Back To CoursePhysical Science Textbook
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Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.
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 called aromatic 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 as cyclic compounds. When a hydrocarbon contains one or more double bonds, and therefore, does not contain the maximum number of hydrogens, it is known to be unsaturated.
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 called resonance. 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.
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.
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 that 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 the one on the left before, then you've been in biology! Where do you recognize this from?
What about the one on the right? 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, 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.
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 heme groups that help our blood cells carry oxygen. The spicy compounds in hot peppers, ginger and black pepper are all aromatic compounds.
Not so spicy (but delicious) compounds like vanillin (artificial vanilla) and nutmeg are aromatic compounds that contain a phenol group. A phenol is a benzene ring with an -OH group attached in place of a hydrogen.
Many aromatic compounds are used as solvents to remove or thin out oil- or grease-based compounds. Toluene, for example, is an ingredient in paint thinners. Benzene is a gasoline additive that reduces knocking in engines.
Benzene and toluene are widely used to make other chemicals including dyes and plastic products. Plastic products made from benzene range from PVC pipes to the thin plastic used in packaging. That Styrofoam that's keeping your to-go food warm? It's made of aromatic compounds.
Aromatic compounds are abundant in medicines. Prozac, an antidepressant, is an aromatic compound.
Phenol and its derivatives have quite an interesting history. Mentioned previously, a phenol consists of a benzene ring with an -OH group attached in place of a hydrogen. Phenol was originally used by Joseph Lister to make antiseptic pastes in the mid to late 1800s. Though the paste worked to an extent, patients had to endure the toxicity of phenol as a side effect. Their skin cracked, turned white and became numb. Today, much safer derivatives of phenol are used in medicine for their antimicrobial properties.
The first plastics - Bakelite - were created from phenol by a man named Leo Baekeland in the early 1900s. Interestingly, Bakelite was first synthesized as an alternative to ivory to meet demands of billiards players. The fantastic properties of Bakelite launched it into many other uses, from buttons to furniture. Many modern plastics are still made from phenol derivatives.
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Back To CoursePhysical Science Textbook
33 chapters | 341 lessons | 1 flashcard set