Tabla de Contenidos
Conformers, or conformational isomers, are a class of isomers that differ from each other only by rotation around a single covalent bond. This means that they are molecules with the same molecular formula (since they are isomers), with the same connectivity between all their atoms, and that they can be converted into one another by means of a simple rotation around a bond.
The conformers correspond to the most stable conformations that a molecule can assume. This means that they are those conformations that correspond to potential energy minima, which is why the molecule has more opportunities to stay in that conformation.
The rotational energy barrier
Interconversion from one conformer to another is governed by a rotational energy barrier which must be overcome in order to fall to a new potential energy minimum (ie, to a new stable conformation or conformer).
If the energy barrier to rotation is very low, then the molecule will spin freely around the bond and will not stay in any particular conformation. This happens when the groups attached to the carbon atoms are very small. However, when the substituent groups on the chain are considerably large or bulky, a large steric hindrance is generated that does not allow rotation about the axis. In these cases there is the possibility that there are conformers stable enough even to be isolated at room temperature.
Conformational isomers and Newman projections
Rotation around a chemical bond is measured by the dihedral angle, which is the angle between two planes formed by the two bond carbons and a substituent on each carbon. The easiest way to clearly observe the dihedral angle is by means of Newman projections.
A Newman projection consists of looking at the relative position of the substituents when the molecule is observed along the bond that we want to analyze. When you look at the molecule this way, there are three groups attached to the nearest carbon that point toward us and three that point toward the back of the molecule, away from us.
When we analyze a bond using Newman projections, it is easy to see that as we rotate the bond, the molecule goes through different conformations, in some of which all groups are eclipsed. These conformations are the least stable, so they do not represent conformers.
On the other hand, those conformations in which the substituent groups remain forming angles of approximately 60º with respect to each other are the least hindered, so they correspond to the most stable conformations, that is, to the different conformers.
Types and examples of conformational isomers
The conformers can be very varied, but there are some that are more common than others. In the case of butane and similar molecules, two different types of conformers can be distinguished depending on the dihedral angle between the largest groups (methyls, in the case of butane).
Gauche conformation
When the angle between the two methyls of butane is approximately 60° in either direction, these conformational isomers are called Gauche conformation. Both correspond to energetic minima compared to the eclipsed conformations (angles of 0º and 120º), but they are not the most stable conformations, since there is another one in which they are further away.
Anti Conformation
When the dihedral angle is 180º, both bulky groups are as far apart as possible, occupying opposite positions with respect to each other. This is the most stable conformer and is called the anti conformation.
The conformers of cyclohexane
The cycles do not have full rotational freedom due to the presence of a closed carbon chain. However, they do have some rotational freedom which allows them to assume different stable conformations. In the case of 6-carbon cycles (cyclohexane and substituted cyclohexanes), there are a couple of conformations that are much more stable than all the others, and are equivalent to the stable conformations of linear alkanes.
The saddle conformation
The chair conformation is the most stable of all the 6-carbon ring conformations. Looking at the Newman projection along the “flat” part of the chair, it can be seen that this conformation is similar to the Gauche conformation of linear alkanes, forming angles of approx. 60º.
When a cyclohexane assumes the chair conformation, both the carbons and hydrogens lie as far apart as possible. There are two possible chair conformations in which, alternatively, 6 hydrogens or substituent groups remain in axial positions (aligned with the axis of symmetry of the cycle that passes through its center), while the other 6 remain in equatorial positions, pointing away from the cycle. These are the preferred positions of the large substituents where they are present.
The twisted boat conformation
This is another relatively stable conformation that corresponds to a deformed Gauche conformation in which the carbon atoms are not eclipsed, but also not in Gauche position. This causes the ring to twist in one of two possible directions.
For obvious reasons, a 6-carbon ring cannot assume an anti conformation between its carbons without breaking the bonds of the cycle.
References
http://www.ehu.eus/biomoleculas/moleculas/confor.htm
