Butane has a molecular formula of C4H10.
The four carbons are connected by C-C single bonds. Its line-angle
structure is shown below on the left, with its "live" structure
showing to the right. The two "live" structures are viewed
from the top and the left side of the line-angle structure respectively.
Again the rotation around C2-C3 creates an infinite number
of conformations. The most interesting ones are shown below:
Now rotate each conformation so that you are looking
along the C2-C3 (i.e. you will only see C2 carbon since C3 is directly
behind C2). Note that all three conformations are derived from C2-C3
rotation of the "live" molecule displayed above on the right
side. After you examine these conformations, test yourself on the
following questions:
a) What are the terminologies given to each of the above
three conformations? (gauche, total eclipse, anti?)
b) Rank order the relative energy level of the three
conformers. For the two conformers with higher energy, what factor(s)
is(are) responsible for the extra energy? (torsional strain, steric
hindrance?)
c) One of above three has the lowest energy of all butane
conformations. Can you provide a reason why this is true?
d) Draw on a piece of paper the structure of each of
the above conformations, using solid line for , wedged-line and dashed-lines.
