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What Are Aromatic Hydrocarbons?

Once upon a time, smell was one of a chemist's best tools. Some chemists even went so far as to

taste their chemicals! Some people think that the Swedish chemist Carl Scheele died because he

happened to taste an especially poisonous chemical while working alone in his lab. Robert Bunsen,best known for his development of the Bunsen burner, was partial to smelling the noxious arsenic

compounds he worked with. Just the smell of these compounds would make his hands and feet

tingle, and at least once the smell turned his tongue black!

It was during this era that chemists started using the term aromatic to describe certain carbon-based

compounds with distinct odors. Thanks to the odors of compounds, like toluene and benzene (which

apparently smell sweet), certain compounds made primarily of carbon and hydrogen are now

calledaromatic hydrocarbons.

The definition of aromatic hydrocarbon is quite specific. Generally speaking, aromatic hydrocarbons

are especially stable, unsaturated cyclic compounds made primarily of hydrogen and carbon atoms.

Let's take a look at what it means when a compound is especially stable. I bet you already have a

good idea.

Check out these three hydrocarbon structures. Each structure contains six carbons. Which one do

you think is more stable (meaning harder to break) A, B or C? Why do you think so?

If you said C is more stable, well done! What made you think so? Was it the presence of double

bonds? Or that it is a ringed compound? It turns out that both factor into the stability of aromatic

hydrocarbons.

Just to review, we refer to ringed structures sometimes made of more than one ring ascycliccompounds. When a hydrocarbon contains one or more double bonds, and therefore, does not

contain the maximum number of hydrogens it is known to beunsaturated.

The unsaturated quality of aromatic hydrocarbons is special. The double bonds in these compounds

are not fixed in one place; the electrons making up the double bonds are delocalized and can move

from parent atom to parent atom. This delocalization happens in a rather organized fashion, so that

double bonds are always alternating with single bonds. The phenomenon of delocalization or having

more than one acceptable bond structure is calledresonance. It lends to an average bond length in

between that of a single bond and double bond.

The cyclic nature of aromatic hydrocarbons allows these bonds to click and clack back and forth

between atoms that are all in the same plane. I say atoms instead of carbons because sometimes

the ring structure may include a nitrogen, oxygen or sulfur.

Before we move on, let's look at delocalization in a tiny bit more detail. The electron orbitals involved

in double bonds are called p orbitals. They extend at 90-degree angles from each atom in the ringed

structure. The p orbitals in aromatic compounds overlap, creating a cloud that allows the electrons to

move back and forth.

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Aromatic hydrocarbons are known to be hard to break in chemical reactions. Unlike their cousins the

aliphatic hydrocarbons, aromatics don't readily undergo addition reactions in which an element is

added to the structure.

Examples

Not that it's a popularity contest, but benzene is probably the most well-known aromatic hydrocarbon.

Benzene has the formula C6H6. It is a six-carbon ring; each carbon is bonded to two other carbons

and one hydrogen. There are three double bonds in benzene - but these bonds are not fixed in

place! The electrons in the bonds are delocalized and constantly are moving between the p orbitals

of neighboring carbon atoms.

The two different bond structures of benzene - together known as resonance structures - are often

shown together with a double-headed arrow between them. This double-headed arrow lets us know

that both of these structures are favorable and possible. Far more often, though, we see benzene

depicted as a hexagon with a circle in the middle. This circle lets us know that the double bonds are

moving between positions. Notice that there are no hydrogens drawn into this structure. It is

assumed that they are there.

Toluene is another example of an aromatic hydrocarbon. It's just like benzene, but with a methyl

(CH3) group attached in place of one of the carbons. The double bonds on the toluene ring are

constantly moving, so toluene is represented with a circle in the middle.

Naphthalene is an example of an aromatic compound made of two benzene rings fused together.

Anthracene is made of three benzene rings fused together.

Furan is an example of an aromatic hydrocarbon than contains an element other than carbon and

hydrogen. Oxygen makes up part of this five-sided ring.

If you've ever seen a structure like this before? Then you've been in biology! Where do you recognize

this from?

What about this one? Maybe DNA?

DNA is made of nucleotide bases that are in turn made up of aromatic hydrocarbons. Purines

(Guanine and Adenine) are made of two rings, one is hexagonal, and one is a pentagon. Notice that

there are four nitrogens that are part of the rings.

Pyrimidines (Cytosine and Thymine) are made of one hexagonal ring that contains two nitrogen

atoms.

Uses

Aromatic hydrocarbons are everywhere, literally. They occur naturally in compounds like DNA and

within some amino acids that make up proteins. Chlorophyll, a pigment used by plants to absorb

light, contains aromatic groups, and so do the same groups that help our blood cells carry oxygen.

The spicy compounds in hot peppers, ginger and black pepper are all aromatic compounds.

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