The document discusses the conformations of ethane and butane molecules. It explains that ethane can exist in staggered or eclipsed conformations, with the staggered being more stable due to less repulsive interactions. Six conformations are possible when one methyl group rotates, with three staggered and three eclipsed. Staggered conformations have lower potential energy. Butane also has staggered and eclipsed conformations, with the anti conformation being the most stable due to the methyl groups being furthest apart.

2. Ethane

3. Conformation of Ethane
• When an ethane molecule rotates about it's carbon-
carbon single bond two extreme conformations can
be result, the staggered and eclipsed conformation.
• Here we will use Newman projection to discuss the
confirmation.
H
H
H
H H
H
H H
H
HH
H
60°
0°
Staggered Eclipsed

4. Rotation around bond
• If one of the methyl group is allowed to rotate
keeping the rest of molecule undisturbed and
number of possible arrangement can be
obtained.

5. 60° 120°
360° 300°
240°
180°
H
HH
H
H
H
HH
H
H
H
H
H
H
H
HH
H
H
H H
H
H
H
H
H
HH
H
H
H
H
H
H H
H
H
H
H H
H
H
(I)- Eclipsed
Repulsion more
Energy more
Less stable
(II) - Staggered
Repulsion less
Energy less
More stable
(III)-Eclipsed
Repulsion more
Energy more
Less stable
(VI)-Staggered
Repulsion less
Energy less
More stable
(V)-Eclipsed
Repulsion more
Energy more
Less stable
(VII)-Eclipsed
Repulsion more
Energy more
Less stable
(IV)-Staggered
Repulsion less
Energy less
More stable

6. Energy level
• By doing so six possible confirmation can be
obtained out of which 3 are staggered (II,IV,VI)
and 3 are eclipsed form (I/VII,III,V).
• Potential energy of I,III,V,VII is greater than
that of II,IV,VI due to repulsive forces.
• Stability of II,IV,VI is greater than that of
I,III,V,VII.
I , III, V, VII > II , IV , VI
II, IV, VI > I , III , V , VII

7. Energy level diagram

8. n-Butane
CH3-CH2-CH3
C – C – C – C
H
H
HH
HH H
H
H
H
1 432
Kekule structure
Skeletal structure
condensed structure

9. Possible rotation for n-Butane
C – C – CH2-CH3
H
HH
HH
1 2
H3C – C – C – CH3
HH
H H
32
Rotation around C1 and C2
Rotation around C2 and C3
Structure I Structure II *

10. Conformations of n-Butane
• Butane has three carbon-carbon single bond
and molecule can rotate about each of them .
• If the rotation will be about C2 and C3 bond
then confirmation will be symmetrical.
• Different conformation of butane can be
obtained by rotating about its middle carbon-
carbon single bond.

11. 60° 120°
360° 300°
240°
180°
H3C
H3CH3C
H3C
CH3
CH3
CH3CH3
CH3
H3C
CH3
CH3
H3C
CH3
H
HH
H
H
H H
H
H
H
H
H
HH
H
H
H
H
H
H H
H
H
H
H H
H
H
(I)-Fully Eclipsed
Very high Repulsion
Very high Energy
Least stable
(II) – Staggered(Gauche)
Moderate Repulsion
Moderate Energy
Moderately stable
(III)-Partly Eclipsed
Repulsion high
Energy high
Less stable
(IV)-Staggered (Anti)
Repulsion less
Energy less
More stable
(VII)-Fully Eclipsed
Very high Repulsion
Very high Energy
Least stable
(VI) – Staggered(Gauche)
Moderate Repulsion
Moderate Energy
Moderately stable
(V)-Partly Eclipsed
Repulsion high
Energy high
Less stable

12. Conformers of n-Butane
• Butane has 3 staggered (II,IV,VI) conformer.
• IV (Anti conformer), in which the two methyl
group are as far apart as possible is more
stable and require very less energy.
• II , VI (Gauche conformer)
• Butane has 3 eclipsed (I/VII,III,V)
• I and VII are fully eclipsed
• III and V are Partly eclipsed

13. Energy level
• Potential energy of I,VII is greater followed by
III,V and II,VI. Conformer IV require very less
energy because bulky group CH3 is far apart.
• Stability of IV is greater than that of II,VI
followed by III, V and I,VII.
I ,VII > III, V > II, VI > IV
IV > II, VI > III, V > I, VII

14. Energy level diagram

15. Conformation for n-Butane
C – C – CH2-CH3
H
HH
HH
1 2
Structure I

16. 60° 120°
360°
300°
240°
180°
H3C
H3C
H3C
CH2-CH3
CH3
CH3
CH3
H3C-H2C
CH3
H
HH
H
H
H H
H
H
H
H
H
HH
H
H
H
H
H
H H
H
H
H
H H
H
H
CH2-CH3CH2-CH3
CH2-CH3
H3C-H2C
H3C-H2C
(I)- Eclipsed
Repulsion more
Energy more
Less stable
(II) - Staggered
Repulsion less
Energy less
More stable
(III)-Eclipsed
Repulsion more
Energy more
Less stable
(IV)-Staggered
Repulsion less
Energy less
More stable
(VI)-Staggered
Repulsion less
Energy less
More stable
(V)-Eclipsed
Repulsion more
Energy more
Less stable
(VII)-Eclipsed
Repulsion more
Energy more
Less stable

17. Energy level
• By doing so six possible confirmation can be
obtained out of which 3 are staggered (II,IV,VI)
and 3 are eclipsed form (I/VII,III,V).
• Potential energy of I,III,V,VII is greater than
that of II,IV,VI due to repulsive forces.
• Stability of II,IV,VI is greater than that of
I,III,V,VII.
I , III, V, VII > II , IV , VI
II, IV, VI > I , III , V , VII

18. Thank you my dear students...!!!