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Which of the following compounds does not show tautomerism?

The correct answer is (a), and I'm curious if this is because to show tautomerism, there can't be any net charge.

For instance, ketones can tautomerize to enols, and more importantly, enamines can tautomerize to imines. In (a), we would get an imine-like thing, but the nitrogen would be positively charged. Does this still count as tautomerization?

EDIT: Thank you everyone for your helpful clarifications, but I'm still a bit confused as to why, mechanistically, we need a hydrogen on the nitrogen.

When I Google the mechanism for imine tautomerization, what if find is this:

enter image description here

Where the nitrogen lone pair pushes down and the double bond reaches out to pick up a proton. There isn't a transfer of hydrogen from the nitrogen to the solvent in the case shown below, yet the molecule still tautomerizes.

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  • $\begingroup$ As this is a homework question, please share your thoughts and attempts towards the solution. It'll make us certain that ‎we aren't doing your homework for you. Additionally, all I am seeing a black box where the image should be. $\endgroup$
    – user15489
    Sep 1 '15 at 21:30
  • $\begingroup$ Hi, sorry about that. I had just posted it to see if the black box that I was seeing would correct itself, but obviously it hadn't. I wasn't actually done writing my post when you found it, but I appreciate your dedication to answer questions. $\endgroup$
    – coloratura
    Sep 1 '15 at 21:33
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    $\begingroup$ @coloratura Sometimes you have to use the HTML workaround to resize images when it fails via imgur. Have a look at the source when you want to know how it is done. $\endgroup$ Sep 1 '15 at 23:14
  • $\begingroup$ Thank you for fixing the image and adding a considerable amount of your thinking - close vote and downvote retracted. $\endgroup$
    – user15489
    Sep 1 '15 at 23:23
  • $\begingroup$ What's you're talking about is simply another mesomeric structure... $\endgroup$
    – Mithoron
    Sep 1 '15 at 23:27
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Tautomerization is a chemical reaction whereby constitutional isomers interconvert. The interconversion process involves the shift of a hydrogen atom accompanied by the shift of a double bond. Here are a few examples.

enter image description here

In molecule (a) pictured above, there is no hydrogen attached to a nitrogen (it has been replaced by a methyl group), so there is no hydrogen atom available to shift.

Response to OP's edit:

The picture you posted in your edit is incorrectly labeled. The charged molecule on the left is an iminium ion, not an imine. So this is not an example of an imine-eneamine equilibrium. You are correct that in this equilibrium (the one pictured in your edit) there is no hydrogen transferred to nitrogen, but again, this is not an example of an imine-eneamine tautomerization. I think this error is the source of your confusion.

Here is a picture of an imine-eneamine tautomerization.

enter image description here

As you can see, a hydrogen is transferred between nitrogen and carbon. This hydrogen transfer can occur with or without solvent molecules playing a role. That is to say, the hydrogen can be transferred directly between the carbon and nitrogen atoms, or it can be transferred from one of the atoms to a solvent molecule which can then transfer it back to the other atom involved in the tautomerization. The image source also provides a nice explanation of tautomerization.

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  • $\begingroup$ Does the hydrogen necessarily need to be transferred from one nitrogen to the other nitrogen? In mechanisms, it's portrayed that the hydrogen comes from solvent. $\endgroup$
    – coloratura
    Sep 2 '15 at 14:55
  • $\begingroup$ Tautomerization can also occur in solvents that can't supply a hydrogen. In those cases the reaction can be intramolecular with the hydrogen transferring from one nitrogen to the other. $\endgroup$
    – ron
    Sep 2 '15 at 14:59
  • $\begingroup$ I see, so in this problem, we must assume that the solvent can't donate a hydrogen? $\endgroup$
    – coloratura
    Sep 2 '15 at 15:01
  • $\begingroup$ No, the solvent doesn't really matter in your question. Even if the solvent is involved, the first step would involve transfer of the hydrogen on the nitrogen to solvent and then from solvent it would eventually return to the other nitrogen. $\endgroup$
    – ron
    Sep 2 '15 at 15:05

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