London forces between benzene molecules

In summary, the conversation discusses the interaction of benzene molecules with London forces. The large surface area and pi-ring of the molecules allow for strong London forces, resulting in the formation of pi-stacks. A benzene dimer has a binding energy of -1.8 kcal/mol due to these forces. The strength of this interaction is compared to other compounds, such as hydrogen bonds. The conversation also mentions the potential properties of a polymer made of benzene rings, and the influence of entropy on the stability of these stacks. It is noted that the strength of London forces depends on the electronic structure of the molecules involved and can be studied using potential energy models.
  • #1
espen180
834
2
How easily do benzene molecules interact with london forces?

I imagine that the relatively large surface area of the aromatic ring coupled with the pi-ring, which is a pretty large orbital makes london forces of a pretty large magnitude possible, but I am not certain.

Does anyone have any insight into this?

Thanks in advance.
 
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  • #2
Well, I'm not sure what kind of answer you expect to 'how easily'... They do certainly interact via london forces, just like everything else.

Being nonpolar (and as you said, pi-conjugated) molecules, they certainly do interact, forming nice little stacks (cutely named pi-stacks!)
I guess you could put it this way: You can't get an very useful description of the physical chemical properties of benzene without taking into account dispersion forces.

And since I happen to have a relevant paper right here on my desk, here are some facts and figures:
A benzene dimer with two benzenes stacked directly on top of each other has a binding energy (due to London forces) of -1.8 kcal/mol at optimal distance, which is 3.70 Å.
 
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  • #3
Thank you for your answer. I guess the wording "How easily..." was flawed. What I had in mind was information about how stongly they interact relative to other compunds.

I found the figures you supplied to be quite ammusing, as I immagine a polymer of suck rings. I wonder what properties such a polymer would have?
 
  • #4
espen180 said:
I found the figures you supplied to be quite ammusing, as I immagine a polymer of suck rings. I wonder what properties such a polymer would have?

Well, it's not a very strong force keeping it together really.
For comparison (which I should've given earlier), a single hydrogen bond is typically ~5 kcal/mol.

I doubt you could make a really big stack, given the low energy involved, the forces of entropy would overcome it pretty quickly.
 
  • #5
It would have the property of a very low temperature thermotropic liquid crystal, very likely of the http://en.wikipedia.org/wiki/Liquid_crystal" type.
 
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  • #6
The Hydrogen atoms in Benzene molecule are inactiv and usualy they are involved only in Electrophylic aromatic substitution mechanism. But uper or down Benzene ring electrons act all together and make some Mettalocenes sandwich aromatic compounds as
Chromocene Cr(CH6)2.
 
  • #7
The interaction between molecules related to London forces is weak. But, the quantitative London interaction depends on the electronic structure of the molecules involved, i.e., for the simpler case, London forces depend on the ionization energy of the atoms.

To study these interactions one should introduce a potential energy model like the simple point charge model or any of its modifications.

Interesting question.
 

Related to London forces between benzene molecules

1. What are London forces between benzene molecules?

London forces, also known as dispersion forces, are a type of intermolecular force that exists between non-polar molecules, such as benzene. They are caused by temporary dipoles that arise due to the constant motion of electrons in the molecule, creating an attractive force between neighboring molecules.

2. How do London forces affect the properties of benzene?

London forces play a significant role in determining the physical properties of benzene, such as its boiling point and melting point. The strength of these forces increases with the size of the molecule, so larger benzene molecules will have stronger London forces and therefore higher boiling and melting points.

3. Can London forces be stronger than other intermolecular forces?

In general, London forces are the weakest type of intermolecular force. However, in certain cases, they can be the dominant force between molecules. This is especially true for non-polar molecules, as they do not have any other intermolecular forces (such as dipole-dipole or hydrogen bonding) to compete with.

4. How do London forces contribute to the solubility of benzene?

Since London forces are present in all molecules, they also play a role in the solubility of benzene in different solvents. Non-polar solvents, such as hexane, have similar London forces as benzene, making them miscible. However, polar solvents, such as water, do not have strong enough London forces to overcome the strong intermolecular forces in benzene, making it insoluble.

5. Can London forces be broken?

London forces are considered to be weak intermolecular forces, and they can be broken by increasing the temperature or applying pressure. This is because increasing the energy of the molecules can overcome the attractive forces between them. Additionally, polar molecules or ions can also disrupt London forces by inducing temporary dipoles in non-polar molecules.

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