lecture 5,6 (1)
TRANSCRIPT
1
Lecture 5
(2) Geometrical isomerism
Geometrical isomerism which results from restricted rotation about double
bond or cycle e.g. 2-butene CH3CH=CHCH3 exists in two geometrical
isomers, cis-form (when the two methyl groups are on the same side) and
trans- form (when the two methyl groups are on opposite sides)
cis-2-butene trans-2-butene
Fumaric acid
COOH
Maleic acid
COOH
COOHCOOH
Fumaric acid
COOH
Maleic acid
COOH
COOHCOOH
The restricted rotation about the double bond makes it possible to isolate the
two geometrical (cis- , trans-) isomers.
Geometrical isomerism cannot exist if either carbon atoms carry identical
groups. Thus:
a a a a
dc cd
2
Geometrical isomers are diastereomers and thus possess different physical
properties.
The E-Z system of labeling alkene diastereomers:
It is difficult to use the cis-trans system for trisubstituted or
tetrasubstituted alkenes. E.g. how would we distinguish?
E Z
and
Cl
Br
CH3
H
Br
Cl
CH3
H
E Z
and
Cl
Br
CH3
H
Br
Cl
CH3
H
For such compounds the E-Z system is used: the two groups attached to each
carbon of the double bond are arranged in the order of their priorities
according to sequence rules explained before. We then take the group of
higher priority on one carbon and compare with the group of higher priority
on the other carbon. If the two groups of higher priority are on the same side
of the double bond the alkene is labeled Z (German: Zusammen = together).
If the two groups of the higher priority are on opposite sides of the double
bond, the alkene is designated E (German: Entgegen = opposite).
OH
Vitamine A
E
E,E-heptadieneE,Z-heptadiene
ZE
E
3
Geometrical isomerism in cyclic compounds:
Rings having at least two substituents at different carbon atoms give
rise to geometrical isomers. The “rigid” ring plays the same role as double
bond in alkenes: the substituents may be arranged on the same side of the
ring or on opposite sides of it.
Illustrations are:
CH3
H
CH3
H
H
CH3
CH3
H
CH3
H
CH3
H
CH3
H
H
CH3
HO2CCO2H
CO2H
CO2H
Cis
Trans
4
(3)CONFORMATIONS (ROTATIONAL
ISOMERS)
They are different forms of spatial arrangement of atoms in a molecule of a
given constitution and configuration as a result of either rotation around
single bonds or flipping or inversion of cyclohexane without affecting the
constitution or the configuration of this compound. These forms (of the same
molecule) are called conformations (or rotomers or conformer s). For
example, the molecule of ethane
(CH3-CH3) show free rotation around single bond and the Newman
projection for its conformations (eclipsed and staggered) are outlined as
such:
H H
H
H
HH
H H
H
HH
H
60°
Staggered Eclipsed
These Newman projections are obtained by viewing the molecule along the
bonding line of the two carbon atoms with the carbon atom nearer to the eye
being designated by equal space radii and the carbon atom further from the
eye by a circle with three equal space radial extensions. The rotation around
the C-C bond will change the dihedral angle (Ǿ)
Y
X
Y
X
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In the staggered (anti-periplanar) conformation, the hydrogen atoms of
ethane are as far apart as possible (with dihedral angle Ǿ = 60° or 180°), while in the eclipsed (syn-periplanar) conformation, the hydrogens are as
close together as possible (with dihedral angle Ǿ= 0°).
The difference between eclipsed and staggered conformers is in the energy
level between them (i.e.) there is an energy barrier between them. In case of
ethane the energy barrier between its conformers is much too small to the
extent that they are readily interconvertible and hence neither can be
isolated.
However, the staggered conformation is the preferred form (i.e.) its ratio
is greater than that of the eclipsed form. It should be noted that molecules in
its normal condition will exist largely in the conformation of the lowest
energy content.
etc
fully eclipsed Gauche, skewpartially eclipsed fully staggered
(anti)
ClH
Cl
Cl
Cl
HH
Cl
HH
H
H
H
Cl
H H
HH
H
Cl
H H
Cl
Hetc
fully eclipsed Gauche, skewpartially eclipsed fully staggered
(anti)
ClH
Cl
Cl
Cl
HH
Cl
HH
H
H
H
Cl
H H
HH
H
Cl
H H
Cl
H
H
HH
H
H CH3
HH
CH3CH3CH3
H
Rotation
Eclipsed
Rotation Rotation
CH3
H H
HH
CH3
Staggeredmore stable
H
CH3H
CH3H
H
partial eclipsedLess stablehigh energy
Q: Draw all conformations of butane? rotation around single bond
60 60 60
Gauche (skew)
A B C D
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In case of ethylene glycol or ethylene chlorhydrin, the most stable conformer
is the gauche (Ǿ= 60°), due to the high stabilization induced by intramolecular hydrogen bond.
