: 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
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
36
Summary: Types of Strain
Angle strain - expansion or compression of bond angles away
from most stable
Torsional strain - eclipsing of bonds on neighboring
atoms/gps
Steric strain - repulsive interactions between nonbonded
atoms in close proximity
Cyclopentane
38
angle strain: the C-C-C bond angles are
compressed from 109.5° to 60°
torsional strain: there are 6 sets of eclipsed
hydrogen interactions
strain energy is about 116 kJ (27.7 kcal)/mol
39
Cyclobutane
The ring strain of a planar cyclobutane results from two factors:
1.angle strain from the compressing of the bond angles to 90° rather than the tetrahedral angle of 109.5°
2. torsional strain from eclipsing of the bonds.
Internal bond angle ~88o (~21o deviated from the normal
109.5o tetrahedral angle)
Cyclobutane ring is not planar but is slightly folded. It is
slightly bent out of plane - one C atom is about 25°
above.
If cyclobutane ring were planar, the angle strain would be
somewhat less (the internal angles would be 90o instead
of 88o), but torsional strain would be considerably larger
because all eight C–H bonds would be eclipsed
puckering from planar cyclobutane reduces torsional
strain but increases angle strain
the conformation of minimum energy is a puckered
“butterfly” conformation
strain energy is about 110 kJ (26.3 kcal)/mol
42
Cyclopentane
Planar cyclopentane would have no angle strain but very high torsional strain
Actual conformations of cyclopentane are nonplanar, reducing torsional strain. Puckering from planar cyclopentane reduces torsional strain, but increases angle stain
Four carbon atoms are in a plane
The fifth carbon atom is above or below the plane – looks like an envelope
the conformation of minimum energy is a
puckered “envelope” conformation
strain energy is about 42 kJ (6.5 kcal)/mol
Measuring Strain in Cycloalkanes
Heats of combustion can be used to compare stabilities of
alkanes & cycloalkanes.
Heats of combustion increase as the number
of carbon atoms increase.
Therefore, divide heat of combustion by number
of C’s and compare heats of combustion
on a "per CH2 group" basis.
45
CnH2n + O2 n CO2 + (n+1) H2O + heat cycloalkane (can be measured)
Total Strain
Energy =
Sample
ΔHcomb per -CH2- _
Reference
ΔHcomb per -CH2- • n
Heats of Combustion of Cycloalkane: the more strained a compound is, the more is the heat released upon combustion
Cycloalkane
Cyclopropane
Cyclobutane
Cyclopentane
Cyclohexane
Cycloheptane
Cyclooctane
Cyclononane
Cyclodecane
Cyclohexadecane
Alkane reference
Ring Size (n)
3
4
5
6
7
8
9
10
16
Hcomb KJ/mol
2091
2721
3291
3920
4599
5267
5933
6587
10466
Hcomb per CH2- KJ/mol
697
681
658
654
657
658
659
659
654
654
Total Strain Energy
129
108
20
0
21
32
45
45
0
0
strained rings
commonrings
mediumrings
large rings (> 12)
(43)
(27)
(4)
(0)
(3)
(4)
(5)
(5)
(0)
According to Baeyer, cyclopentane should have less
angle strain than cyclohexane.
Cyclopentane 3,291 658
Cyclohexane 3,920 653
The heat of combustion per CH2 group is less for
cyclohexane than for cyclopentane. Therefore,
cyclohexane has less strain and more stable than
cyclopentane.
Heat of combustion suggests that angle strain
is unimportant in cyclohexane.
Tetrahedral bond angles require nonplanar
geometries.
Conformations of Cyclohexane
Cyclohexane is by far the most common
cycloalkane in nature and also in organic
chemistry.
The cyclohexane ring is free of angle strain and
torsional strain. Zero ring strain implies the bond
angles must be 109.5°. (no angle strain) and also
no eclipsing interactions between the C-H bonds
(no torsional strain).
