In organic chemistry, you have to understand how the conformation is done. This is because organic compounds have a three-dimensional structure, not a flat surface. By studying stereochemistry, we can often predict whether or not a chemical reaction will take place.
Also, even for the same molecule, the conformation of the same molecule will be different. Depending on where the substituents are located, they have different energies.
One of the first things you will learn in stereochemistry is the conformation of alkanes. Alkane stereochemistry uses the Newman projection. The Newman projection is used to describe the state of molecules by focusing on the rotation of the axes.
By using Newman projection, we can understand the conformation of alkanes. In this section, we will explain the steric structure of alkanes, which is the first step in stereochemistry.
Table of Contents
- 1 Relationship between Fischer and Newman projection
- 2 For Single-Bond (σ-Bond), the Axis Is Rotated and the Dihedral Angle Is Considered
- 3 In Butane, the Stable Structure Differs Between Anti and Gauche Forms
- 4 Figuring Out the Energy in Stereochemistry
Relationship between Fischer and Newman projection
When writing the structural formula of an organic compound, there are several methods available. Among them, the Fischer projection is often used.
Fischer projection is used to describe the structure of a molecule in two dimensions, even in the presence of chiral carbons.
When there is a chiral center, a technique often used is dashed-wedge notation. Although the molecule is marked in the plane, a dashed line means that the substituents are in the back. In wedge-shape notation, the substituents are in the foreground. However, this is not the case, and the Fischer projection is to write them all in the plane.
-Fischer Projection Does Not Show the Conformation
However, the Fisher projection has the disadvantage that the conformation of the molecule cannot be determined at all. For this reason, the Newman projection is often used when considering stereochemistry.
The Newman projection is used to show the positions of substituents in the same molecule. For example, we have the following.
Even if you can’t understand it in Fisher projection, you will be able to understand the conformation of the molecule in Newman projection.
Considering the Alkane Conformation in the Newman Projection
It is impossible to understand how alkanes work in stereochemistry without using the Newman Projection. So how can we use the Newman Projection? First, let’s think of the Fischer projection in three dimensions.
All molecules are three-dimensional. The Newman projection is one of the methods to think of molecular structures in three dimensions. Another characteristic of Newman projection is that the molecule is viewed from the side.
For example, the Newman projection for ethane looks like the following.
The Newman Projection is used to check what substituents are present around the C-C bond (carbon-to-carbon bonds) at the center.
At the center of the Newman projection, two carbon atoms are overlapped on each other. The Newman projection is what it looks like from the side of the axis. Understanding this method of description will help you to understand how the energy differences are caused by the conformation of the molecule.
For Single-Bond (σ-Bond), the Axis Is Rotated and the Dihedral Angle Is Considered
Why do we need to consider the Newman projection? This is because in a single bond (σ bond) the axis is free to rotate. For example, in ethane, the axis of the C-C bond rotates as shown below.
In a double or triple bond, there is no rotation of the axis. Therefore, we don’t write Newman projection for compounds with double bonds. The Newman projection is used only to study the conformation of a compound due to the rotation of the axis, and there is no point in using the Newman projection for a double or triple bond without rotation of the axis.
In a single bond where the axes are rotated, we need to consider the dihedral angle. Looking at the molecule from the side of the axis, as in Newman projection, can produce strain due to the dihedral angle.
The ideal angle (θ: theta) for a dihedral angle is 60°. In some cases, however, the dihedral angle can be zero degrees. Since the axis rotates freely, the angle of the dihedral angle varies from situation to situation. The Newman Projection makes it easier to understand what the dihedral angle strain is.
Think of the Newman Projection as a tool to understand what dihedral angle strain looks like.
Staggered Conformation Is a More Stable Structure Than the Eclipsed Conformation
What kind of strain is produced at the dihedral angle by the rotation of the molecular axis? Let’s consider the example of ethane.
Let us consider the case in which the rotation of the C-C bond in ethane results in a dihedral angle (θ) of 60° and a case in which the dihedral angle (θ) is 0°. In this case, the case where the dihedral angle is 60° is called staggered conformation. On the other hand, when the dihedral angle is 0°, it is called eclipsed conformation.
The rotation of the axis can take two forms: staggered conformation and eclipsed conformation. Incidentally, about the eclipsed conformation, it actually overlaps perfectly. However, if you write them superimposed in Newman projection, you can’t see the substituents bound to the carbon atoms behind them, so they are usually described with a slight shift like this.
