Is there no alternative mechanism for carbocation rearrangements rather than hydride shifts? I've been killing myself over this thing for over two days, and I do believe that hydride/alkyl shifts are not only unfavourable, but unnecessary as well.
What follows might be a little weird to comprehend at the first try, but please, stay with me here...
Let us deal with one of the simplest situations we can think of, say, 1-butanol in acidic medium.
The H+ ion will interact with the lone pair in oxygen and H2O goes out. Till here we are all in the same boat. Now comes the interesting part.
We're all familiar with the classical theory, so I shall not waste time discussing that mechanism (kindly refer to the enclosed series -1 if in doubt).
I shall only point out what seems, to me, a major flaw.
When the primary carbocation is formed, it causes a number of things to happen:
(1) The electronegativity of the carbon holding the positive charge increases drastically.
(2) This increase in electronegativity causes a dissatisfaction in the adjacent carbon, increasing, in turn, the second carbon's electronegativity significantly.
As it is, carbon has a higher electronegativity than hydrogen. now, you've gone and increased it even further. Now, when the carbon(2)-hydrogen bond is cleaved, what would one expect? One would expect that the electron pair is retained by the carbon and hydrogen goes out as H+. The classical theory suggests that the opposite of this is true. How, then is that kind of bond cleavage feasible?
I say that it is not. It is my strong conviction that nature, in fact, doesn't work that way. Let us look at an alternative mechanism to this kind of a reaction.
(I request all of you to kindly assist me in finding the flaws in what I shall put forth now, for if I can think of it, so can every other person on the planet, and it is impossible that some organic chemist didn't think of it before me. Since it has not been accepted, there HAS to be a flaw. What I think has been expressed in the enclosed Series-2.)
According to the alternate theory, the carbon(2)-hydrogen bond is cleaved as one would expect it to. A comparatively stable intermediate, namely 1-butene, is formed.
There is a comparatively higher electron density in the region of the pi bond. Hence, in absence of any other option, the proton stays in the vicinity of this pi bond, due to electrostatic force of attraction.
This system, when given enough energy(via collisions with other molecules), cleaves the pi bond and the electron pair, now retained by one of the carbons (most of the time with carbon(1) since a primary carbanion is much more stable than a secondary carbanion), is given to H+, thus giving you your desired secondary carbocation.
After the formation of the secondary carbocation, the formation of 1-butene/ 2-butene is but a trivial question, which happens in the same way as what the classical theory suggests.
So... Is there any other theory as well? Is what I propose feasible? We can never know the absolute truth, of course, but how close are we to it?
PostScript: I am very much aware that what I state is not something new. It must have crossed many other minds before mine.