Conformation of ethane and n butane

This slide includes details about conformation conformations of Ethane and n-Butane

1. Conformation of Ethane and n-Butane Lecture by S.Mathivanan ., M.Pharm Asst Professor Pharmacy

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 H H 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 H H H H H H H H H H H H H H H H H H H H H H H H H H H 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 CH 3 -CH 2 -CH 3 C – C – C – C H H H H H H H H H H 1 4 3 2 Kekule structure Skeletal structure condensed structure

9. Possible rotation for n-Butane C – C – CH 2 -CH 3 H H H H H 1 2 H 3 C – C – C – CH 3 H H H H 3 2 Rotation around C 1 and C 2 Rotation around C 2 and C 3 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 C 2 and C 3 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° H 3 C H 3 C H 3 C H 3 C CH 3 CH 3 CH 3 CH 3 CH 3 H 3 C CH 3 CH 3 H 3 C CH 3 H H H H H H H H H H H H H H 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 CH 3 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 – CH 2 -CH 3 H H H H H 1 2 Structure I

16. 60° 120° 360° 300° 240° 180° H 3 C H 3 C H 3 C CH 2 -CH 3 CH 3 CH 3 CH 3 H 3 C-H 2 C CH 3 H H H H H H H H H H H H H H H H H H H H H H H H H H H H CH 2 -CH 3 CH 2 -CH 3 CH 2 -CH 3 H 3 C-H 2 C H 3 C-H 2 C (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...!!!