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In most high school chemistry books, it is said that the delocalised electrons in benzene makes it particularly stable. However, how does it contribute to stability? Is there any simple way of understanding this?

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In physics (or you can google it) you will learn about the particle in a box experiment. At the atomic level, if you confine a particle to a box, it will have some energy. It turns out, if you make the box longer, the energy of the particle will be lowered, if you make the box smaller, the particle will have a higher energy. The same concept applies to benzene. Basically you start with 3 separate acetylene molecules in which the pi electrons are confined to a 2 carbon segment. Once you trimerize the acetylene and form benzene, the pi electrons can now roam over 6 carbon atoms, a much longer "box" if you will. So the pi electrons in benzene will be stabilized (lower energy, longer box) compared to the pi electrons in the starting acetylene (higher energy, shorter box). Molecular Orbital theory tells us the same thing in a slightly different manner. In each acetylene (let's just focus on the 2 pi electrons in the acetylene that will be the pi electrons in benzene) we have 2 pi electrons in the HOMO and none in the LUMO. As we bring these 3 acetylenes closer together to form benzene, the orbitals interact. The 3 initially degenerate HOMOs in the 3 acetylene molecules split (they are no longer degenerate) as they move closer to becoming benzene and a net energetic stabilization results.

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For high schoolers, this is how I explain it. Remind them, that stability = less reactive or more set in place.... i.e. the electrons aren't going to go wondering off to another atom.

In our scenario... the carbons are moms/dads and the extra electrons are excitable little children who are prone to run off (think of little preschool brothers/sisters/cousins at a family reunion in a park). Each individual mom/dad would be spending all their energy keeping their energized little kid from running off. In the same way each carbon has trouble holding on to the extra energy electrons.... again, just as parents have a hard time holding their excitable children in place.

However, these carbons have linked up and are supporting each other and their extra energy babies (i.e. delocalized electrons) who have to move so they zip around in this ring of carbons. Just like energized children zip around but stay in the safe part of the park when there are extra adults looking out for them and giving them physical safety boundaries. By linking up, each of the carbons share the extra load of holding on to the extra electrons. This stabilizes the whole group meaning the extra energy electrons won't go running off i.e. try to react with another molecule. The electrons are happy and content staying within the ring of carbons. AND (for my AP students, this keeps the carbons happy having the 4 bonds it needs without having to have a higher electronegativity number to keep them bonded; and how the stability of benzene makes it hydrophobic because it prevents the hydrogen bonding in water...)

I also remind them that if the double bonds in benzene were just double bonds, there would be a complete pi bond between two of the carbon atoms - i.e. these two carbons would own/hold the extra bonding energy and covalently share it. But with a benzene ring, it is more like a co-op of carbons sharing the extra electron load and that is why we say that in reality it isn't 3 single and 3 double bonds in the ring, but more like (6) one and a half bonds. Then we talk about bond lengths a little bit - comparing bond lengths of a single, double, and then a benzene ring bond.

AND (for my AP students, we talk about how conjugation stabilizes other organic molecules like antioxidants, etc.)

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  • $\begingroup$ The hydrophobicity of benzene doesn't have anything to do with delocalization. It's just do to with the fact that the bonds are essentially non-polar. $\endgroup$ – bon Sep 26 '17 at 10:38
  • $\begingroup$ But you go from two parents with two children to six parents with six children. The number of children per parents has not changed. How does this make it more stable? $\endgroup$ – Jan Sep 26 '17 at 11:16
  • $\begingroup$ Non-polarity alone does not account for the hydrophobicity of benzene; it is markedly more hydrophobic than cyclohexane, for instance. $\endgroup$ – user41033 Sep 26 '17 at 11:53
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    $\begingroup$ A chemical reaction is a kidnapping then? The analogy is terrible, unscientific and probably much harder to understand than introducing a little bit of molecular orbital theory. It's explanations like this, that lead to an underdeveloped abstract thinking and to serious misunderstandings in more advanced courses. $\endgroup$ – Martin - マーチン Sep 29 '17 at 11:06

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