A Happy New Year to you all. In my last post of 2013, I gave a problem that provides part of the plot of my ebook novel Athene's Prophecy: how could a Roman prove the heliocentric theory? I shall give the answer in due course, but in the interim I commented on another post that I would start a discussion on how to get an idea so here goes. I should mention that I intend to follow the procedure of my first ebook, Elements of Theory 1, and the example I am going to use, that of the 2-norbornyl cation, had a chapter in that devoted to it, and I suggested an answer. The example has gone on through my chemical career: the non-classical carbenium ion.
 
What is the first step in having an idea? In my view, identifying a reason to have one. If you are satisfied that all is well, your brain will not devote time to the problem of what if it is not well. So, let me start on this non-classical ion. In Chemistry World (August, p 20, and January 2014, p 26) we see that a German group had isolated the ion and found that it was symmetrical. As Chemistry World put it, "Case closed!" Or is it? Recall that in "The Hitch-hiker's Guide to the Galaxy" the final answer was given, but what was the question? My first point is, if you accept the "Case Closed" situation, you will never have a contrary idea because you are not looking for one. The first step is to recognize you need it. This must be closely followed by the asking of questions of what you know.
 
The original question regarding the non-classical 2-norbornyl ion was clear: why did the exo 2-norbornyl derivatives solvolyse much more rapidly than the endo derivatives? Accordingly, the first question is, is this symmetrical ion pertinent to the original question? It is reasonably obvious that Chemistry World thinks so, but let us ask a further question:  if the activated state is the fully developed carbenium ion, then how does exo and endo give dramatically different rates of solvolysis, because the substituent is now lost? If the activated states for exo and endo derivatives are different (which they must be to get different reaction rates, unless our concept of reaction rates is entirely wrong), then in what way, and why? The why would appear easiest: in the activated state the anion has yet to fully leave. Another question: how can a species on an energy maximum be isolated and live long enough to have its nmr spectrum measured? The answer to that question, surely, is that the carbenium ion must be in an energy well, not at an energy maximum. If so, under standard activation theory, the energy maximum is before the fully developed ion forms, in accord with the previous conclusion that the anion is still present.
 
Thye next question is, was there any pertinent evidence to question whether the activated state was a partially developed symmetric ion? The answer is, yes there is. In the symmetric ion, C1 has an equal exposure to the positive charge, but Brown had shown by substitution that in the activated state there was no particular positive charge at that site, and that was his biggest point against the "non-classical ion". Now, back to the question, how to have an idea? The activated state is now defined as having the leaving group to have partially left. What does that mean? Surely there is a significant dipole between C2 and the leaving group, along the bond axis. The partial positive charge is located at C2, not at C1 (by substitution data) with C6 unclear at this point. The next question is, what mechanism can conceivably stabilize this system and not be available to the endo substituent? That requires you to think about all the possibilities available, and list them. There are not that many. The answer to that, in my view answers, the question, and the case is not closed by the existence of a symmetric ion, as previously claimed. That does not mean the determination of the symmetric ion is wrong, but rather that while it exists, it does not actually answer the original question.