Post on 31-Jan-2016
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BITSPilani, Pilani Campus
BITS Pilani Pilani Campus
Lecture-31
CONFORMATIONS
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CONFORMATIONS
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Two methyl groups in ethane are not fixed in a single position.
They rotate about the sigma bond connecting two carbon atoms,
maintaining the linear bonding overlap.
The different arrangements formed by rotation about single
bond are called conformations and a specific conformation is
called a conformer.
Since conformations differ from each other in the way their
atoms are oriented in space, they are stereoisomers.
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Conformations
Conformations are represented by two kinds of formulas
(1) Sawhorse formula & (2) Newman projection formula
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Newman projection sawhorse
Dihedral angle (θ): Angle between the C-H bond on the front
carbon atom and C-H bond on the back carbon atom in
Newman projection.
Eclipsed conformation: (θ= 00)
Staggered conformation: (θ= 600)
Skew conformation: (θ= anything else)
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Eclipsed Staggered Skew
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Sawhorse
Newman
CONFORMATIONS
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CONFORMATIONS
Conformational analysis: study of energetics of different
conformations.
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Potential energy of the molecule:
Minimum for the staggered conformation
Increases with rotation
Maximum at the eclipsed conformation
Torsional Strain: It is the resistance to twisting (torsion) as
molecule rotates toward an eclipsed conformation.
The energy difference between staggered and eclipsed
conformers is called torsional energy.
At any given moment all molecule do not exist in most stable
conformation; rather a higher percentage of molecule is present
in a more stable conformation than any other arrangement.
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CONFORMATIONAL ANALYSIS
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For ethane molecule torsional energy is about 12.6 kJ/mol
For propane molecule torsional energy is about 13.8 kJ/mol
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• Other factors such as van der Waals attraction or repulsion,
dipole-dipole interaction, hydrogen bonding etc also affect
the stability of conformations.
• But tendency for the bond orbitals on adjacent carbons to
be staggered remains, and any rotation away from the
staggered conformation is accompanied by torsional strain.
• Increase in torsional strain (from 3.0 kcal/mol to 3.3
kcal/mol) in propane is due to more bulky methyl group.
CONFORMATIONAL ANALYSIS
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3.0
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• Highest energy has methyl groups eclipsed.
• Lowest energy has methyl groups anti.
• In between, staggered methyl-gauche & methyl-H eclipsed.
Free rotation around C2-C3 bond in n-butane
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CONFORMATIONAL ANALYSIS
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• Anti conformation is lowest in energy.
• “Straight chain” actually is zigzag.
• A staggered conformation with two larger groups 180° from
each other is called anti.
• A staggered conformation with two larger groups 60° from
each other is called gauche.
• In most of the cases staggered conformations are lower in
energy than the eclipsed conformations.
• The relative energies of the individual staggered
conformations depend on their steric strain.
• Steric strain is an increase in energy resulting when atoms are
forced too close to one another.
• Gauche conformations are generally higher in energy than
anti conformations because of steric strain.
C
H CC
CC
H H H H
H H
H H
HH H
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CONFORMATIONAL ANALYSIS
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Hydrogen bonds have a locking
effect on the conformation
about the C-C bonds.
Anomeric effect in conformation
Preference for gauche conformations about the bond C–R in the system
X–C–R–C where X and R are heteroatoms having nonbonding electron
pairs, commonly at least one of which is nitrogen, oxygen, sulfur or
fluorine (the generalized anomeric effect).
In this case the determining factor is the interaction of the nonbonding
electron pair of the heteroatom with the electron deficient orbital of C-
heteroatom bond.
Hydrogen bonding affect in conformation
CONFORMATIONAL ANALYSIS
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Conformation of amide bonds
The rate of conformational changes can be further lowered by
involvement of the NH group in hydrogen bonding with
neighbouring molecules. e.g. alpha-helix, beta-sheet of protein
CONFORMATIONAL ANALYSIS
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CONFORMATIONS OF CYCLOALKANES
In cyclopropane or cyclobutane, one pair of bonds to each
carbon cannot assume the tetrahedral angle, but must be
compressed to 600 or 900 to fit the geometry of the ring.
These deviations of bond angles from the “normal”
tetrahedral value cause the molecule to be strained, and
hence to be unstable compared with molecules in which the
bond angles are tetrahedral.
An increase in energy when bond angles deviate from the
optimum tetrahedral angle of 109.5° is called angle strain.
Cycloalkanes with more than three carbon atoms in the ring
are not flat molecules. They are puckered to reduce strain.
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Cyclopropane
Angle strain
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Torsional strain because of eclipsed hydrogens
Ring Strain = Angle strain + Torsional strain