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Author: Subject: Question regarding stability of carbocations...
sankalpmittal
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[*] posted on 28-6-2013 at 06:30
Question regarding stability of carbocations...


Here is the question: http://postimg.org/image/gtlq9urwb/

Look at question 62..

Its asking decreasing order of stability.

I think that order is..

IV>III>II>I, due to resonance involved, but this is wrong.

Where did I go wrong ?

Thanks !!
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sonogashira
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[*] posted on 28-6-2013 at 06:33


Did you consider the extra stability of aromatic cations, having 4n + 2 pi electrons?
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sankalpmittal
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[*] posted on 28-6-2013 at 06:40


Quote: Originally posted by sonogashira  
Did you consider the extra stability of aromatic cations, having 4n + 2 pi electrons?


Thanks !! Completely missed that..

Ok, so we have,

IV and II are aromatic, but I and III are not aromatic.

Hence,

IV>II due to resonance..

So our order is..

IV>II>...

How do I compare the stability of I and III ? Both are not aromatic.
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sonogashira
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[*] posted on 28-6-2013 at 06:42


III is more stable than I, due to resonance. Any resonance will stabilize a cation (or anion), but aromatic resonance is especially stable.
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sankalpmittal
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[*] posted on 28-6-2013 at 06:46


Quote: Originally posted by sonogashira  
III is more stable than I, due to resonance. Any resonance will stabilize a cation (or anion), but aromatic resonance is especially stable.


So the order should be,

IV>II>III>I, that is (b) should be the answer. But this is wrong..:(

As per the book, its (b). That is

IV>II>I>III
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sonogashira
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[*] posted on 28-6-2013 at 06:51


I isn't more stable than III. The answer in the book may be wrong. Who's the author of the book?

[Edited on 28-6-2013 by sonogashira]
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Mercedesbenzene
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[*] posted on 28-6-2013 at 07:25


for I and III its quite simple, I is just a random carbocation but III is anti aromatic, 4n=4.

so I should be more stable than III


[Edited on 28-6-2013 by Mercedesbenzene]
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sankalpmittal
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[*] posted on 28-6-2013 at 07:35


Quote: Originally posted by sonogashira  
I isn't more stable than III. The answer in the book may be wrong. Who's the author of the book?

[Edited on 28-6-2013 by sonogashira]


Isn't I non aromatic and III anti aromatic ? And anti aromatics are less stable than non aromatics.
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sankalpmittal
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[*] posted on 28-6-2013 at 07:37


Quote: Originally posted by Mercedesbenzene  
for I and III its quite simple, I is just a random carbocation but III is anti aromatic, 4n=4.

so I should be more stable than III


[Edited on 28-6-2013 by Mercedesbenzene]


Oh ok. But you and sonogashira give different answers. And your answer agrees with the book. Thanks.

sonogashira, the author of the book is Indian, and his name is OP Tandon.
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sonogashira
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[*] posted on 28-6-2013 at 07:39


Use his answer, I've never heard of anti-aromatic!
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sankalpmittal
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[*] posted on 28-6-2013 at 09:25


Quote: Originally posted by sonogashira  
Use his answer, I've never heard of anti-aromatic!


Thanks everyone!!

More comments are welcomed.
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[*] posted on 29-6-2013 at 00:27


What is the reference claiming a higher stability of cyclohexyl carbocation vs. cyclopentadienyl carbocation?

Just because some guy claims a certain stability order in a textbook, it does not mean that this corresponds to experimental truth. Textbooks for students contain blatant lies and obvious bullshit, because they are intended to teach concepts - their intention is not to teach a truth (they don't even cite references!). If you are interested in experimental truth, then you need to verify what you learn in the scientific literature (all the way to the primary literature). On the other hand, if you are only interested in passing the exam, then ignore the science and just learn what your professor want you to believe.

Antiaromaticity only means the planar cyclic structure is less stable than the nearest open ring structure (e.g., cyclopentadienyl carbocation vs. divinylmethyl carbocation). Nothing more than that.

