CH 106 - Lesson 2
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Structure

The molecular formula of benzene is C6H6. Its structure is very important, and I think in a way, elegant.

Benzene Ring

It has a six-carbon ring. Each of the carbons has sp2 hybridization. That gives it 120 degree bond angles all around the molecule. That makes it a perfect hexagon with a hydrogen atom sticking out from each point. Note that all these atoms are in the same plane. But that in itself is not what makes it special. Orbital diagram of benzene from top. [62orb12.JPG]
If we turn the molecule on its side you can see that the sp2 hybridization leaves each carbon atom with one half-filled p orbital. Each of these forms pi bonds to neighbors on both sides, and the pi bonding goes all the way around the molecule. Orbital diagram of benzene from the side showing p-orbitals. [62orb13.JPG]
There are no individual localized pi bonds, just a big super pi bond. It is actually called a delocalized pi bond to distinguish it from the pi bonds that are localized alongside two carbon atoms to form the double and triple bonds that we dealt with earlier in this lesson. This super or delocalized pi bond is unusually stable. It will not react with bromine like the localized pi bond that is found in alkenes. Another way of saying this is that because the pi bond is not localized between, or associated with, just two carbon atoms, we have not really formed a double bond as such. Because of this, the electrons are not concentrated enough or available enough to react with the bromine. Orbital diagram of benzene showing pi bonding rings. [62orb14.JPG]
This picture shows a close-up view of how the p orbitals overlap one another to form pi bonding all the way around the benzene molecule. The sigma bonds between adjacent carbon atoms are not shown in this picture. Orbital diagram of benzene showing overlapping p-orbials and pi bonding ring. [62orb15.JPG]

 

Resonance

Some chemists use a different approach to describing the bonding in benzene which they call resonance. I think it is unnecessary and I mention it primarily because you may run into it in other books or in other classes.

If you take a look at this diagram (or the one in Example 14a in your workbook) you can see one way that is sometimes used to show the bonding in benzene. This way is not really very good because it shows three double bonds and three single bonds between the carbon atoms. Those single and double bonds are just not there. Diagram of benzene with Cs, Hs and double bonds. [62benz01.JPG]
Quite often the benzene ring is shown as a hexagon with alternating single and double lines. Each corner represents a carbon atom with a single bond to a hydrogen atom, but neither the C nor the H are written. You may run into that symbol for the benzene ring quite often. Diagram of benzene as hexagon with double bonds. [62benz02.JPG]
This idea of resonance is indicated in this way. Notice that two diagrams are shown here. The only difference between them is where those double bonds are shown. At one time it was thought that the locations of the double bonds oscillated back and forth. Next it was thought that the benzene molecule is something like the double-bond arrangement on the left and something like the double-bond arrangement on the right, but more like a merging of both of these. The double-headed arrow is intended to indicate this. Diagram of benzene as two hexagons with double bonds in resonance. [ 62benz03.JPG]
I tend to make fun of resonance diagrams but many organic chemists have found them very useful in determining the location of the best reacting site for some kinds of reactions involving derivatives of benzene. Resonance diagrams helped make sense of a lot of organic reactions after they were introduced.
It was not until sometime later that people came up with the idea of the pi bonding going all the way around, rather than alternating back and forth. One way of showing this is shown here (and in Example 14c in your workbook). That dotted or dashed line going all the way around the inside of the hexagon is used to indicate that the pi bonding is going all the way around the molecule. Usually, it is easier to draw a continuous line than a dotted line, so the diagram to the right is the one you will quite frequently see associated with benzene. That symbol is the structural formula for benzene. Notice that it does not show any of the carbon or hydrogen atoms. You have to get used to realizing that each corner of the hexagon is a carbon atom with a hydrogen atom attached to it unless something else is shown. Diagrams of benzene as hexagons with circles in them. [62benz04.JPG]
Please note that the structure of benzene is the same whichever symbol is used to represent it.

 

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