Advices For Studying Organic Chemistry

G

G G G

H H

H H

H

H

H H

These conformers cannot be isolated except at extremely low temperatures.

4.9 CONFORMATIONAL ANALYSIS OF BUTANE

4.9A A CONFORMATIONAL ANALYSIS OF BUTANE

1. Ethane has a slight barrier to free rotation about the C—C single bond.
   
   
   1. This barrier (torsional strain) causes the potential energy of the ethane
      molecule to rise to a maximum when rotation brings the hydrogen atoms into an
      eclipsed conformation.
   
   
   
   


2. Important conformations of butane I – VI:

CH 3

H CH 3

H

H

H

HH

CH 3

CH 3 H

H

H CH^3

3 C

H H H

H

HH

H 3 CCH 3

HH

CH 3

CH 3

H H

H

H HH

CH 3

H CH 3

H

I II III IV V VI
An anti An eclipsed A gauche An eclipsed A gauche An eclipsed
conformation conformation conformation conformation conformation conformation

1) The anti conformation (I): does not have torsional strain ⇒ most stable.

2) The gauche conformations (III and V): the two methyl groups are close

enough to each other ⇒ the van der Waals forces between them are repulsive

⇒ the torsional strain is 3.8 kJ mol−^1 (0.91 kcal mol−^1 ).

3) The eclipsed conformation (II, IV, and VI): energy maxima ⇒ II, and IV

have torsional strain and van der Waals repulsions arising from the eclipsed

methyl group and hydrogen atoms; VI has the greatest energy due to the large

van der Waals repulsion force arising from the eclipsed methyl groups.