Benzene Derivatives
Monosubstituted Benzene Derivatives
There are several common names of benzene derivatives that you should know.
Disubstituted Benzene Derivatives
Ortho, Meta, Para
When two groups are attached to a benzene ring, there is a common way to name them using the prefixes ortho, meta, and para. The way to remember the order of the prefixes is to remember “ROMP”. Let’s look at a generic R group on a phenyl ring. The carbon atom attached to the R group is called the ipso carbon atom. The carbon atom next door to the R group, the 2nd carbon atom, is the ortho carbon atom. The 3rd carbon atom is meta, and the 4th carbon atom is para. This spells “ROMP” around the ring.
There are some common disubstituted benzenes that you should know, toluic acid, cresol, and xylene.
Benzene groups as substituents
Benzene rings groups can be named as substituents. A benzene ring attachment is called a phenyl group. A group with a CH2 followed by a benzene is called a benzyl group.
8. Name the following compounds using the common prefix (ortho, meta, para) when appropriate.
a)
b)
c)
d)
e)
f)
g)
h)
i)
Polyaromatic hydrocarbons
Polyaromatic hydrocarbons are aromatic compounds that are benzene rings fused together. Some simple ones are:
Polyaromatic hydrocarbonds, PAHs
Naphthalene smells like mothballs. This is for good reason. Mothballs are made of naphthalene!
Larger fused benzene ring systems are often called polyaromatic hydrocarbons (PAHs). PAHs are implicated as cancer causing agents. PAHs are produced when substances like cigarettes or meat are burned. The tasty, burned edges of meat on the grill contain these. For that matter, the bits of charred material left on the grill contain PAHs. It is believed that PAHs actually become cancer-causing agents when they are oxidized into epoxides in the liver. These epoxides are electrophiles that can be attacked by nitrogen bases on our DNA. These PAHs then stick to our DNA causing a mutation.
Allotropes of Carbon
Imagine a fused ring system that is not only four or five benzenes fused together, but an infinite number of benzene rings fused together into an infinite place. Then, imagine several of these infinite planes stacked on top of each other. These planes would be made out of only carbon since it has no edges where the hydrogens can be. This stacked group of infinite planes of fused benzenes is called graphite.
Graphite
Each carbon atom of this infinite plane has an unhybridized p-orbital on it that can overlap with the others. Electrons are shared in the molecular orbitals formed by these p-orbitals giving a plane where electrons can freely move around. These p-orbitals cause the layers to loosely stack on each other. Here is a set of three benzenes stacked on top of each other.
Pi stacking
Because of this loose stacking, graphite us used as pencil “lead”. It is called pencil lead because long ago, lead metal was used to make the dark marks on paper. But, since lead is toxic, the nontoxic compound graphite is used. As the graphite is rubbed on paper, the bottom layer of graphite easily shaves off. The graphite is deposited one layer at a time onto the paper.
Graphite is one of the several orderly forms of pure carbon. These are called allotropes of carbon.
Another allotrope of carbon is diamond. The carbon atoms in diamond are all sp3 hybridized. A diamond structure is a series of cyclohexane chairs stacked on top of each other. Because each carbon atom in diamond is a perfect tetrahedral, diamond has tremendous strength. Of course, it is one of the hardest materials.
Structure of diamond
For many years, these were thought to be the only allotropes of carbon. But, in the 1980s, scientists discovered another allotrope of carbon. These are the fullerenes. Fullerenes are 5- and 6- membered aromatic rings arranged to form a “soccer ball” structure. The most common fullerene is C60. This is a molecule that contains 60 carbon atoms and zero hydrogen atoms. It was named the Molecule of the Year in 1990 by the journal “Science”. Yes, believe it, there is such a thing as the Molecule of the Year
C60, Buckyball
Even more recently, another form of all carbon is being developed, the nanotubes. You can think of carbon nanotubes as one layer of graphite wrapped into a cylinder. Nanotubes are incredibly strong. Nanotubes are nearly 120 times stronger than steel. One hope is to eventually make extremely strong, lightweight cables out of carbon nanotubes.
Carbon nanotube