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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