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KekulÚ models

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swot
939819.  Mon Sep 17, 2012 11:46 am Reply with quote

I can't fathom why, but in my first year, my friends and I really enjoyed learning about benzene.

Anyway, one of the things we learned was about the KekulÚ model, which describes the shape of the molecule and explains the lower than expected number of hydrogen atoms.

A benzene ring is made of 6 carbon atoms and 6 hydrogen atoms, with the carbon atoms arranged in a hexagonal 'ring' structure, each attached to a single hydrogen atom. As some people will know, a hydrogen atom can bond to one other atom only, whereas a carbon atom has to have four single bonds, or a combination of single and double bonds adding up to four to be stable. The KekulÚ model describes two separate ring structures with alternate single bonds, giving each carbon atoms 4 bonds each.



The above diagrams are known as Lewis structures, which represent all the possible arrangements of the electrons, and as such, the bonds between atoms. The more Lewis structures, the more stable a molecule is. Benzene is unusually stable for an alkene, and unusually, its bond lengths are all the same - 1.39 . Usually, a carbon to carbon single bond is 1.54 ┼, and a carbon to carbon double bond is 1.34 ┼. The stability provided by the Lewis structures is described as 'resonance'.

Benzene may also be drawn as a hexagon with a circle in the middle, representing the alternate double bonds. In the case on benzene and other aromatic compounds, these bonds are delocalised electrons.

 
swot
939820.  Mon Sep 17, 2012 11:54 am Reply with quote

A benzene ring is planar, with the s (sigma) orbitals above and below the plane (so the 'corners' of the hexagon, i.e. the carbon atoms in the centre of the upright dumbbell shape), with the delocalised p (pi) electrons forming a a torus above and below the s orbitals.

 
krollo
947730.  Sat Oct 27, 2012 7:27 am Reply with quote

Along with the song 'Yesterday', the sewing machine and the fortune of the first self-made female millionaire in the USA*, the structure of the benzene ring came to Kekule in a dream.

*Very interesting woman in her own right - Madam CJ Walker, the child of African-American slaves, found the formula for her hair-replenishment product in a dream.

 
Efros
947738.  Sat Oct 27, 2012 8:08 am Reply with quote

The story behind the structures is interesting in that KekulÚ maintained that he came up with the structures whilst daydreaming about a snake eating its tail.

An aromatic compound has 4n+2 pi electrons where n is an integer, in benzene you have effectively 3 double bonds each of which has 2 pi electrons, so 6 pi electrons with n=1 this satisfies the 4n+2 rule. I say effectively as the resonance at room temperatures is so fast that observation gives all the bonds of equal length with a bond order near to 1.5, I like to think of it as double bonds whirling round like a propeller. Recent work has shown that degrees of aromaticity can exist in compounds not containing 4n+2 pi electrons. For example the aromaticity of arsenic substituted pyrrole rings have long been the subject of discussion, and as they don't satisfy the 4n+2 rule they have been labelled non aromatic, however some work has suggested that they are at least half as aromatic as pyrrole itself so the claim that arsoles are not aromatic should be replaced with arsoles are somewhat aromatic. (snigger, I can almost hear Mr.Fry saying this).

 
swot
947834.  Sun Oct 28, 2012 6:36 am Reply with quote

Thanks Efros. I did the best I could, but I was hoping that someone cleverer than me could pick up the slack, as it were.

 
Efros
947841.  Sun Oct 28, 2012 7:49 am Reply with quote

Not cleverer just a bit more up in this area.

 
WordLover
947888.  Sun Oct 28, 2012 1:27 pm Reply with quote

swot wrote:
Benzene is unusually stable for an alkene, and unusually, its bond lengths are all the same - 1.39 . Usually, a carbon to carbon single bond is 1.54 ┼, and a carbon to carbon double bond is 1.34 ┼. The stability provided by the Lewis structures is described as 'resonance'.

