In this post, conformers are back in town to spin the carbon bonds around.  

We know that a compound with single carbon-carbon bond is free to undergo rotation about the bond due to the flexibility of the sigma bond between the carbons. The different arrangements of the compound are called conformations, while the relatively more stable ones are called conformers, short for conformational isomers. 

You must be wondering what I mean by ‘relatively stable’ earlier. A ‘relatively stable’ conformations have a lower potential energy compared to their neighboring conformations. For the more mathematically inclined, this simply means the potential energy of the conformer is a local minimum amongst the adjacent conformations [1]. 

The potential energy of each conformation is determined by three types of strains, which is essentially the deviation of the ideal geometry. The first type, steric strain, occurs when two relatively large substituent groups are too near to each other. This causes the electron clouds to repel each other, increasing the potential energy. The second type, torsional strain, occurs when two substituent groups repel each other as they pass by each other due to rotation around a bond. Steric strain and torsional strain do sounds very similar, but a point to note is that torsional strain is typically minimum in the gauche configuration and maximum in the staggered configuration. The last type is angular strain which occurs when the bond angle is forced to deviate from the ideal angle. In the case of an sp3 carbon, anything more or less than 109.5° will cause an angular strain. 

Before we go into the discussion, let’s learn some of the terms we will be using. You may realize that in the textbooks, most of the bonds are drawn in 15° or 30° increments, but that is rarely the case in real molecules with all sorts of intramolecular and intermolecular interaction going on [2]. Hence, we divide the possible dihedral angles into different regions and give them some name to facilitate discussions.

This is a very nice illustration that helps to summarize the entire terminology. First, look whether the bond is within ±90° of each other. If yes, it has the prefix ‘syn–‘. Else, it has the prefix ‘anti–‘. Next, if the lines drawn through each bond are within ±30° of each other when viewed in Newman projection, then it has the suffix ‘-periplanar’. Otherwise, it has the suffix ‘-clinal’. For those of you who prefer tables, here is another version of the summary.

You might have also heard of the term gauche. This is a very specific term referring to the dihedral angle of 60°. However, it is often used quite interchangeably with the term synclinal.

Back to the main discussion on potential energy, we will first start with an example on butane. 

In conformation C (anticlinal), we can see than the molecule is experiencing a fair bit of torsional strain. In conformation D (synperiplanar) this is made worse with additional steric strain from the two methyl groups being very near to each other. No matter how you rotate the methyl group about carbon 1 or 4, they will still repel each other. Conformations C and D are known as the eclipsed conformations since the substituent groups and/or hydrogen are right behind one another. In general, eclipsed conformations are not very stable. 

In conformations A (antiperiplanar) and B (synclinal a.k.a. gauche), there is not much torsional strain since the groups are staggered, hence the name staggered conformations. However, since the methyl groups are closer to each other in conformation B, there are more steric strain, causing it to be less stable than A. Yet, conformation B is still a local minimum in terms of potential energy. As a result, conformations A and B are conformers of butane, with A being the most preferred conformation. In general, staggered conformations are more stable than eclipsed conformations. 

You may notice that we have not used angular strain at all. This is because in an open chain alkane, the bonds stay pretty close to the sp3 ideal of 109.5°, but this is not the case for cyclic compounds. Let’s take a look at cyclohexane. 

It is apparent that cyclohexane carbons are all sp3 hybridized. Hence, the flat ring with bond angles of 120° between the carbons is not it’s most ideal form. In order to get close to the ideal 109.5°, the bonds rotated to give the chair conformation shown in A with bond angles of 111°. This is the most stable conformer of cyclohexane, with minimal angle, torsional and steric strain. 

However, in conformations C (boat) and D (half-chair), the partially flattened out ring causes the bond angle to deviate significantly from 109.5° as well as introduces both steric and torsional strain as hydrogens end up right behind or too near to one another. Hence, these are unstable conformations of cyclohexane. 

In conformation B (twist-boat), cyclohexane is slightly less unstable than the boat conformation, but there is still quite a fair bit of angular, torsional and steric strain. However, in order to reach the chair conformation from the boat conformation, cyclohexane will have to transit through the twist-boat and half-chair conformations. Some of the molecules do not have sufficient energy for those transitions and will remain in the twist-boat conformer. 

For more complicated compounds, the thinking is pretty much the same as that of butane and cyclohexane. Ask yourself about how the bonds could rotate to minimize the three strains. 

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References

1. Burrows, A., Holman, J., Parsons, A., Piling, G., and Price, G. (2017) Isomerism and stereochemistry, in Chemistry³: introducing inorganic, organic and physical chemistry 3rd ed. essay Oxford University Press Oxford. 
2. Hardinger, S. Illustrated Glossary of Organic Chemistry – Periplanar, coplanar, anti-periplanar, syn-periplanar. Organic Chemistry at UCLA.