Conformations of Butane

Conformations of Butane

In n-Butane, there are four extreams. A fully eclipsed conformation (syn-periplanar) and partially eclipsed conformation (anticlinal). A fully staggered conformation (antiperiplanar) and another staggered conformation (gauche or syn-clinal).


In ethane, all the staggered conformations are equivalent. However, in n-butane, there are two different staggered conformations. In one of these (synclinal) there is a gauche relationship between methyl groups while in other conformation the relationship is anti (antiperiplanar). Two groups are said to be gauche when the dihedral angle between them is 60° and in an anti conformation the groups are maximum distance apart and the dihedral angle between them being 180°. These staggered conformations are free of torsional strain however, the anti periplanar conformer is the most stable, because the methyl groups are far apart.
The methyl groups in the gauche conformations are close enough to each other, so that the van der Waals forces between them are repulsive. This repulsion causes the gauche conformations to have approximately 0.9 kcal/mol (3.8 kJ/mol) more energy than the anti conformation. The eclipsed conformations represent energy maxima in the potential energy diagram. Eclipsed conformations not only have torsional strain, they have additional van der Waals repulsions arising from either eclipsed methyl groups and hydrogen atoms or between the eclipsed methyl groups.
While it takes only about 3 kcal/mol for ethane to rotate from staggered to eclipsed conformation, it takes about 5 kcal/mol for n-butane to rotate from anti to synperiplanar conformation.
In n-butane (A-CH₂-CH₂-A or A-CH₂-CH₂-B type molecule), the anti conformation is the most stable.
If a molecule contains small electronegative atoms like fluorine and oxygen gauche form is predominant. This probably due to intramolecular hydrogen bonding.

Practice Questions

The energy required to rotate n-butane molecule about the carbon-carbon bond is called

A. Rotational energy
B. Torsional energy       ✓
C. Enantiomeric energy
D. Potential energy

The rotation about the (C2 – C3) carbon-carbon bond of n-butane requires the energy of about

A. 100 kJ/mol
B. 50 kJ/mol
C. 10 kJ/mol
D. 3 kJ/mol       ✓

Which of the following conformer of n-Butane is associated with minimum potential energy

A. Skew conformations
B. Staggered conformations       ✓
C. Eclipsed conformations
D. Gauche

Which of the following conformer of n-Butane is associated with maximum potential energy

A. Partialy eclipsed conformations
B. Staggered conformations
C. Fully eclipsed conformations       ✓
D. Gauche

Gauche conformation is less stable due to

A. Hydrogen bonding
B. Covalent bonding
C. Vander Waal's repulsion       ✓
D. Torsional strain

Which of the following is most stable conformation of 2,3-butanediol

A.
B.       ✓
C.
D.

Conformations of Ethane