Need help preparing for the Organic Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about conformational isomers. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Conformational Isomers
Conformational isomers are also called conformers or rotamers. Conformational isomers differ in spatial arrangement of atoms by rotation about a single bond. This is different from most other forms of isomerism in that interconversion between conformational isomers is possible without breaking and re-forming any bonds. For this reason, conformational isomers are considered to be the same molecule, except in a different state.
A word of caution: Anomers are NOT conformational isomers, as the process of mutarotation cannot occur without the breaking and reforming of bonds.
Drawing Conformational Isomers
The making and breaking of bonds in configurational isomers provides a significant energetic barrier to interconversion - this is why R-glyceraldehyde doesn’t suddenly become S-glyceraldehyde, for instance. In conformational isomers however, the barrier between two forms is much smaller, and interconversion can happen much more readily. Barriers between states do still exist, however, and some conformers are energetically lower (more favorable) than others.
Consider the case of butane. Our conventional perspective seeks to gain a sort of structural overview of all the groups present in butane, and for this reason we pick essentially a lateral perspective and arrange all the sp3 carbons within the plane of the page. But if we want to gain some insight about the conformational behavior of butane, we need to imagine it in all 3 dimensions. A useful tool to do so is the Newman projection, a way of visualizing substituents of a molecule around an arbitrarily selected axis between two atoms. Here, we selected the two central atoms of butane as our axis. This is what we are looking down the bond between carbon 2 and carbon 3 of butane.
In a Newman projection, the group closer to the observer is presented unobstructed and it’s ligands are indicated with clear lines coming off the central atom. The group further back is represented by a large white circle with a black outline (representing the rear-ward carbon), and its ligands are in turn represented by lines that are partially obscured by the circle. The location of each substituent is then represented in its angular position relative to all others. This provides a fairly clear picture whether two substituents are in an energetically unfavorable or favorable state, without the need for drawing a more complex 3-dimensional representation.
Types and Energetics of Conformational Isomers
Now that we know how to draw diagrams that can readily represent conformational isomers, let’s take a look at what kinds of conformations we might encounter, and which ones are most and least energetically favorable. Energetic favorability will be determined largely by minimizing steric strain, the property that substituents separated by at least 4 bonds will repel each other when brought closer than their electron clouds really allow (their van der Waals radius, a concept not on the MCAT).
In the Newman projections we observe the same butane molecule in 2 different conformations and 2 transition states between conformers. Recall that methyl, the largest substituent, can be expected to have the largest radius of repulsion - and therefore we would expect the left-most arrangement, the staggered antiperiplanar conformation, to be most favorable and lowest in energy. This turns out to be true. Leaving this conformation to enter less favorable ones is however possible. In the next Newman projection in our series, we observe the eclipsed transition state. This state will exist only briefly and readily arrange itself to either the prior staggered antiperiplanar conformation, or to our next conformation on the right, staggered gauche. Finally, rotating the two methyl substituents past each other provides the largest energetic barrier - a situation called an eclipsed synperiplanar conformation. This is the least stable transition state, although ultimately full 360-degree rotation around a single bond remains possible if this energetic barrier is overcome.
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