48
Cyclohexane adopts a puckered structure. A planar arrangement of the six methylene groups in cyclohexane
does not give a tetrahedral shape for every carbon atom - this is
achieved by puckering the ring. Cyclohexane does this by
adopting mainly two conformations the CHAIR and the BOAT.
49
Chair Conformation
Most stable conformation. Each carbon is in the
staggered conformation
All the bond angles are 109.5° and all the C-H bonds
are staggered. (Zero ring strain) .
More stable than a boat conformation by 27 kJ (6.5
kcal)/mol.
50
Boat Conformation
51
52
The boat is just a chair with the footrest flipped up.
C-1, C-4 are bent toward each other.
Four sets of eclipsed C-H interactions & one
flagpole interaction
This also has bond angles of 109.5° and thus avoids
any angle strain, but there is torsional strain.
The two hydrogens at the ends of the boat are in
close contact, causing torsional strain. These flagpole
hydrogens are eclipsed.
53
Twist-boat conformation To avoid these unfavorable interactions, the boat
conformation skews slightly, giving a twist boat
conformation. The twist boat conformation has a
lower energy than the pure boat conformation, but is
not as stable as the chair conformations
approximately 41.8 kJ (5.5 kcal)/mol less stable
than a chair conformation
approximately 6.3 kJ (1.5 kcal)/mol more stable
than a boat conformation
Half-chair
Half-chair
Skew boat
Half-chair
Skew-boat
45
kJ/mol
45
kJ/mol
23
kJ/mol
59
The chair is the lowest energy conformation, although since
the energy barrier to ring flip is fairly small, there will always
be some other conformations present.
The half chair is the point of highest energy, and is not a
stable conformation.
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
61
• 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
How to Draw Cyclohexane
Step 1: Draw two parallel lines slanted
downward
Step 2: Draw two lines starting from the
parallel lines slanting upward
and intersecting at a point.
Step 3: Draw two lines downward
starting from the other end of
the parallel lines and intersecting
at another point.
63
How to make Axial bonds and
Equatorial bonds
64
Chair–Chair Interconversion/
Ring Flip
An most important phenomenon in chair
conversion is that any substituent that is axial in
the original conformation becomes equatorial in
the new conformation (exchange of axial and
equatorial positions by a ring-flip )
65
All axial bonds become equatorial
All equatorial bonds become axial
All “up” bonds stay up
All “down” bonds stay down
66
Example:
Axial-up becomes Equatorial-up
67
Equatorial Conformation is Preferred……WHY????
A Conformational Analysis of Methyl cyclohexane
Substituted cyclohexane
• Exists in two different chair forms
H
G
HG
69
Axial Methyl in Methylcyclohexane
70
Equatorial Methyl Group
71
Cyclohexane ring rapidly flips between chair conformations at room temp.
Two conformations of monosubstituted cyclohexane aren’t equally stable.
The equatorial conformer of methyl cyclohexane is more stable than the axial by 7.6 kJ/mol
72
1,3-Diaxial Interaction
5% 95%
Van der Waals/ steric repulsions between axial
substituents on a cycloalkane ring
73
The axial substituent interferes with the axial
hydrogens on C1 and C3. This interference is called
a 1,3-diaxial interaction.
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
. Difference between axial and equatorial conformers
is due to steric strain caused by 1,3-diaxial
interactions
74
Tert-butylcyclohexane
Substituents are less crowded in the equatorial
positions.
Mono substituted Cyclohexane
Less than 0.01% Greater than 99.99%
40% 60%
Crowding is less pronounced with a "small"
substituent such as fluorine.
Size of substituent is related to its branching.
Fluorocyclohexane
F
F
Keq = [equatorial conformer]/[axial conformer]
• The larger the substituent on a cyclohexane ring, the
more the equatorial substituted conformer will be
favored
77
Substituent Axial – equatorial energy
difference kJ mol-1
% equatorial
H 0 50
OMe 2.5 73
Me 7.3 95
Et 7.5 95
iPr 9.3 98
tBu >20 >99.9
110 11.7 99
Substituted cyclohexanes:energy difference
78
Chapter 4
Disubstitued Cycloalkanes Can exist as pairs of cis-trans stereoisomers
– Cis: groups on same side of ring
– Trans: groups on opposite side of ring
80
Cis-1,3-dimethylcyclohexane
Cis-1,3-dimethylcyclohexane can have both methyl
groups in axial positions or both in equatorial positions.
