Chemical Thoughts

Two things that have been revealed to me tonight:

1) Emily was a valedictorian in her school! OMG! That makes her the second person I know, after Mingmei, who had been valedictorians in high school, and who I had competed with to beat the curve in Chem 4a. You know, the funny thing is, when the Prof. asked all the valedictorians back in high school to raise their hands, about 1/4 of the lecture hall did. I thought they were bluffing. I have been proven wrong tonight.

Question is, why does the CoC out of everything attract these valedictorians? I never thought I’d see such a high concentration of them in one room.

2) What your professor never told you – (and the main purpose of my post) HYPERCONJUGATION. Aha, envisioning Frechet saying this in his French accent right now. Anyway, I have always been intrigued about this phenomenon, ever since my SLC tutor, Kathy, labeled it as “bs”. Well finally tonight I decided to take a closer look, since I stumbled upon its wiki article. And I was shocked.

This part is what we already know:

An instance where hyperconjugation may be overlooked as a possible chemical explanation is in rationalizing the rotational barrier of ethane. It had been accepted as early as the 1930’s that the staggered conformations of ethane were more stable than the eclipsed. Wilson had proven that the energy barrier between any pair of eclipsed and staggered conformations was approximately 3 kcal/mol, and the generally accepted rationale for this was the unfavorable steric interactions between hydrogen atoms.

But then, the surprise:

In their 2001 paper, however, Pophristic and Goodman[3] revealed that this explanation may be too simplistic.[5] …. These results [from the study; not copied/pasted here for brevity] demonstrate that Coulombic forces [i.e. steric effects] do not explain the favored staggered conformations, despite the fact that central bond stretching decreases electrostatic interactions.[3]

I really wish I had total access to the (3) and (5) articles, but this is truly crazy that, even to this day, the basics of organic chemistry are still being refined. Upshot? Vollhardt needs to update his textbook for the 7th edition!

(Disclaimer: I may have advertised this as our professors lying, but I’m obviously being exaggerative. The hyperconjugation wiki article shows a diagram of the “accepted” view of hyperconjugation and steric effects on eclipsed vs. staggered conformations below the lines containing what I quoted above.)


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