O
H
H- Bond O H- Bond
HH
OH
H
Cl
H
H
H H
H
HO
H
H- Bond O H- Bond
HH
OH
H
Cl
H
H
H H
H
H
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Lecture 6
Cyclohexane
All C-C-C bond angles in the hypothetical planer form of cyclohexane
are 120° a value considerably larger than the tetrahedral angle of 109.5°. So
cyclohexane can be twisted into a number of nonplanar, or puckered,
conformations; (The chair and boat). The most stable of which is the chair
conformation in which all C-C-C bond singles are 109.5°, and C-H bonds on
adjacent carbons are staggered (gauche) with respect to one another. The
boat conformation is considerably less stable than the chair conformation
because of two factors;
1- Four sets of eclipsed hydrogen interaction along C-C bonds labeled
2-3 and 5-6
2- One set of “flagpole” interactions between hydrogens on carbon 1
and carbon 4.
The difference in potential energy between chair and boat conformations
is approximately 7 Kcal/mole that is, the interconversion of chair and
boat conformations by twisting or flipping about a C-C bonds need high
energy. This large difference in the potential energy between chair and
boat conformations means that at room temperature, chair conformation
make up more than 99.99% of the equilibrium mixture. For cyclohexane,
the two
equivalent chair
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conformations can be interconvert by twisting (flipping) first to a boat
and then to the other chair
aa
aa
ae
a
e
a
a
e
a
e
ea
ee
ee
ea
e
ae
chairChair
1
234
5 6
aa
aa
ae
a
e
a
a
e
a
e
ea
ee
ee
ea
e
ae
chairChair
1
234
5 6
Equatorial and axial bonds of cyclohexane:
When we look more closely at all the atoms constituting cyclohexane, we
see that the 12 hydrogen atoms do not occupy equivalent positions. In chair
conformation six hydrogen atoms are perpendicular to the molecular plane
and parallel to each other are called Axial bonds (a) and the other six
hydrogen atoms extend outward from the ring are called Equatorial bonds
(e). Each carbon atom of the cyclohexane ring posses’ one axial bond and
one equatorial bond directed toward opposite sides of the molecular plane.
That is, there are three axial bonds and three equatorial bonds in each side of
the ring.
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1, 3-Diaxial interactions in cyclohexane:
The chair conformation of cyclohexane is very stable, but it suffer from
little steric-repulsion (or opposition interactions) induced by the atoms or
groups present in the axial bonds in each side. This numbering do not refer
to the relationship between any two axial bonds in each side of the ring
which have the 1, 3-position i.e. 1,3 & 3,5& 5,1& 2,4& 4,6& and 6,2. That
is, groups or atoms occupy axial bonds will be suffered from
1,3- diaxial interaction. However, groups or atoms which occupy
equatorial bonds have not any interactions (since they are oriented outside
the ring which means that they are very far from any steric repulsion).
Indeed, when one chair is converted to the other ( by flipping or twisting the
ring), a change occurs in the relative orientations in space of the hydrogen
atoms attached to each carbon. A hydrogen atom Axial in one chair becomes
in the other vice versa. In non substituted cyclohexane, where the two chairs
are readily interconvertible and so they are equal energy; and each hydrogen
will be axial half of the time and equatorial the other half of the time.
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As mentioned above, the boat form of cyclohexane has very low population
(0.01%) in its equilibrium with the other two chair conformations. This
means that it has no existence but it may be a transient of flipping process.
a
a
aa
a
a
e
e
ee
e
e
a axial suffer from 1,3- diaxialinteraction that will decrease stability
if bulky groups
e equatorial with no relation between them
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Conformations of Monosubstituted Cyclohexane:
When a substituent group replaces one hydrogen atom of
cyclohexane, the difference between equatorial and axial positions can
become significant. For example, the methyl group of methylcyclohexane
rapidly interconvert between the equatorial and axial positions but is
energetically more favorable in equatorial position.
Measurements show that, at equilibrium the methyl group is 95 %
equatorial conformation
and 5 % axial conformation
It is clear that the t-butyl group [C(CH3)3], because of its large size, is
far more stable in the e-than in the a-position. Thus almost only the e-form is
present and consequently this position is “locked” alternatively, the t-butyl
group is referred to as an “anchor”, or anchoring group and the molecule is
said to be conformotionally “Biased”.