In this case, which is more stable, the staggered conformation or the eclipsed conformation? Molecules are more stable in the staggered conformation than in the eclipsed conformation.
Rotation of the Axis Causes Differences in Potential Energy
Repulsion of electrons is involved in the reason for instability in the eclipsed conformation. By sharing electrons, molecules form covalent bonds. When they do so, electrons with a negative charge repel.
In the eclipsed conformation, the electrons involved in the bond repel each other, making it unstable. If the dihedral angle is zero degrees, the energy is greater because of the strain produced.
So, although the axis can rotate freely, the energy in the molecule is higher or lower depending on whether the dihedral angle is 0° or 60°.
When the dihedral angle is 0°, the potential energy is high. On the other hand, when the dihedral angle is 60°, the potential energy is low. The energy diagram is as follows.
If you represent the dihedral angle in the Newman projection, you should understand that the potential energy varies with the angle, as shown here.
In Butane, the Stable Structure Differs Between Anti and Gauche Forms
What about compounds that have more substituents than ethane? Butane is frequently used as an example of this.
Propane is a potential energy diagram of the same form as ethane. Therefore, both ethane and propane have the same energy difference due to their conformation. Butane, on the other hand, has a more complex stable structure.
When writing the structure of butane in the Newman projection, there are two ways to consider in the staggered conformation.
Among the staggered conformation, one can write a structure in which the methyl group is in the 180° position on one side. This conformation is called anti form.
On the other hand, one can also write a Newman projection with the methyl group at 60°. This conformation is called the gauche form.
Is there any difference in the potential energy between the anti-form and the gauche-form? For this, the potential energy is higher in the gauche form. The reason for this is that the methyl group is nearby and is sterically repulsive.
When you consider stability in stereochemistry, you have to check what the steric hindrance is.
In the anti forms, the methyl groups are at 180 degrees, so there is no steric hindrance. On the other hand, in the gauche form, the methyl groups are at 60° to each other and are sterically repelled. Therefore, the strain in the gauche form is higher than in the antique form, and the energy is higher. The anti form is more stable than the gauche form.
Staggered and Eclipsed Have Different Energies in the Substituents
The same is true for the eclipsed conformation as well as the staggered conformation configurations. In the same way that the energy of the staggered conformations differs depending on the position of the methyl group in the staggered conformations, the energy state of the eclipsed conformations differs depending on the location of the substituent.
In the eclipsed conformation, we can consider the following two conformations.
When considering steric hindrance, a form with overlapping methyl groups has a very high energy state and is unstable. Methyl groups are much larger in shape than hydrogen atoms. When the large substituent group is closest to each other, the energy state is high.
On the other hand, when the methyl groups are separated from each other in the eclipsed conformation, the energy state is lower. Of course, since the bonds are overlapping, the energy state is still high and unstable. However, the energy state is more stable than the state where the methyl groups overlap each other.
Potential Energy Diagram of Alkanes That Varies with the Position of the Methyl Group
When we consider the Newman projection for butane, what does the potential energy look like as the C-C axis (single bond of carbon) rotates? The stable (or unstable) state can be shown in the energy diagram as follows.
The potential energy of butane varies in this way depending on what kind of conformation it is in.
In the staggered conformation, the energy is lower and more stable. However, as shown in the above figure, when the methyl group is in the gauche form, the energy is slightly higher. On the other hand, the energy is the lowest and most stable when the methyl group is in the anti form because there is no steric hindrance.
In the eclipsed conformation, the energy is most unstable where the methyl groups overlap. In contrast, when the methyl groups do not overlap, the energy is slightly lower.
Figuring Out the Energy in Stereochemistry
In most cases, in organic chemistry, the structural formula is written in a planar form. For example, by using the Fischer projection, one can write the structural formula in a plane, even if there are stereoisomers.
However, all molecules are three-dimensional. Also, the axis of a single bond can rotate freely, although this is not relevant for double or triple bonds. Therefore, the energy possessed by the molecule differs depending on the conformation of the molecule. In such cases, the Newman projection is used to make it easier to understand.
The first two types of conformations to be considered in the Newman Projection are eclipsed and staggered. The ideal angle of the dihedral angle is 60°; any deviation from this angle will result in high energy and instability.
Not only that, but if the molecule has more substituents, understand that there are anti form and gauche form. Once you learn about these different energies, you will be able to understand in more detail what shape the molecule is in. You will also be able to guess how easy it is for synthetic reactions to occur.