The conformation with both methyl groups being
equatorial is more stable. However, both conformations
are equal in energy.
81
82
Trans-1,3-dimethylcyclohexane
Both conformations have one axial and one equatorial
methyl group so they have the same energy.
Methyl groups are on opposite faces of the ring 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
both methyl groups equatorial
•no 1,3-diaxial interactions
•both methyl groups axial
• four 1,3-diaxial interactions
CH3
ring
flipH3C
CH3H3C
H3C
H3C CH3
(more stablebecause largegroup isequatorial)
(less stablebecause largegroup isaxial)
CH3
Trans-1-tert-Butyl-3-methylcyclohexane
2 | 84
Cis-1,3-Disubstituted Cyclohexanes
ring
flip
(more stable)
CH3
H
CH3
H
CH3CH3
H H
(less stable)
2 | 85
Trans-1,2-Disubstituted Cyclohexanes
ring
flip
trans-1,2-Dimethylcyclohexane
CH3
CH3(eq)
(ax)
(ax)
(eq)
CH3
CH3
diequatorial(much more stable)
diaxial(much less stable)
Cis-1,4-Disubstituted Cyclohexanes
H
HH
H3C
CH3 CH3
HCH3
ring
flip
Equatorial-axial Axial-equatorial
chair-chair
CH3
CH3
ring
flipH3C
CH3H3C
H3C
H3C CH3
(more stablebecause largegroup isequatorial)
(less stablebecause largegroup isaxial)
Cis-1-tert-Butyl-4-methylcyclohexane
89
Cis-1,4-ditertbutylcyclohexane
The most stable conformation of cis-1,4-di-
tertbutylcyclohexane is the twist boat. Both chair
conformations require one of the bulky t-butyl groups
to occupy an axial position.
90
cis 1,3-Dimethylcyclohexane
91
trans 1,3-Dimethylcyclohexane
92
cis 1-Chloro-4-t-butylcyclohexane
CH3
ring
flipCH3
CH3CH3
cis-1,2-Dimethylcyclohexane(equal energy and equallypopulated conformations)
(equatorial-axial) (axial-equatorial)
(eq)
(ax)
(eq)
(ax)
Cis-1,2-Disubstituted Cyclohexane
94
Cyclohexane Stereochemistry
Cis -Trans Isomers
Position cis trans
1,2 e,a or a,e e,e or a,a
1,3
1,4
a = axial; e = equatorial
e,a or a,e e,e or a,a
e,e or a,a a,e or e,a
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
97
Trans-fused cyclohexane ring is more stable than cis-
fused cyclohexane ring
99
Problems
• Is this the most stable conformer?
101
Problem- 1
A. Draw both chair conformations of cis-1,2- dimethylcyclohexane, and determine which conformer is more stable?
B. Repeat for the trans isomer.
C. Predict which isomer (cis or trans) is more stable.
102
A. There are two possible chair conformations for the
cis isomer, and these two conformations interconvert
at room temperature. Each of these conformations
places one methyl group axial and one equatorial,
giving them the same energy.
103
B. There are two chair conformations of the trans isomer
that interconvert at room temperature. Both methyl
groups are axial in one, and both are equatorial in the
other. The diequatorial conformation is more stable
because neither methyl group occupies the more
hindered axial position.
104
C. The trans isomer is more stable. The most stable
conformation of the trans isomer is diequatorial and
therefore about 7.6 kJ/mol (1.8 kcal/mol) lower in
energy than either conformation of the cis isomer,
each having one methyl axial and one equatorial.
Remember that cis and trans are distinct isomers and
cannot interconvert.