Benzene may also be drawn as a hexagon with a circle in the middle, representing the alternate double bonds. In the case on benzene and other aromatic compounds, these bonds are delocalised electrons.
Thank you for that. That's QI. One consequence of the structure of benzene being as you describe is that there are only 3 isomers of, say, dichlorobenzene: one each of 1,2-, 1,3- and 1,4-. If the bonds around the ring were alternately single and double bonds, then there'd be two 1,2-dichlorobenzene isomers: one with a single, and one with a double, bond between the carbons that the chlorine atoms are bonded to. But no; thanks to the delocalisation of the electrons, there's just the one.

 
Efros
947890.  Sun Oct 28, 2012 1:43 pm Reply with quote

ortho meta and para isomers.

 
iansonuk
955378.  Mon Dec 10, 2012 5:50 pm Reply with quote

Efros wrote:
The story behind the structures is interesting in that KekulÚ maintained that he came up with the structures whilst daydreaming about a snake eating its tail.

An aromatic compound has 4n+2 pi electrons where n is an integer, in benzene you have effectively 3 double bonds each of which has 2 pi electrons, so 6 pi electrons with n=1 this satisfies the 4n+2 rule. I say effectively as the resonance at room temperatures is so fast that observation gives all the bonds of equal length with a bond order near to 1.5, I like to think of it as double bonds whirling round like a propeller. Recent work has shown that degrees of aromaticity can exist in compounds not containing 4n+2 pi electrons. For example the aromaticity of arsenic substituted pyrrole rings have long been the subject of discussion, and as they don't satisfy the 4n+2 rule they have been labelled non aromatic, however some work has suggested that they are at least half as aromatic as pyrrole itself so the claim that arsoles are not aromatic should be replaced with arsoles are somewhat aromatic. (snigger, I can almost hear Mr.Fry saying this).



I'm not sure what you mean by suggesting that arsole doesn't follow the 4n + 2 rule, as it is isoelectronic with pyrrole. As long as the lone pair is able to hybridise in such a way as to contribute to the pi system, there are 6 pi electrons in total and the rule is obeyed. (The issue here I think is to what extent it is energetically favourable for the lone pair on arsole to be sp2 hybridised).

 
Efros
955381.  Mon Dec 10, 2012 6:31 pm Reply with quote

and if it isn't sp2 hybridized then it can't be a pi electron and consequently will not contribute to aromaticity. Evidence suggests that aromaticity in arsole is not nearly as complete as in pyrrole, and the reluctance of the As lone pair to hybridize is probably the reason.

 
iansonuk
955537.  Tue Dec 11, 2012 10:14 am Reply with quote

Indeed

 
Hernammi
978119.  Sat Mar 02, 2013 7:52 pm Reply with quote

Efros wrote:
I say effectively as the resonance at room temperatures is so fast that observation gives all the bonds of equal length with a bond order near to 1.5, I like to think of it as double bonds whirling round like a propeller.


This is true and furthermore, since a benzene ring contains electrons moving in a circular motion via unhybridized p-orbitals of the carbon atoms, the benzene ring induces a magnetic field through the center of the molecule. This is a quite useful trait in recognizing benzene rings in unknown molecular structures in chemical analyses, most notably NMR (nuclear magnetic resonance) spectroscopy because this induced magnetic field dramatically affects the chemical shifts of the 13C and 1H nuclei.

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Posital
978156.  Sun Mar 03, 2013 3:51 am Reply with quote

Crumbs - from my basic quantum mechanics I never thought of an electron orbital having a direction - but it makes sense when trapped like that.

I suppose it's even more interesting for irtho and trans isomers as you could potentially separate them by direction of orbit. And I wonder if they would exhibit differing chemical properties.

Assuming that these orbits are relatively stable, that is...

On reflection - I think this is more of a thought experiment than anything.

 
PDR
978211.  Sun Mar 03, 2013 7:28 am Reply with quote

There is nothing more effective in showing others their limitations than a phrase like

"from my basic quantum mechanics"

As far as I'm concerned ALL quantum mechanics is "advanced" stuff that I can barely grasp the outline of, and the idea that is has a "Basic" or "simplified" form which is above this level just depresses me!

:-)

PDR

 
Jenny
978356.  Sun Mar 03, 2013 1:51 pm Reply with quote

I learned everything I know about thermodynamics from Flanders and Swann.

 

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