The document discusses the conformational analysis of n-butane. It defines conformations as different spatial arrangements of a molecule generated by rotation about single bonds. It then discusses various types of isomers including constitutional and stereoisomers. It explains concepts like Newman and sawhorse projections that are used to represent conformations. Finally, it analyzes the different conformations of n-butane including anti, eclipsed and gauche structures and discusses their relative energies.

1. Topic:-
conformational analysis of n-
BUTANE
Made by:-
Sophia Mubashir
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2. 2

3. Isomerism
‣Isomers are non-identical compounds with the same molecular
formula.
Types
‣The two main classes of isomers are called structural isomers and
stereoisomers.
‣Structural (constitutional) isomers
‣Stereoisomers
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4. ISOMERS
Compounds with the
same molecular formula
Conformational
Isomers
rotation about
single bonds
with chiral centers
Stereoisomers
Meso
Compounds
Enantiomers
Constitutional
Isomers
Cis,Trans
(E,Z) Isomers
(can be called
diastereomers)
Conformations
rotation
restricted
different
connectivity
Diastereomers
stereocenters
but no chiral centers
Enantiomers
one chiral center
m ore than
one chiral center
chiralachiral
not mirror
images
mirror
images
Atropisomers
same
connectivity
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5. Conformations or Conformers or Rotamers:-
‣Different spatial arrangements of a molecule that are generated
by rotation about single bonds.
Conformational analysis :-
‣Conformational analysis is the study of the effect of
rotation on the properties of a molecule.
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6. Andiron or Sawhorse
Sawhorse Projections are very similar to Newman Projections, but are used
more often because the carbon-carbon bond that is compressed in a
Newman Projection is fully drawn out in a Sawhorse Projection.
Newman Projection
The molecule is viewed from front to back in the direction of bond linking
the two carbon. The bond joining the two carbons is hidden. ). The front
carbon atom is called proximal, while the back atom is called distal
Representation:-
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7. Sawhorse Projection
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8. Newman Projection
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9. Conformations
 Torsional strain
◦ also called eclipsed interaction strain
◦ strain that arises when nonbonded atoms 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
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10. Conformations
 Steric strain (nonbonded interaction strain):
◦ the strain that arises when atoms separated by four or
more bonds are forced closer to each other than their
atomic (contact) radii will allow
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11.  Angle strain:
◦ strain that arises when a bond angle is either
compressed or expanded compared to its optimal
value
 The total of all types of strain can be calculated
by molecular mechanics programs
◦ such calculations can determine the lowest energy
arrangement of atoms in a given conformation, a
process called energy minimization
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12. Conformations
 Dihedral angle the angle created by two
intersecting planes
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13. Anti Butane
 Energy-minimized anti conformation
◦ the C-C-C bond angle is 111.9° and all H-C-H bond
angles are between 107.4 and 107.9°
◦ the calculated strain is 9.2 kJ (2.2 kcal)/mol
CH3
H H
H H
CH3
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14. Eclipsed Butane
◦ calculated energy difference between (a) the non-
energy-minimized and (b) the energy-minimized
eclipsed conformations is 5.6 kJ (0.86 kcal)/mol
H
H H
H
H
H
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15. C C
CH3
H3C
H
H
H
H
HH
CH3
H H
H3C
Staggered: anti
C C
CH3
H
H3C
H
H
H
CH3H
H
H H
H3C
Staggered: gauche
3 KJ/mol
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16. Butane Conformers C2-C3
 Highest energy has methyl groups eclipsed.
 Steric hindrance
 Dihedral angle = 0 degrees
totally eclipsed
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17. Butane Conformers (2)
 Lowest energy has methyl groups anti.
 Dihedral angle = 180 degrees
anti
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18. Butane Conformers (3)
 Methyl groups eclipsed with hydrogen
 Higher energy than staggered conformer
 Dihedral angle = 120 degrees
eclipsed
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19. Butane Conformers (4)
 Gauche, staggered conformer
 Methyl closer than in anti conformer
 Dihedral angle = 60 degrees
gauche
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20. Conformational Analysis
=>
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21. E2 is an anti-elimination. They are stereo specific. The hydrogen and
the halogen must be on opposite sides of the molecule before the E2
elimination can take place. This makes sense as both the base and
the leaving group are negatively charged. Therefore they would try to
be as far apart as possible. In addition, the leaving group is large and
there is more room for the removal of the adjacent proton if it is on
the opposite side from the leaving group.
Reactions
If the anti-arrangement is not possible, syn-arrangement may take
place.
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22. Mechanism = elimination, bimolecular E2
100% anti-elimination!
base:
C
X
C
H
C C + H:base + :X
RDS
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