[devvaibhav]

Hi
I have a little problem in tautomerism. My sir gave me some questions which i have tried and don't match with the answer.

Q:- Will these compounds show tautomerism or not? If yes, draw and if not why not?

(a)
(b)

My attempt to ques (a):-

Sir's answer to (a) :-

My support for my attempt:- I have converted the keto form to enol form.
Sir's reason:- When the H bond with CH₂ is broken and the electron pair is taken by CH₂, there occurs n-π(pi) conjugation due to which the molecule is exceptionally stable.At last when the lone pair of electrons come to O^-,H⁺ will attack it and the keto form is converted to enol form.
Is my attempt wrong only because by my mechanism, the compound is not stable by resonance or  is there something else missing?

My attempt to (b):- 

I am stuck at last here. There is a "-" charge on "C" and  "N" both. Where would H⁺ attack?My friends said that it will attack at "N" because it is more electronegative than "C". Yeah i know that if H⁺ will attack at "C" the same compound will be formed. But i have a doubt that "N" is more electronegative that "C" yet why will H⁺ attack at "N" as both have same attraction for a proton. As far i am concerned that electronegative is the relative tendency of an atom to attract electrons towards itself but H⁺ is a proton and not an electron.
The final product which we will get is this :

How will we justify this dative bond?

Thanks and regards
Dev Vaibhav

[CaverKat]

Hi Dev!

a) Your tautomer is not resonance stabilised.  When the double bond reforms from your proposed enol on its way back to keto form, there is nowhere for the pi electrons of enol double bond to go.
(just a note on your mechanism.  The second step, you've got an arrow going from the water hydrogen to the oxygen.  Notice anything funny about it?)
Other than the funky arrow, your mechanism is sound (sort of I think), but it very, very, very unlikely.  Very.

b) When the C-H bond breaks, and the electrons go to the C, it gains a formal negative charge (the water becomes hydronium and charge is conserved).  The nitrogen picks up a proton and becomes positively charged.  The group is called isonitrile (i.e. isomer of nitrile) and it is a particularly nasty and smelly compound.  (I had a horrific time making it here)

Apologies if I'm not quite making sense (let me know if you would like a picture instead).  It's quite late (or very early) here at the moment.

[jm250]

Hi,
Basically the compounds exist as 2 tautomers. This tautomerisation will actually be a reversible reaction (shown with equilibrium arrows). However, the tautomer that you have drawn as the product is hydrogen isocyanide (or isonitrile) and this molecule also has two resonance structures. The one you have drawn with a triple bond is actually a zwitter ion and should show a formal negative charge on the carbon and a formal positive charge on the nitrogen. The other resonance form is a neutral species but is drawn with a double bond and a lone pair on both carbon and nitrogen.
It might also be worth noting that Hydrogen isocyanide (isonitrile) the molecule you have drawn as the product is actually the minor tautomer of hydrogen cyanide (the nitrile molecule shown as the reactant).
You show the exact question but I would guess that the point is something to do with the fact that there is both resonance and tautomeric forms.
Hope this is helpful.

[devvaibhav]

Hi Dev!

a) Your tautomer is not resonance stabilised.  When the double bond reforms from your proposed enol on its way back to keto form, there is nowhere for the pi electrons of enol double bond to go.
(just a note on your mechanism.  The second step, you've got an arrow going from the water hydrogen to the oxygen.  Notice anything funny about it?)
Other than the funky arrow, your mechanism is sound (sort of I think), but it very, very, very unlikely.  Very.

b) When the C-H bond breaks, and the electrons go to the C, it gains a formal negative charge (the water becomes hydronium and charge is conserved).  The nitrogen picks up a proton and becomes positively charged.  The group is called isonitrile (i.e. isomer of nitrile) and it is a particularly nasty and smelly compound.  (I had a horrific time making it here)

Apologies if I'm not quite making sense (let me know if you would like a picture instead).  It's quite late (or very early) here at the moment.

I would like to focus Why Nitrogen picks up a proton and becomes positively charged and why not Carbon? And please justify the Dative Bond.
Thanks and please post the pictures. Hope they would help more.

[CaverKat]

Tautomerisation of hydrogen cyanide and the two resonance forms of the formed isonitrile:



The carbon does pick up the proton.  When it does, it just goes back to the starting material, the hydrogen cyanide (after the nitrogen stabilises the carbocation - Nitrogen is happier having a +ve charge, because it still has a full octet).  It is much more likely that this will happen, but then you won't show the tautomer.  Right?



That's why the cyanide is the major tautomer and the isonitrile is the minor.

[CaverKat]

Couldn't be bothered to finish off the last step: losing the proton and neutralising the charge.  But then it was bugging me, so here's the last step:


Back to square one.

[orgopete]

I would like to focus Why Nitrogen picks up a proton and becomes positively charged and why not Carbon? And please justify the Dative Bond.
Thanks and please post the pictures. Hope they would help more.

I would like to suggest using a model I am fond of using. I suggest that reactivity follows the inverse square law. The further a pair of electrons extends from a nucleus, the more reactive they are. In this case, it should be the carbon atom.

The inverse square law should be familiar to anyone with photography experience. When using a flash, if you double the separation, one-fourth the light returns.

I describe this concept in more detail here.

[devvaibhav]

@CaverKat
The images are not visible though i have created an account at Dropbox. Would you please upload them on photobucket or imageshack??
Thanks

[CaverKat]

Ah stupid web link, I'll attach them.

[devvaibhav]

I think is not formed.

[CaverKat]

http://en.wikipedia.org/wiki/Hydrogen_isocyanide

[devvaibhav]

@Caverkart
I am an 11^th grade student. The higher and practical stuff is not in my syllabus for IIT-JEE. I am a Morrison and Boyd level student. In the last you have formed . The H is attached to C and not to N. And the wikipedia link, its totally out of my mind. Its a higher level(i think college) stuff.
Thanks and please reply...

[CaverKat]

That's just the last step of the mechanism in the picture above it.  I got lazy, and didn't do the last step, but then it was bugging me, hence the last attached picture.

Short answer.  Hydrogen cyanide will show tautomerism and its tautomer is the isocyanide.  The "iso" means isomer of cyanide.  It does look odd, yes.  Especially in its zwitterionic form (i.e. has formal +ve and -ve charges), but that's generally how its represented.

As jm250 mentioned, you can also draw it without the formal charges, but then it's represented with a double bond, and the lone pair electrons on the C and N (charge is zero, hence conserved).

Mechanism was just a representation of how to get from one tautomer to the other.

[devvaibhav]

@CaverKat
Where are the images? i don't see them.
Thanks