Benzene – Aromatic Compounds and Eas

00:00 So, all carbon-carbon bonds are actually an intermediate between a single and a double bond. If you recall when we were talking about mesomeric effects, right back at the first module, we were talking about the ability of a double bond to resonate throughout a particular structure where more than one double bond exists.

00:20 As you can see from the diagram shown here at the bottom, you can see that we… that it is possible for the double bonds to move around in almost snake-like fashion. This is due to the fact that the real structure of benzene is a hybrid between these two resonant forms; a standard carbon-carbon double bond is shown here as 1.34 angstroms, a standard carbon-carbon single bond is shown as having a bond length of 1.5 angstroms. However, the half way house length in benzene is 1.39 angstroms, somewhere between the two, suggesting it is not quite as long as a carbon-carbon alkane, but not nearly as short as the conventional carbon-carbon double bond.

01:05 So, the problem is that once these two structures are effectively degenerate, you may have recalled that phrase when we were talking about orbitals because you can superimpose them, they are also equivalent to make equal contributions to the hybrid. However, the benzene structure is badly represented in fairness by either of these two examples structure one or two.

01:27 It actually has a structure which is an intermediate between the two being more stable than both of them. And so, what you will sometimes see is a benzene ring… benzene ring being abbreviated as effectively a hexagon with a circle in the center such as that shown here on the board.

01:43 The reality when we actually look at the orbital system for benzene shouldn’t come as a huge surprise. As you can recall from hybridisation, you should be aware that SP2 hybridisation correlates to the hybridisation of 1S and 2P orbitals. This results in a single P orbital being unhybridised. And as you can see here, each carbon on our benzene ring has that P orbital which has been unhybridised… not been hybridised.

02:14 And what this effectively allows to happen is the electron in each of those is able to delocalise around the entire ring structure such as that which is shown on the diagram in the top corner. As all of the carbon atoms are SP2 hybridised, you effectively end up with something which is a planar model… molecule with bond angles of 120 degrees.

02:42 Contrast that, if you will, with the actual structure and geometry of a cyclohexyl ring.

02:49 We haven’t necessarily covered that geometry, but look it up. You will see it exists as a chair or boat or somewhere in between, something that forces the carbons and hydrogens to be as far apart as possible to minimise steric interference. Benzene rings, on the other hand, are planar.