Introduction
This article on Newman projections will be explaining all the key information to take note of about these projections. It is important to know how to identify and understand these projections as they are highly useful for us to understand important concepts such as strain and stericity. We will also be touching on related concepts, syn-anti and cis-trans isomerism.
In our previous article, which explained 10 factors affecting a molecule’s reactivity, we noted that one of the factors was conformational strain. It is found between single bonds, but never any other type of bond (double or triple bonds), because only single bonds in a molecule are able to rotate about the bond.
An easy way to visualize this would be to take the example of butane. For butane we will focus on the center two carbons, as in 1,2-dimethylethane (note that this is an incorrect way to name butane). Each of these center two carbons will have one methyl group bonded to it.
We realize that since rotation is permitted about the single bond, it is possible for the two methyl groups to be either on the ‘same side’ of a molecule, of on ‘opposite sides’ of a molecule (Fig. 1, there is, in fact, proper nomenclature to describe this, but this will be discussed later).
Fig. 1: Methyl groups on ‘same sides’ and ‘different sides’.
If they can be either on the ‘same side’ or ‘different sides’ of a molecule, does that mean that they are different molecules? Yes, these are indeed two different molecules, and they are known as conformational isomers. This is part of a larger class of isomers, known as stereoisomers.
We realize it is not just as simple as whether the methyl groups are on the ‘same side’ or ‘different sides’. What about a 60o angle between the methyl groups? That would be difficult to draw if we simply applied skeletal formulae, because one of the methyl groups would be ‘sticking out’ awkwardly. Another solution is to use dash and wedge bonds, but those are too cumbersome to draw and it is not easy to see the interactions between the groups.
The solution would be to use Newman projections. We can demonstrate why by explaining what a Newman projection is first. Simply, instead of looking from the ‘front’ of the molecule, we look from the ‘side’, allowing the carbon atoms to overlap. The illustration below in Fig. 2 demonstrates this.
Fig. 2: Three-dimensional Newman projections.
The illustration, however, only depicts partial overlap of the carbons. The true Newman projection will have complete overlap, and this means that only one, single circle (carbon atom) is depicted, with the other covered by the adjacent carbon atom completely.
What about the methyl groups? Wouldn’t they overlap each other? Well, to prevent this from happening, the methyl groups in a Newman projection (should they be on the ‘same side’) are arranged at a slight angle relative to each other. With that, the Newman projection of butane (the carbons depicted are the center two) with the methyl groups on the ‘same side’ is shown below (Fig. 3).
Fig. 3: Actual Newman projections.
Since it may seem that it is difficult to predict the number of conformations that may form, let us begin with the simplest molecule, ethane. For ethane, since all the hydrogens are equivalent, we may predict that only two conformational isomers may form.
The two isomers will either have the hydrogens overlapping each other, or spaced out in an orderly fashion. When the hydrogens overlap each other, it is known as the eclipsed conformation (one hydrogen eclipses the other). When the hydrogens are spaced apart from each other, it is known as the staggered conformation.
A more complex case comes from disubstituted molecules, for example butane (if we consider that the center two carbons are substituted each by one methyl group) or 1,2-dichloroethane. We would expect it to have a larger number of conformations since it is no longer the case that all substituents on the carbons are equivalent.
Conformations
In fact, the case of disubstituted molecules is not really that complex at all, because there are only four possible energies that the conformations can assume. The lowest energy conformation would be the most stable one, while the highest energy conformation would be the most unstable one.
Before we begin looking into the conformations of benzene and why they are either stable or unstable, let us first take note of the nomenclature first. The distance between two groups on a molecule, one on the front and the other on the back, is known as the dihedral angle. For example, the eclipsed conformation of the 2,3-bond in butane would have a 0o CH3-CH3 dihedral angle (since the groups overlap completely).
Now, what makes a conformation stable? In the case of disubstituted molecules it is likely to be steric hindrance between the substituents on each carbon. Obviously, the position of lowest steric hindrance would have the methyl substituents as far away from each other as possible. This is known as the antiperiplanar conformation (it is also a type of staggered conformation, Fig. 5).
Fig. 5: Antiperiplanar conformation of butane.
There are also two other types of staggered conformations; these are known as the gauche conformations (Fig. 6), which possess either 60o or 300o CH3-CH3 dihedral angles. This means that one of the methyl substituents is arranged at either 60o to the left or 60o to the right of the other methyl substituent.
