Conformations
Newman Projections
There are various ways to look at the conformation of a molecule. Let’s look at ethane, CH3CH3 from different angles. The first two drawings are good (from the side and the sawhorse structure). The third (end-on) is not. This is not a good way to draw an end-on version of a molecule because two of the carbon atoms down our line of sight are on top of each other. This makes the molecule look like one carbon atom with six hydrogen atoms on it. We need a better way to draw molecules end-on.
There is a better way to look at the conformations, end-on. We can use a system called Newman projections to describe what we see. Newman projections look down one carbon-carbon bond. First, let’s briefly look at how Newman projections are drawn. First, we draw a circle to represent the back carbon atom. We then draw three bonds on it evenly spaced out to represent the three groups attached to it.
We use a point to represent the front carbon atom and draw three bonds on it to represent the three groups attached to the front carbon.
So, if we are looking at ethane from his vantage point, we would make this Newman projection.
Rotation of sigma bonds
Sigma bonds can rotate. Because of this, alkanes are always twisting and spinning around. Let’s look at two of these conformers.
The eclipsed form with its bonds overlapping in the same plane is higher energy than the staggered form, which does not have overlapping bonds. The eclipsed form is not as happy. There are electrons in each C-H bond, so when they overlap, as in the eclipsed form, the negative electrons in one C-H bond repel the negative electrons of another C-H bond. In the staggered form, these negative electrons in the C-H bonds are as far away from each other as is possible. This extra energy the eclipsed form has over the staggered form is called torsional strain.
Let’s look at the Newman projections of butane, looking down the second and third carbon atoms.
As we look at the various conformations, notice that the lowest energy, best conformation, is when the large methyl groups are as far away from each other as possible. This is called the anti-conformation. The highest energy, worst conformation, is when the methyl groups are closest to each other and overlapping in the totally eclipsed form.
Newman projection comformers of butane
When two groups come close like this, it is a high-energy interaction. This bad interaction caused by large groups overlapping and interacting is called steric hindrance. Molecules will twist or rotate to avoid steric hindrance when they can.
Steric Hindrance
The eclipsed conformer is not as bad as totally eclipsed because the methyl groups are not overlapping, but the methyl groups are overlapping with hydrogen atoms, so it is still bad. Gauche (French for “left”) is better than eclipsed because the bonds are not overlapping, but the methyl groups are only 60° apart in the gauche conformer, while they are 180° apart in the lower energy anti conformer.
Energy diagram for conformers of propane
8. Draw the Newman projections for pentane to represent the view from which they are seen.
9. Label the Newman projections in the previous problem with their appropriate labels of Anti, Eclipsed, Totally Eclipsed, and Gauche.
10. Rank the Newman projections of pentane, as shown in problem 8, from lowest energy to highest energy. Circle the correct ranking below.
a < b < c < d
a < c < d < b
a < d < c < b
b < d < c < a
c < a < d < b
6.11 Review problem 8. Count the five carbons in pentane and notice that the Newman projections drawn were looking down the bond of the 2nd and 3rd carbon atoms (C2-C3). Now draw a lowest energy Newman projection looking down the C1-C2 bond.
Because of this easy rotation about sigma bonds, most molecules will often twist and turn. This twisting and turning of a molecule does not change what the molecule is. For instance, we could place a length of string down on the table, and it could look like the image on the left. We could place another piece of string, the same length, on the table, and it look like the image on the right.
In both cases, we have the exact same strings. It is only that their conformations are different. These would be called conformations or conformers of each other. These are not isomers of each other.
12. Identify each pair of compounds as conformations of each other or as structural isomers.
a)
b)
c)
d)
e)
f)
Answers
8.
9. a) Anti b) Totally eclipsed c) Gauche d) Eclipsed
10.
11.
12. Identify each pair of compounds as conformations of each other or as structural isomers.
a)
b)
d)
e)
f)
