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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.

The Classical Theory

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.)

An Alternative Approach

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.

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    $\begingroup$ Please do not propose primary carbocations as intermediates without extreme reaction conditions. On top of that, two cations in a single species is even worse. $\endgroup$ – Zhe Nov 8 '16 at 17:45
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    $\begingroup$ In defence of the classical mechanism: obviously the carbon with the full positive charge is hungrier for electrons. So it tugs on the bond pair of an adjacent carbon-hydrogen bond. Now both the carbons in a tug of war, the carbon more desparate for the electrons is secondary carbon (or what your example wants). The hydrogen, with its electron pair, tags along onto the other carbon, and the rearrangement is complete. $\endgroup$ – FreezingFire Nov 8 '16 at 18:16
  • $\begingroup$ For the protonation you are proposing with butan-1-ol, I would assume $\mathrm{E2}$ elimination to generate but-1-ene, which can later be protonated to give the but-2-yl cation (and which can then undergo semi-$\mathrm{E1}$ to give but-2-ene). That would be a better choice than proposing primary carbocations. $\endgroup$ – Jan Nov 8 '16 at 18:20
  • $\begingroup$ @FreezingFire, that's JUST what I'm saying. The roguish carbon(1) would take away the bond pair of carbon(2)-H and make a C-C pi bond. Why is the hydride justified? $\endgroup$ – Shekhar Upadhyay Nov 9 '16 at 2:34
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    $\begingroup$ Series 1 has an intermediate with two +'s... $\endgroup$ – Zhe Nov 9 '16 at 14:49
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Wagner-Meerwein rearrangement of a proton
Scheme 1: Orbital model of a Wagner-Meerwein rearrangement.

You may think you are proposing something new, but you aren’t.

While these Wagner-Meerwein rearrangements are typically referred to as ‘hydride shifts’, that name should not be taken too literally (as in your first scheme). Instead, it is a concerted orbital interaction. There is never any hydrogen cation or hydride anion formed, hydrogen remains in the vicinity. The entire process is concerted and thus proceeds suprafacially.

Writing the central structure as a double bond with proton in the vicinity, as you did, or as ‘two cations coordinating a hydride anion’ is really only a formality. It is not an intermediate in any way, it is a transition state.

Being a transition state, by the way, means that it is a directed process. If the hydrogen went half way, it is going to proceed on (law of inertia, if you wish) and not go back. However, given a sufficiently stabilised cationic system, it may reverse at a later moment in time.

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  • $\begingroup$ I must admit that I'm in the dark as regards the semi E1 mechanism... And I'm not able to comprehend the inertia thingy. An Alkene is definitely much more stable than a carbocation, and a primary one at that. So, unless otherwise agitated, why would it want to do anything on its own? And about the actual mechanism for the Wagner Meerwein rearrangement, I admit I didn't know that. I'm just a fresher, who had come into this field of study around 3 months ago. It hadn't been taught to us as yet. Thanks for your help. By the way, there wasn't a double carbocation anywhere in what I thought. $\endgroup$ – Shekhar Upadhyay Nov 9 '16 at 2:48
  • $\begingroup$ I'm really sorry if I gave an impression that I wanted to propose something new. I know it must have crossed many people's minds, and there had to be an error, otherwise it would've been the generally accepted theory. $\endgroup$ – Shekhar Upadhyay Nov 9 '16 at 3:18
  • $\begingroup$ About the Wagner Meerwein rearrangement: Is an alkene not much more stable than a carbocation? When the system has an opportunity to steal the C2-H bond pair and utilise it as a C-C pi bond, why would it prefer to get into a comparatively higher potential energy state by making another carbocation? $\endgroup$ – Shekhar Upadhyay Nov 9 '16 at 3:48
  • $\begingroup$ @Shekhar An alkene is much more stable than a carbocation. But here, we also have a "naked" proton, and we have to consider the stability of the molecule along with the proton. It is presumably more stable for the molecule to have the proton bonded covalently than remaining loosely hanging. $\endgroup$ – FreezingFire Nov 9 '16 at 4:28
  • $\begingroup$ @FreezingFire Do you not have naked protons hanging around in an aqueous solution of HCl? There, it's stable. Here, it's not... How? $\endgroup$ – Shekhar Upadhyay Nov 9 '16 at 4:32

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