According to JACS, 89, 1112–1119 (DOI: 10.1021/ja00981a015), the cyclopentadienyl carbocation forms easily from cyclopentadienyl alcohol with strong acids even at cryogenic temperatures. The same cannot be said for cyclohexanol. This is good enough indication that the cyclopentadienyl carbocation might be more stable than the cyclohexyl carbocation. None of them has been isolated in a stable compound/salt, because they rapidly decompose further, but this only means they are less stable than the very stable ones like the tropylium, trityl carbocations and similar which form stable salts. To definitively say which one of them is more stable, one needs to find a study giving their thermodynamic properties.

PS: Please open homework or school related questions only in the Beginnings section. Preferably, use only one thread for all your homework questions.

[Edited on 29/6/2013 by Nicodem]




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sankalpmittal
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[*] posted on 29-6-2013 at 06:29


Quote: Originally posted by Nicodem  
What is the reference claiming a higher stability of cyclohexyl carbocation vs. cyclopentadienyl carbocation?

Just because some guy claims a certain stability order in a textbook, it does not mean that this corresponds to experimental truth. Textbooks for students contain blatant lies and obvious bullshit, because they are intended to teach concepts - their intention is not to teach a truth (they don't even cite references!). If you are interested in experimental truth, then you need to verify what you learn in the scientific literature (all the way to the primary literature). On the other hand, if you are only interested in passing the exam, then ignore the science and just learn what your professor want you to believe.

Antiaromaticity only means the planar cyclic structure is less stable than the nearest open ring structure (e.g., cyclopentadienyl carbocation vs. divinylmethyl carbocation). Nothing more than that.

According to JACS, 89, 1112–1119 (DOI: 10.1021/ja00981a015), the cyclopentadienyl carbocation forms easily from cyclopentadienyl alcohol with strong acids even at cryogenic temperatures. The same cannot be said for cyclohexanol. This is good enough indication that the cyclopentadienyl carbocation might be more stable than the cyclohexyl carbocation. None of them has been isolated in a stable compound/salt, because they rapidly decompose further, but this only means they are less stable than the very stable ones like the tropylium, trityl carbocations and similar which form stable salts. To definitively say which one of them is more stable, one needs to find a study giving their thermodynamic properties.

PS: Please open homework or school related questions only in the Beginnings section. Preferably, use only one thread for all your homework questions.

[Edited on 29/6/2013 by Nicodem]


Ok thanks !!

So the answer should be,

IV>II>III>I, and resonance still dominates ?

This guy : http://www.askiitians.com/forums/Organic-Chemistry/21/56195/... says that cyclopentadienyl carbocation is unstable, less stable than cyclohexyl carbocation, because cyclopentadienyl carbocation is anti-aromatic..
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[*] posted on 30-6-2013 at 03:25


Found something interesting..

There was a question:

Which carbanions are most stable:

(a) Allylic carbanion
(b) -CH2-NO2
(c) Cyclopentadienyl carbanion
(d) CH3--CH-CH3

Answer:
They said it is cyclopentadienyl carbanion although its anti-aromatic !!!! Reason: Resonance.

Antiaromatics are unstable, but we can compare them to the nearest ring structures only, where there is conjugation.

Any comments ?
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[*] posted on 30-6-2013 at 04:34


Quote: Originally posted by sonogashira  
Did you consider the extra stability of aromatic ANions, having 4n + 2 pi electrons?
:D
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[*] posted on 30-6-2013 at 12:39


Quote: Originally posted by sankalpmittal  

Which carbanions are most stable:

(a) Allylic carbanion
(b) -CH2-NO2
(c) Cyclopentadienyl carbanion
(d) CH3--CH-CH3

Answer:
They said it is cyclopentadienyl carbanion although its anti-aromatic !!!! Reason: Resonance.

Antiaromatics are unstable, but we can compare them to the nearest ring structures only, where there is conjugation.

Any comments ?

The cyclopentadienyl carbanion is aromatic, not antiaromatic!

The pKa values for all those anions can easily be found by a five minutes search, so I suggest you to find the answer yourself and then ask for specifics you don't understand.




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