Taking a Ride in a Benzene
Something really nifty happens when single and double bonds alternate within a molecule. They start to kind of bleed together, forming a planar region within the molecule where the atoms have a triangular geometry and the bonds behave more like "one-and-a-half" bonds than strictly single or double bonds. Alternating single and double bonds are called conjugated. This is one example of when a single bond isn't totally free to rotate; if it can rotate at all, it is under strong pressure to remain planar with the double bonds. The quintessential example of a conjugated molecule is benzene:
Even though benzene is usually depicted with single and double bonds, in practice the bonds are constantly switching places so fast that the whole ring acts as one homogenous entity. It is perfectly planar. Even the hydrogen atoms are coplanar with the ring.
Conjugated molecules have some interesting properties. Two of the useful things they do are they conduct electricity and they absorb light. Conductive polymers are made from long strands of conjugated atoms, or from stacks of conjugated rings. These are used to make OLEDs (organic light emitting diodes), among other applications.
Light absorption happens when a photon strikes a molecule and kicks one of its electrons into a higher energy level. For alkanes, this energy level is very high, so it takes an energetic photon with a short wavelength to provide the electron enough energy. But conjugation has an effect on the energy levels, requiring less powerful a photon to excite the molecule. Cyclohexane absorbs only light with wavelengths below about 180 nanometers, or billionths of a meter. That's so far into the ultraviolet that even air and water are opaque. Cyclohexene, with one double bond, absorbs below 215 nanometers, still a really short wavelength but a noticeable effect for just one bond. Benzene starts to absorb at 280 nanometers, which is similar to ozone. The more conjugated bonds there are in a molecule, the longer the wavelength it will absorb. Every organic dye, every food coloring, every natural plant or animal pigment, all of these colorants work because of conjugation. Even the vitamin A in our eyes that detects light does so by means of conjugation.
Benzene's conjugation in fact meets an even higher classification, called aromaticity. An aromatic compound doesn't necessarily have a smell, but the phenomenon is so named because many compounds with aromatic rings have characteristic smells. Aromatic rings are more stable than would be expected; it takes much more energy to break open or oxidize a benzene ring than a cyclohexane ring. Consequently, the vast majority of reactions involving aromatic rings have to do with substituting them or changing their existing substituents. Very few reactions can remove aromaticity from a molecule. Another feature of aromatic rings is they can stack together. Two benzene rings can stack like pizza boxes, although they tend to prefer to be slightly offset so that a hydrogen of one ring is directly over a carbon of the other. They can also stack in a T shape, with a hydogen of one ring pointing directly at the center of the other. This is called pi stacking. There's a subatomic physics reason for the name. Ordinary alkenes and their derivatives can also participate in pi stacking, though not as strongly as aromatic rings can.
Substituents are possible on all of benzene's ring atoms. Some of the singly substituted benzenes have their own names:
toluene |
aniline |
phenol |
benzyl alcohol |
cresol |
When there are two substituents, IUPAC conventions determine which constituent is considered to be on atom 1, and the other atoms are counted in the direction that gives the other substituents the lowest numbers. For example, the cresol molecule shown above can also be called 4-methyl phenol. A less formal way to indicate the positions of the two groups is with a dedicated set of prefixes:
ortho-cresol |
meta-cresol |
para-cresol |
ortho-xylene |
meta-xylene |
para-xylene |
para-dichlorobenzene is sometimes used to repel moths,
however it is mildly carcinogenic.
The three prefixes are often abbreviated o-, m-, and p-.
Some compounds always occur as only one of the isomers,
such as salicylic acid ,
which is always the o- form.
Some disubstituted benzenes have different names for the three isomers,
for example phthalic acid which is usually the o- isomer
while p-phthalic acid is called terephthalic acid.