Chair and Boat Form of Cyclohexane

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Chair and Boat Form
of Cyclohexane
Outline
1 History
2 Chair Conformation
3 Boat Conformation
4 Other Comformation
5 Connection to Biochemistry
1
1. History
1.1 Baeyer Strain Theory
Chemists in the late 1800s knew that cyclic
molecules existed. But the conformations
were not known.
Baeyer suggested that cycloalkanes are flat
and he calculate the angle strain of
cycloalkanes.
2
Adolf von Baeyer
Adolf von Baeyer(18351917),born in Berlin,
Germany.
German chemist who
synthesized indigo in
1880 and formulated its
structure in 1883.
He was awarded the
Nobel Prize for
Chemistry in 1905.
Figure 1-1
Angel Strain energy(kJ/mol)
60
50
40
30
20
10
0
3
4
5
6
7
8
9
Ring Size
“Angel strain” =(109°28’-bond angel)/2
3
From Baeyer’s theory, cyclopentane is stainfree and cyclohexane has some angle stain.
But the truth is opposite. The date in next page
show that cyclopentane is more strain than
predicted, and cyclohexane is strain-free.
Strain energy(kJ/mol)
Figure 1.2
140
120
100
80
60
40
20
0
3
4
5
6
7
8
9
Ring Size
Heat of combustion per CH2 of cycloalkanes.
4
1.2 Three-dimensional
conformations
Combustion data shows that cyclohexane
is strain-free, with neither angle strain nor
torsional strain. How could it be?
The answer was first suggested in 1890 by
Hermann Sachse and later expanded on by
Ernst Mohr. Cyclohexane is not flat as what
Baeyer assumed; instead, it is puckered into
a three-dimensional conformation that
relieves all strain. The C-C-C angles of
cyclohexane can reach the strain-free
tetrahedral value if the ring adopts a chair
conformation.
5
Figure 1.3
The chair form of cyclohexane
2.Chair Conformation
6
2.1 Free Angle Strain
In cyclohexane, all bond angles are 109°28’,
and all hydrogens are staggered to each
other ,it makes the stain energy is very small.
2.2 Axial and Equatorial Bonds
In cyclohexane, two distinct types of
hydrogens are evident; one set is
perpendicular to the ring plane and the
other one is “within” the ring plane.
7
2.3 Ring-flip
Cyclohexane rings are conformationally mobile at
room temperature.
Different chair conformations readily interconvert,
resulting in the exchange of axial and equatorial
positions.
This interconversion of chair conformations usually
referred to as a ring-flip.
Ring-flip
8
2.4 Conformations of
monosubstituted cyclohexane
Although cyclohexane rings rapidly flip between
different conformations at room temperature,
the two conformers of a monosubstituted
cyclohexane aren’t equally stable.
A substituent in an equatorial position is always
more stable than it in an axial position.
9
10
2.5 Conformations of Polycyclic Molecules
What is the structure for a polycyclic
molecule? For example, decalin
2
10
1
3
9
8
4
6
7
5
11
Decalin consists of two cyclohexane rings joined
to share two carbon atoms and a common bond.
Decalin can exist in either of two isomeric forms,
depending of whether the rings are trans fused
of cis fused.
In cis-decalin, the hydrogen atoms at the
bridgehead carbons are on same face of the ring,
in trans-decalin, the bridgehead hydrogens are
on opposite faces.
H
H
=
H
H
cis-Decalin
12
H
H
=
H
H
trans-Decalin
3.Boat conformation
13
3.1 Boat cyclohexane
Boat conformation is
also free of angle
stain. But boat
cyclohexane is less
stable than chair
cyclohexane, having
both steric strain and
torsional strain.
3.2 Interconversion between Boat
Cyclohexane and Chair Cyclohexane
For the boat cyclohexane
is unstable, it can change into
chair comformation
through ring-flip.
The energy is released during
the process.
14
Figure 3.1
H
H
H
H
H
H
H
H
3.3 Position of Function Group
and the Most Stable Structure
When the function group presents in the
structure, the steric or torsional strain may
become large.
It is relatively difficult to decide the most
stable structure.
15
Figure 3.2
Cl
Cl
Cl
Cl
Cl
Cl
The three different stereoisomers
have different stabilities, the
first one is most unstable, and
the third one is most stable.
4. Other Conformations
16
4.1 Half-Chair Conformation and Twist Conformation
They are specific conformations between boat
conformation and chair conformation.
Usually we use them as intermediates in the ringflip process.
4.2.1 The Process of Ring-flip and Twist Conformation
First, the boat conformation changes into the twist
conformation in one step of ring-flip.
Ring-flip
Boat Conformation
Twist Conformation
17
4.2.2 The Process of Ring-flip and Half –chair Conformation
Then, the twist conformation changes into the
half-chair conformation through the ring-flip.
Ring-flip
Half-chair Conformtion
Twist Conformation
Finally, half-chair conformation changes into
chair conformation through the ring-flip.
Ring-flip
Half-chair Conformtion
Chair Conformtion
18
Ring-flip
Boat Conformation
Twist Conformation
Ring-flip
Half-chair Conformtion
Chair Conformtion
5.Connection to Biochemistry
19
5.1 The Chair Conformation Exists in The Nature Widely
In organic chemistry, we usually discuss the sixmembered ring which may have the chair
conformation.
But truth is that any sp3 hybridized atom
connecting to the other two atoms in the sixmembered ring can also present the chair
conformation.
5.2 The Carbohydrates and Their Classification
The carbohydrates are the most important energy
resource of living organisms.
There are three major size classes of the
carbohydrates:
Monosaccharides
Oligosaccharides
Polysaccharides
20
5.3 The structure of Glucose unit
The Glucose unit is the most important subunit
of carbohydrates. It presents in chair or boat
conformations, but usually, in the
oligosaccharides and polysaccharides it presents
in the chair conformationas for lower energy.
5.4 Starch and Glycogen:
Stored Fuels of Living Organisms
Starch contains two types of glucose polymer, amylose and amylopectin.
Amylose consists of long, unbranched chains of D-glucose units connected
by (α1→4) linkages. Amylopectin is highly branched,
Glycogen is the main storage polysaccharide of animal cells. Like amylopectin,
glycogen is a polymer of (α1→4)-linked subunits of glucose, with (α1→6)linked branches, but glycogen is more extensively branched (branches occur
every 8 to 12 residues) and more compact than starch.
21
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