Question

In class, we discussed the conformational behavior of n-butane. In particular, we said that it could adopt two staggered conformations– gauche or anti–about its 2,3 bond. We did not talk about the conformational behavior of n-butane about its 1,2 or 3,4 bonds, because every staggered conformation about these bonds is the same. (To convince yourself of this, build a model of n-butane, look down the 1,2 bond or the 3,4 bond and rotate about it).

In contrast to n-butane, n-pentane has two central C–C bonds: the 2,3 bond and the 3,4 bond. Both of these are important for conformational analysis.

Taking both its 2,3 and 3,4 bonds into consideration,

Depict the highest energy staggered conformer of n-pentane in a very clear 3-D drawing or with a molecular model kit

Label the conformation of the 2,3 and 3,4 bonds with appropriate designations (gauche, anti...)

Annotate the image to illustrate the most significant steric interaction present in the conformer

Answer 1

Conformational analysis

n-pentane can be looked from two sides. One with 2,3 bond and another with 3,4-bond. Both are identical.

The staggered conformation for the molecule in the Newman projection is drawn below

In the above shown staggered conformation for n-pentane, the ethyl (Et) and the methyl (Me) groups are in a gauche-gauche interaction. This thus gives it a high energy value. The highest steric interaction is between the hydrogens of the -CH2 group of ethyl with the methyl hydrogens.