[helper]

hello,

I draw the resonance strucures and od not find n explantion to this statement. I need a help and have no idea how to explain it.( consistutional isomers have the same molecular formola but are link diffently).

[Archer]

Is (A) aromatic?

[helper]

no, resonance can occur only in aromatic compounds?

[Babcock_Hall]

No, resonance can occur in nonaromatic chemical species.

[Archer]

Think about the structure of aromatic compounds (such as naphthalene) and consider the structure of cyclobutadiene. If it resonance occurs is it likely to alter the structure? or would it remain the same as with aromatics?

If a substance resonates between two non-identical forms what is another word for this process?

[helper]

can it be becase the p is not perpendicular the plane of the molecule or it has a different reason.
Can you please explain the reason because I don't understandit clearly and I whould like too.

please and thank you a head.

[magician4]

in aromatic systems, the double bonds are not "fixed" but completely delocalised instead

hence, in example B to draw two different Lewis structures is ... meaningless

... and as a consequence, there is no such thing as  two different positions for the fluoride atoms

structure A on the other hand is a cyclobutadiene derivative, and would be an antiaromat if it went for resonance ( 4 [itex]\Pi[/itex] electrons: look at Hueckel's rule), and that would be very bad from an energetical point of view.
this the molecule tries to avoid, showing localised, individual double bonds instead:


(hope this picture talks, though it's labelled in German: "Verzerrung" = "distortion" , "lokalisierte Doppelbindungen" = "localised double bonds" )

(cyclobutadiene and most of its derivatives are very unstable hence)

Nevertheless, if we look at the fluoride atoms involved, we had two isomers here: on with a double bond inbetween the fluoride atoms, and one without


the whole point with these two molecules hence is, whether they'll show delocalised double bonds or not.
one does (as it's aromatic) , the other doesn't (as else it would be antiaromatic)


regards

Ingo

[helper]

Thank you, I got the idea.

[Archer]

in aromatic systems, the double bonds are not "fixed" but completely delocalised instead

hence, in example B to draw two different Lewis structures is ... meaningless

... and as a consequence, there is no such thing as  two different positions for the fluoride atoms

structure A on the other hand is a cyclobutadiene derivative, and would be an antiaromat if it went for resonance ( 4 [itex]\Pi[/itex] electrons: look at Hueckel's rule), and that would be very bad from an energetical point of view.
this the molecule tries to avoid, showing localised, individual double bonds instead:


(hope this picture talks, though it's labelled in German: "Verzerrung" = "distortion" , "lokalisierte Doppelbindungen" = "localised double bonds" )

(cyclobutadiene and most of its derivatives are very unstable hence)

Nevertheless, if we look at the fluoride atoms involved, we had two isomers here: on with a double bond inbetween the fluoride atoms, and one without


the whole point with these two molecules hence is, whether they'll show delocalised double bonds or not.
one does (as it's aromatic) , the other doesn't (as else it would be antiaromatic)


regards

Ingo

What he said! 

The word I was asking for is tautomer. If it does resonate between the two then they would be chemically different and ¹³C NMR would show 2 substance present in the sample (i.e. double the expected peaks, bearing in mind that ¹⁹F splits carbon peaks as they are not usually decoupled)

[magician4]

it's an interesting question, indeed, whether those two substances in A are tautomers, i.e. if the energetic barrier of rearrangement (which should belong to an aromatic compound ?) is sufficiently low at room temperature, or if they're just isomers.

with a halflifetime in the ballpark of 5 sec at room temperature for cyclobutadiene this should be difficult to investigate, but by no way impossible

... esp. if the difluoro-derivatives were somewhat stabilised by the + M effects of the halogenes, which I think they should be

unfortunately, I don't hold literature about this : maybe someone with access to CAS-online might wish to take a look...


regards

Ingo

[Archer]

It would be interesting to investigate but I don't think this substance exists. I too would like to see some literature data.