Need help preparing for the Organic Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about chair conformation for isomers. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
The Problem with Conventional Projections
Visual representations of compounds are often chosen with convenience in mind rather than fidelity in conveying information about the structure of these compounds. Our habit of representing cyclohexane rings and similar structures in the same hexagonal arrangement as benzene rings, and the conventions of the Haworth projection, are two instances where this disparity between the actual geometry of the molecule and the projection are most evident. Consider a cyclohexane ring represented as a planar hexagon with substituents coming in and out of the page, and the Haworth projection of glucose. Any 2 neighboring carbons of cyclohexane must have hydrogen substituents that are repelling each other by being effectively ‘lined up’, rather than staggered. The effect must be even more pronounced if the hydroxyl substituents of glucose were arranged as is implied - the hydroxyl substituents of carbon 1 and 2 would be repelling rather significantly. If we were to draw Newman projections for these cases, we would find eclipsed conformations in these cases. All this has to lead us to believe that our conventional projections simply don’t represent the true geometry of 6-membered sp3 hybridized rings well at all.
Chair Conformations: Representing Tetrahedral Geometry Accurately
To find a more sensible projection, we need to reason about the geometry of sp3 hybridized atoms. We know that they should have bond angles close to 109.5° and that their electronic geometry should be tetrahedra. So, we need to for starters reject any approach that suggests 120° bond angles, such as hexagons, and instead we need to look at non-planar shapes.
This is where the chair conformation comes in handy - it is one of several somewhat stable arrangements for a six-membered ring of sp3 hybridized atoms. It is not the only possible non-planar arrangement that satisfies our requirements: Boat and twisted-boat arrangements exist too, but these are not explicitly tested on the MCAT. Of the possible solutions, the chair conformation is the most energetically favorable by far.
Picture a sample chair conformation for both cyclohexane and glucose. Imagine substituents now staggered, a situation that is far more favorable than eclipsing each-other. However, it is not entirely favorable yet. Potential for a certain type of repulsive interaction still exists. There are the two types of substituents: axial substituents and equatorial ones.
Whenever large, bulky substituents are present - such as methyl or hydroxyl groups, are present there is a potential for another kind of interaction within the chair conformation. If one of these larger substituents is in an axial position, they are will be relatively close to other axial substituents. This gives rise to the potential for these larger substituents to still have repulsive interactions with other axial substituents. We call these 1,3-diaxial interactions, because they occur across substituents separated by a single additional member of the ring. If the larger substituents were instead positioned equatorially, no such problem would exist, and both the molecule in question and the conformation would be more stable.
Luckily, they can enter this more stable state because chair conformations can flip without breaking any bonds. And as one would expect, they will go through more energetically expensive and unfavorable transition states to do so - but they can ultimately reach another chair conformation in which all prior axial substituents are now equatorial, and all prior equatorial substituents are now axial. This is called a chair flip. The most stable chair conformation is always the one with all large substituents equatorial, or if that is not possible, as many of the larger substituents in the equatorial position as is possible.
A good piece of strategic advice is that when you are presented with some very similar looking arrangements of cyclic molecules with potential for these kinds of interactions or shown a set of similar-looking chair conformations, you will want to consider diaxial interactions and make sure all the largest substituents are positioned equatorially. On the MCAT, you are most likely to encounter this in the context of carbohydrate chemistry.
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