Question

2. Make n-butane (n- stands for “normal”, which means straight chain). Use the hydrogen atoms for this one. Looking down the C2-C3 bond examine the different conformations as the bond is rotated. Draw Newman Projections for all of the staggered and eclipsed conformations. Comment on torsional and steric strain for each conformation.

Answer 1

In n-Butane there are three energy barriers to to rotation in C2 -C3 bond and three minimum energy conformers are distinguishable.

The newman projections of the conformers are

[w is torsion angle]

d - Anti staggered ; lowest energy conformation ( bulky methyl groups are at 180⁰)

b,f - gauche; Energy difference between anti and gauche is about 3.3 KJ/mol. Due to repulsion between two methyl groups.

c, e - partially eclipsed, High energy due to repulsion between methyl and H .

energy difference between anti and partially eclipsed = 14.0 KJ/mol

a - fully eclipsed ; highest energy conformer ,.Highest energy due repulsion between two eclipsed methyl group.

Energy difference anti and fully eclipsed is 19.0 to 25.6 KJ/mol

Potential energy diagram against torsion angle is given below.