Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Alkanes, Cycloalkanes, and Alkenes: Isomers, Conformations, and Stabilities
4.3 Conformational Isomers of Alkanes
At room temperature, rotation about a carbon—carbon single bond of molecules occurs freely. For some molecules that have bulky groups bonded to adjacent carbons of a carbon—carbon single covalent bond, rotation is a bit more difficult compared to molecules that have smaller groups bonded to similar adjacent carbon atoms. Owing to the restricted rotation that results around the carbon—carbon bond of such a molecule that has bulky groups, the bulky groups spend more time in specific regions of the molecule, typically as far away as possible from each other, compared to being closer to each other resulting in different conformations of the molecules. The terms conformers and rotamers are used to describe these types of isomers. By definition, conformers are isomers with the same atom connectivity but have different arrangements of specific groups about a single bond, which comes about due to rotation about a carbon—carbon single bond. The term isomeric conformer is used to describe conformers in which groups around a carbon—carbon single bond are in different locations. The illustration of this three-dimensional concept on two-dimensional paper can be challenging and specific representations must be used to convey the orientation of the different conformational isomers. There are two types of representations that are typically used to represent conformers in organic chemistry: dashed/wedge and the Newman projection.
4.3.1 Dashed/Wedge Representation of Isomers
For the dashed/wedge representation, imagine that there are four groups bonded to a central carbon and that the molecule is oriented in such a way that two groups, along with the central atom are in the same plane, i.e. the plane of the paper. Solid lines are used to represent the bonds from the central atom to the two groups in the plane. With this orientation of molecules, one bond will be in front of the paper, while the other will be in the back. A dashed line is used to represent the group that is behind the plane of the paper and a wedged line represents the group that is in front of the plane of the paper. The dashed/wedge representation is illustrated in Figure 4.1.
Figure 4.1 Illustration of the dashed-wedge representation of 2-bromobutane.
Problem 4.2
Draw a dashed-wedge representation of the following molecules.
1. 2-Chlorobutane (use carbon #2 as the central atom)
2. 2-Bromo-2-chloropentane (use carbon #2 as the central atom)
The dashed/wedge representation can be used to show not only the three-dimensional arrangement of atoms about one central carbon atom but also the three-dimensional arrangement about two carbon atoms in a molecule. A careful examination of the molecule shown below, 3,4-dimethylhexane, reveals that there are many possible representations of this molecule in three-dimensional space using the dashed-wedge representation.
A dashed-wedge representation is shown in Figure 4.2, in which the groups around the starred carbons are represented by solid, dashed, and wedged lines.
The use of a model will assist tremendously to effectively visualize the three-dimensional arrangements of the groups of the conformer shown in Figure 4.2.
The conformer shown in Figure 4.2 is only one representation of many possible conformers. Other conformers can be achieved by rotation about the starred carbon—carbon single bond as shown in Figure 4.3.
Problem 4.3
Draw dashed-wedge representations of the molecule shown in Figure 4.2 after rotation about the carbon—carbon bond by 60° and 120°, respectively.
4.3.2 Newman Representation of Conformers
The Newman projection is another representation that is used to represent the arrangements of the atoms or groups of atoms of a molecule in three-dimensional space. For the Newman projection, the molecule is viewed by looking directly down a carbon—carbon single bond. That is, one carbon will be in front (close to the viewer) and another is in the back (further from the viewer). This is illustrated in Figure 4.4, in which the viewer is looking down the carbons indicated in the figure.
Figure 4.2 A dashed-wedge representation for 3,4-dimethylhexane illustrating the three-dimensional arrangement about the two central carbons that are starred in the molecule above.
Figure 4.3 Dashed/wedge conformer that results after the rotation about the C3 and C4 single bond by 180° of the conformer of 3,4-dimethylhexane shown in Figure 4.2.
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Figure 4.4 Example of the orientation of molecules for the Newman projection.
Figure 4.5 The Newman projection of 3,4-dimethylhexane shown in Figure 4.4
The projection that the viewer sees from the perspective shown in Figure 4.4 is the Newman projection and is shown in Figure 4.5. The Newman projection is best visualized utilizing a model to fully appreciate especially this three-dimensional representation.
