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Aromatic Ions. Some questions

Aromatic Ions. Some questions - Chemistry Forum

Aromatic Ions. Some questions - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #1  
Old 07-25-2003, 03:39 PM
Pedro
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Default Aromatic Ions. Some questions



Hello to all:
I was studying the chapter on Aromatics in Organic Chemistry and
came up with some questions and was wondering if I could get your
opinions on the subject.

1) My textboox cites ciclooctatetraene dianion as an example of an
aromatic ion (a dianion in this case). Surely it follows Huckel's rule
with 10e-; however, using Frost's circle I see that while all e- are
paired, the last four are in non-bonding orbitals. Is this really a
particularly stable arrangement?

2) It seems to me that a more stable arrangement would be the
cyclooctatetraene dication with 6e- and that would place all e- in
bonding orbitals and none on non-bonding or antibonding orbitals. The
dianion seems a more common example (looked in several books) that the
dication and I am wondering why. Is it just that authors have made a
poor choice of example or is it that for some reason I cannot see the
anion is indeed a better example?

3) Tropylium bromide is cited as an example of an aromtic cation with
6e-. The book says it is a stable ionic compound. I am wondering about
the existance of other "aromatic salts". Do things like sodium
pentadienide, or the salt of the cyclooctatetraene dianion or cation
exist?

Thanks in advance for your comments

Pedro
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  #2  
Old 07-25-2003, 04:31 PM
Joćo Antonio
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Default Aromatic Ions. Some questions

1) The diagram is right, no problem about the anti-bonding electrons. How do
you plan to take two
electrons away? It may be feasible by electrochemical ways, but I think
that, despite the
aromaticity, it would be very reactive. The same thing with 6 electrons
added. Aromaticity may
increase the stability, but you cannot ignore electron affinity and
carboication rearrangement
while thinking of the products. Cyclooctatetraene + 2e- is a coomnly used
ligand in coordination
chemistry, and the examples you saw tend to point a real life application.

2) The salts do exist with cyclopentadienyl(1-) and cyclooctatetraenyl(2-)
and they are also
great for complex forming, like metallocenes (eg. ferrocene) and uranocene
(with 2
cyclooctatetraenyl rings sandwiching an uranium atom). They are well
explained in any Inorganic
Chemistry textbook, like that of Huheey.

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  #3  
Old 07-26-2003, 02:45 PM
Ivana Fleischer
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Default Aromatic Ions. Some questions

Hi,


these are non-bonding, not anti-bonding electrons


Both is possible. It is possible to add electrons to cyclooctatetraene by
treating it with alkali metals and also to take 2 electrons away.
1,3,5,7-tetrametylcyclooctatetraene was treated with SbF5/SO2ClF (-78°C) and
a dication was formed. This ion is stable in solution at -50°C and it is
planar. But I don't know, which one is more stable.

Ivana Fleischer.




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  #4  
Old 07-26-2003, 05:21 PM
Joćo Antonio
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Default Aromatic Ions. Some questions

> >1) The diagram is right, no problem about the anti-bonding electrons.

that's right

and

If I'm not mistaken COT(2-) forms complexes which are stable at room
temperature.
Do you know what happens if COT(2+) goes above -50? I don't trust
carbocations...
They always have a trick hidden in their sleeves...


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  #5  
Old 07-27-2003, 03:25 PM
Ivana Fleischer
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Default Aromatic Ions. Some questions

>> >1) The diagram is right, no problem about the anti-bonding electrons.




You are right. If COT(2+) goes above -30, there isn't COT(2+) any more. I
like rt, too.

You wrote, that COT(2-) forms uranocene, but afaik there is neutral COT and
it donates 2*8e-.



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  #6  
Old 07-29-2003, 02:27 AM
Joćo Antonio
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Default Aromatic Ions. Some questions

both neutral COT and COT(2-) can be used as ligands
but due to their structure, they will behave differently


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  #7  
Old 07-29-2003, 03:36 PM
Ivana Fleischer
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Default Aromatic Ions. Some questions

I know that COT can act as "eta"-2, -4, -6 or -8 ligand.

What is the oxidation level of U? IV? I thought, if it would be IV and each
COT-ring would donate 8e-, it would be a stable 18e- complex.





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  #8  
Old 07-30-2003, 06:43 AM
dave.lister@web.de
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Default Aromatic Ions. Some questions

"Joćo Antonio" <[Only registered users see links. ].br> wrote in message news:<bg4m2f$oue$[Only registered users see links. ].ntu.edu.tw>...

The key words here are aromatic (4n + 2 pi-electrons) and antiaromatic
(4n pi-electron systems). Benzene, cyclopentadienylanion,
isobutenedianion (Y2-) as simplest aromats and cyclobutatiene (running
out of imagination here ;-) as simplest antiaromats.

Hope i'm not kicking in open doors here..
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