However, these gauche conformations do not have the same energies as the antiperiplanar conformation. Why? In the antiperiplanar conformation, there is completely no steric interaction. In the gauche conformation, although there is no steric interaction between the carbon atoms in the methyl group, there are small steric interactions (known as gauche interactions) between the hydrogens of the methyls. This makes gauche conformations around 1 kcal/mol higher in energy than the antiperiplanar conformation.
Fig. 6: Gauche conformations of butane.
As for the eclipsed conformations, there are two such examples of them. The two lowest energy eclipsed conformations are simply named eclipsed conformations (Fig. 7). There is a CH3-CH3 dihedral angle of 120o to the left or to the right (creating two such conformations). This conformation has a much higher energy than both the gauche (+3 kcal/mol) and antiperiplanar (+4 kcal/mol) conformations, because the steric interaction is between the overlapping CH3 and H substituents (as compared to H-H interactions for the gauche).
Fig. 7: Eclipsed conformations of butane.
Finally, there is the most unstable conformation, the synperiplanar conformation (Fig. 8), which features a 0o dihedral angle between the two CH3 groups (this means that they are overlapping). Obviously, this will be the greatest energy conformation, because the steric interactions are between two overlapping CH3 groups. In fact, the energy difference is around +5 kcal/mol relative to the antiperiplanar conformation.
Fig. 8: Synperiplanar conformation of butane.
Syn, Anti, Cis, Trans
In the earlier part of this article, the example of methyl groups on ‘same sides’ and ‘different sides’ were raised. Notice how the synperiplanar conformation represents butanes with methyl groups on the ‘same side’, and how the antiperiplanar conformation represents butanes with methyl groups on ‘different sides’.
That is also how we may name molecules with substituents on the ‘same side’ or ‘different sides’: syn and anti. For this to be done, however, there are a few requirements. Firstly, the carbon atoms bonded to these substituents must be chiral (otherwise there would be a lack of geometry. Secondly, the substituents cannot be hydrogens. They can, however, be different substituents.
The concept of syn and anti also bears similarities to the concept of cis and trans, which is also detailed in a previous article. However, we will again mention this concept here, because it is important for us to understand the subtle differences between syn-anti and cis-trans isomerisms. Note that both are good and valid ways to describe a molecule.
There lies a key difference between cis-trans isomerism and syn-anti isomerism. Cis-trans isomerism is used to describe double bonds in most cases, because its isomerism is derived from restricted rotation. However, syn-anti isomerism can simply be used to describe adjacent carbons with a different position of substituents.
In the case of cis-trans isomerism, since restricted rotation exists, a cis isomer cannot transition to a trans isomer (unless the pi bond is broken, which would then allow for rotation to occur). However, for syn-anti isomerism, there is no restricted rotation and thus it is possible for a transition to occur.
Let us discuss cis-trans nomenclature. If two similar substituents are on the ‘same side’ of a molecule, the alkene is cis. If the substituents are instead on different sides, the alkene is trans. However, the cis-trans nomenclature is no longer useful if all four substituents attached to the double bond carbons are different, and for that case, the recent E-Z system is employed (although the cis-trans system is still widely used).
E and Z refer to entgegen and zusammen respectively. These correspond to the english words ‘apart’ and ‘together’ (if I remember correctly), but the general meaning of these words should be correct. In E-Z nomenclature, the CIP (Cahn-Ingold-Prelog) system, which is also used to classify groups on chiral centers by priority, is employed. Simply, out of the four groups attached to each of the alkenes, the one with the atom of the highest atomic number directly bonded to the alkene carbon is the one of highest priority.
If atomic numbers are equal (i.e. the same atoms), we look to the second atom bonded to this first atom, and compare the second atoms. If the second atoms are also the same, we continue until the end of the chain. If there is no difference, the E-Z system can also be employed. If those two groups are on the same side of the alkene, it is the E isomer, while if they are on different sides, it is the Z isomer.
It is also possible for us to determine priorities if there are multiple bonds within the groups. In that case, we ‘separate’ the multiple bond into single bonds. For example, if there is a C=C bond (carbon-carbon double bond), it will be considered as two C-C bonds.
After the priority of the substituents are determined, the alkene with the two highest ranking groups on the same side of the double bond is known as the Z isomer (remember, Z means zusammen which means ‘together’), while the alkene with the two highest ranking groups on different sides of the double bond is known as the E isomer (E means entgegen, or ‘apart’).