By James Ashenhurst
Levels of Mastery
Last updated: March 27th, 2019
Here’s an idea I’ve been playing with: levels of mastery. Growing up with video games, it’s an idea that is intuitively familiar, but not often applied in an academic setting. What happens when you start taking specific concepts/skills in organic chemistry and start assigning to them a level of difficulty, where each new level depends not only on the skills from the previous level but in some cases will require applying concepts from different aspects of the course?
As a thought experiment I wondered how this might apply to a pretty central concept in Org 1 – the cyclohexane chair conformation. Cyclohexane chairs are used to teach conformational analysis, but they also demonstrate the huge effects that conformations can have on chemical reactivity.
(These are purely my own subjective assessments of the skills involved, by the way – other chemists will most certainly differ).
Level zero is the starting point: being able to recognize cyclohexane and to understand wedge/dash notation.
Level one would be the proper drawing of a cyclohexane chair skeleton. Getting the skeleton drawn right is a skill in itself, apart from the separate task of drawing in the substituents, which would be level two. I say that there is a different gradation involved in putting on the substituents because many students can draw a perfectly fine cyclohexane chair skeleton while drawing the substituents wrong, and if the substituents are drawn wrong it will be impossible to solve the problems in the subsequent levels. Level two also includes being able to draw a chair flip with a single substituent.
Level three is recognizing cis and trans on a cyclohexane ring and understanding that “up” substituents can be equatorial as well as axial. It’s important to realize that a chair flip does not turn a “wedge” into a “dash” but instead turns an “axial” into an “equatorial”. Hence, the relative orientation (cis vs. trans) is not changed by a chair flip.
From level four onwards, the difficulty is increased as concepts are included from conformational analysis and chemical reactivity.
Level four is seldom practiced but would be the skill of drawing Newman projections along any C-C bond of the cyclohexane ring.
Level five is progressively more difficult as it involves not only transcribing a flat cyclohexane drawing to the chair form (and doing a chair flip) but also making judgements about the relative energies of each chair. This requires additional information (in the form of a provided table of energies) that can allow for a quantitative assessment of which conformation is most favored. For instance the difference in energy between axial and equatorial conformers of 1-t-butylcyclohexane is approximately 20 kJ/mol (4.9 kcal/mol), which represents a huge energetic preference.
Very common testing ground for midterms here.
Finally, level six incorporates these concepts with those from stereoselective reactions such as the SN2 and E2 reactions. The E2 reaction requires that the leaving group and hydrogen are oriented anti ( 180°) to each other. For practical purposes this means that you can only do the E2 in cases where the leaving group is axial. Likewise, the SN2 reaction requires overlap of the antibonding orbital 180° from the leaving group with an incoming nucleophile, which is only accessible when the C-X bond is axial (when it’s equatorial the antibonding orbital points into the ring, making it inaccessible to attack.
These are also extremely common exam type questions. Why? Because they combine concepts from stereochemistry, chemical reactivity, and conformational analysis all in one compact little package.
In the interest of space it might be good to stop here but there are definitely more levels beyond this. I could sketch them out here:
7) drawing and predicting structures of decalins (fused cyclohexanes) and substituted decalins
8) addition of electrophiles to cyclohexenes in the presence of a chiral center (predicting facial selectivity)
9) Drawing out boat conformations for a given cyclohexane
10) Predicting the products from ring opening reactions on a cyclohexane (the Furst-Plattner rule).
Obscure for sophomore organic chemistry? Possibly – but they do come up in some of the more notoriously difficult organic chemistry courses.
The purpose of doing problems, by the way, is at least partially to be able to ascend in level of mastery, from being able to draw simple cyclohexane structures to finally gaining an appreciation of how structure impacts chemical reactivity.
What do you think? Am I missing anything?