Problem 4.4
Give a dashed-wedged representation and its Newman representation of one isomer of CH3CH(Cl)CH(Cl)CH3 (note that carbons 2 and 3 will be the carbons that will be the central atoms).
The energy difference between conformational isomers is approximately 3 kcal mol−1 depending on the size of the groups around the bonds of the molecule. If the groups that are bonded to each other via a single covalent bond are very large however, then rotation about that carbon—carbon single bond is more difficult, compared to a molecule with smaller groups. Owing to the difficulty encountered when two bulky groups pass each other, rotation is restricted, and as a result, the energy difference between conformers is greater, compared to a molecule with smaller groups. For some molecules that have extremely large groups bonded to a single covalent bond, two conformers can be separated from each other, but only at low temperatures.
There are specific names that are used to describe different conformational isomers of molecules. The anti-conformer is the conformer that results if the two bulkiest groups of two adjacent carbons are exactly opposite to each other, or trans to each other. If the largest groups are directly behind each other, this conformer is called an eclipsed conformer. If the two bulkiest groups are not directly behind each other, the staggered conformer results. A specific staggered conformer in which the two largest groups are close to each other is called a gauche conformer. These concepts are illustrated using the Newman projections in Figure 4.6.
Problem 4.5
Using the Newman representation, give the anti, eclipsed, and gauche conformers of 2,3-dichlorobutane.
Figure 4.6 Newman projections of different conformers of 2,3-dimethylhexane.
4.3.3 Relative Energies of Conformers
Another method that is often used to demonstrate the different stabilities of the conformers of a molecule is based on their relative energies. Conformers that are most stable will have the largest group furthest from each other (the anti conformer). Thus, the most stable conformer of 1,2-dibromoethane is the anti conformer. Conformers that have large groups or atoms close to each other are the least stable conformer. Therefore, the least stable conformer of 1,2-dibromoethane is the eclipsed conformer. The relative energies for the various conformers of 1,2-dibromoethane can be represented on an energy diagram as shown in Figure 4.7.
Note that in Figure 4.7 the conformer that is highest in energy is the eclipsed conformer in which both large bromine atoms are directly behind each other (the first conformer at 0°). By rotating about the C1─C2 bond keeping the front carbon stationary and rotating only around the back carbon by 60° results in another conformer, a staggered conformer. For the conformer that results after a 60° rotation, the bromine atoms are not directly behind each other, but beside each other. This conformer is more stable than the eclipsed conformer. As a result, the staggered conformer is lower in energy than the eclipsed conformer. The rotation about the same carbon—carbon bond by another 60° results in another eclipsed conformer. For this eclipsed conformer, however, the bromine atoms are not directly behind each other, but a bromine atom and a hydrogen atom are directly behind each other. Since the hydrogen atom is smaller than the bromine atom, there is less interaction between these atoms, compared to an eclipsed conformer where the large bromine atoms are directly behind each other. As a result, this conformer is less stable than the second conformer, but more stable than the first eclipsed conformer. The next conformer, the fourth conformer, is the most stable conformer and, as a result, is shown in the lowest position on the energy diagram. For this conformer, the large bromine atoms are the furthest from each other; hence, a staggered, and specifically an anti conformer results, since the large bromine atoms are anti across from each other. The rotation of this anti conformer by another 60° results in another eclipsed conformer (the fifth conformer). This conformer is very similar to the third conformer in that the bromine and the hydrogen atoms are directly behind each other, thus they are similar in energy. Rotation of the C1─C2 bond by another 60° results in the sixth conformer, which is another staggered conformer, and is similar to the second conformer, which is also known as a gauche conformer (since the two largest groups are beside each other).
Figure 4.7 Relative energies of the various conformers of 1,2-dibromoethane obtained by rotation about the carbon—carbon bond by 60°.
Problem 4.6
Give the Newman representations of the most and least stable conformers of the following molecules.
1. 3,4-Dimethylhexane (consider rotation about C3 and C4).
2. 3-Methyl-2-pentanol (consider rotation about C2 and C3).