I was consulting Wade and I came across this problem and solution:

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While I agree with the solution manual, I can't help but wonder if my proposed mechanism is correct.

I had the water attack the hydrogens attached to the alkene. I knew that alkenes are slightly electron-withdrawing and this makes the hydrogens an appealing target for a base.

Once the hydrogens were cleaved, I had the remaining pair of electrons attack the carbocation. Is this a valid pathway? Valid as in plausible? Would you accept this mechanism on a test?

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The main problem I see with this mechanism is that yield is exceedingly low as sp2 carbons are not especially acidic, and water is a poor base. On the other hand the original problem does specify that this product is minor. But then, it appears that I chose the more minor of two minor pathways.

  • $\begingroup$ Not a definitive answer, but one problem I see with that mechanism is that it entails a concerted, bimolecular process, which should really be much less probable than any given unimolecular process of comparable energy and leading to the same product. $\endgroup$ – Greg E. Aug 3 '14 at 17:02
  • $\begingroup$ I'm also skeptical that the required configuration of molecular orbitals in the transition state is reasonable. I'm imagining your mechanism would require the electrons of the base flowing into a $\sigma^{*}_{CH}$ antibonding orbital, and I think this would really require the base (and the electrofugal proton) to be roughly coplanar with and on the same side as the newly forming $\ce{C-C}$ bond if it were all to happen concertedly. I could certainly be mistaken, but that doesn't seem at all plausible to me. $\endgroup$ – Greg E. Aug 3 '14 at 17:39
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    $\begingroup$ No, this is a serious mistake, look at the pKa of alkenes. Don't take it in a bad way, it's part of the learning process. What is the main product of the reaction? A pyran or just dehydration? $\endgroup$ – K_P Aug 3 '14 at 20:55
  • $\begingroup$ Acidity of the alkene was mentioned in my OP $\endgroup$ – Dissenter Aug 3 '14 at 21:41

Greg E.'s comments sum up the flaws with your version of the mechanism. While yours contains the same steps, it combines the nucleophilic attach and the proton transfer in a single step. Unless you have experimental evidence for a bimolecular step, the unimolecular step is much more likely. In this case, the unimolecular step is a 5-endo-dig cyclization, which is particularly fast. The alkene nucleophile is already present when the carbocation forms. The rate of the deprotonation is diffusion controlled, as we have to wait for a water molecule to find it.

Also, your logic relating the minority of the product to the unlikeliness of the mechanism is inherently flawed. A minor product does not occur at a lower proportion because the mechanism contains less valid steps. Instead, the distribution of products is related to the relative rates of different mechanisms and the relative energies of the products. That product is a minor product because some other product either forms faster or is more stable or both.

If this was on my exam, and I like the problem so I might use something like it, I would dock you a few points for combining steps that are not likely concerted into a concerted step. For example, if I decided this mechanism is worth 10 points, then you might receive 7 or 8.

Final note: Remember that experimental evidence trumps all else in science.

Regardless of how intuitively and aesthetically unusual your mechanism appears, if experiments support it, then it becomes a more valid mechanism. As an example of some of the non-intuitive things that are in "accepted" mechanisms, check out halonium ions and sigmatropic rearrangements, especially the one in the Baeyer-Villager Oxidation. Evidence in support of your mechanism might include a failure to observe or trap that final carbocation, a demonstration that the rate depends on the amount of water (since final deprotonation steps tend to be very fast), and others.


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