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Topic: Aromatic  (Read 21099 times)

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Offline Will

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Re: Aromatic
« Reply #15 on: May 23, 2006, 01:33:07 PM »
I can't believe anyone has picked up on Dan's point! I think he is completely right.

The notation used means it must be a singlet carbene (if it was triplet the electrons would be one above the carbon and one below).
Effectively it is the same as cycloprop-2-en-1-ylium (cyclopropene with a positive charge) but with a pair of electrons in a sp2 orbital rather than a C-H bond. If that is aromatic then surely D is as well.
I think everything Dan has said is right, there is no formal negative charge etc. etc.
The only problem with this carbene is that it might react with itself, it would make a really cool addition polymer though- like a long chain of back-to-back cyclopropanes, or a chain of "bicyclo[1.1.0]butane"s as ChemDraw calls it!

Offline Dan

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Re: Aromatic
« Reply #16 on: May 23, 2006, 02:02:24 PM »
cyclopropene with a positive charge

Yes! It's just a deprotonated one, I'm glad someone can see what I'm getting at.
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Offline movies

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Re: Aromatic
« Reply #17 on: May 23, 2006, 05:59:21 PM »
Not to over-discuss this topic, but I wanted to throw my two cents in:

As Dan has said, if it is a singlet carbene, it's aromatic, if it's a carbanion, then it's anti-aromatic.

My guess, however, is that it's a typo in the problem and they intended for it to be a carbanion.

By the way, the triplet form of a carbene is about 10 kcal/mol lower in energy than the singlet form, according to calculations.  There are plenty of examples of both types of carbenes, however; that energy difference is for the simplest example: H2C:
« Last Edit: May 23, 2006, 06:55:45 PM by movies »

Offline Borek

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Re: Aromatic
« Reply #18 on: May 23, 2006, 06:06:50 PM »
the singlet form of a carbene is about 10 kcal/mol lower in energy than the singlet form

???
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Offline Winga

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Re: Aromatic
« Reply #19 on: May 23, 2006, 06:30:54 PM »
He means the triplet form of a carbene is about 10 kcal/mol lower in energy than the singlet form.
That what I also found from a book.

Of course, I (everyone discussing in here) got what Dan said, and I also agree what he said.
About the triplet and singlet forms, they should be interchangable, so we can regard it as aromatic.
If under a magnetic field, it is not easy to tell!
 
I wonder where is the source of this question!

Offline movies

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Re: Aromatic
« Reply #20 on: May 23, 2006, 06:59:00 PM »
Sorry, I fixed the mistake.

Whether or not the carbene has time to isomerize to whichever form is more stable will depend on how long it hangs around for before it reacts.

Offline Will

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Re: Aromatic
« Reply #21 on: May 23, 2006, 08:51:03 PM »
Of course, I (everyone discussing in here) got what Dan said, and I also agree what he said.
About the triplet and singlet forms, they should be interchangable, so we can regard it as aromatic.

:-[ My bad! I just read what a couple of people wrote before I posted, dismissing the carbene as antiaromatic, and got a little bit worked up lol!

Another thing in favour of the singlet state in D, as well as the aromaticity, is the smaller bond angles in the molecule, which allow a greater space than normal for the electrons in the sp2 orbital, so the "incentive" for the electrons to be in the triplet state is reduced further. Also, the general idea of the triplet state being favoured (due to stability) is useful, sometimes, but useless when there are electron-rich substituents on the carbene molecule, like in :CCl2, where the electrons would be in the singlet state (but, by coincidence :CCl2 is formed in the singlet state!).

Sorry for going on about carbenes, I just find them really interesting! [Edit: especially the Arndt-Eistert synthesis, and the Hofmann rearrangement!]

I wonder where is the source of this question!

So do I! There seems to be so many mistakes, though, in chemistry books nowadays. I've even found at least 3 'chemistry' errors in my organic chem book (the Clayden, Greeves, Warren, Wothers one), and my current A-Level book has at least 6 'chemistry' errors in it, one being half a page devoted to a mechanism (acid catalysed halogenation of acetone), which is entirely wrong! When my teacher tried to write out the mechanism out for that reaction last year he wrote the one in the book, and without reading up about this reaction I could tell it looked wrong, and suggested a mechanism which happened to be right after checking it out afterwards. Good thing you don't need to know that mechanism for the A-Level, the book was just trying to demonstrate the rate equation and its relation to the RDS.
I think there seems to be a lack of chemistry book editors- I am yet to find a chemistry book without a mistake in it! ;)
« Last Edit: May 23, 2006, 08:53:51 PM by will17 »

Offline arvind1990

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Re: Aromatic
« Reply #22 on: May 27, 2006, 05:23:04 AM »
cyclopropene is also aromatis because the ring is polar.
4n+2=2,pi electrons(n=0).So the answer is E

Offline AWK

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Re: Aromatic
« Reply #23 on: May 27, 2006, 05:57:43 AM »
cyclopropene is also aromatis because the ring is polar.
4n+2=2,pi electrons(n=0).So the answer is E
Cyclopropene  (C3H4) is not aromatic - cyclopropenyl cation (cycloperopenylium) C3H3+ is aromatic according to Huckel rule 4n+2
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