Science Makes Sense-Week35:Organic Chemistry,Benzenoid aromatic compounds

Every year my mother would clean her silk saris by washing them in mild soap, getting them ironed and stack them in the cupboard with a few moth balls. So every time I would open the cupboard to wear her saris I would smell the odor of moth balls and hope it did not cling to me! Moth balls originally contained naphthalene which is a benzenoid compound and we shall look at a few of these today.
Benzenoid compounds contain the benzene ring and last time we had a chance to look at the simplest of these, benzene. The structure of benzene was discovered almost simultaneously by Loschmidt and Kekule around the mid-19th century,(both German scientists) as a six-membered cyclic hydrocarbon containing 6 carbon and 6 hydrogen atoms with alternating double bonds. But it was Kekule who modified the structure to one where the oscillation of the double bonds gave two equivalent structures in rapid equilibrium. Then in 1931, Linus Pauling suggested that benzene had a single structure that was a resonance hybrid of the two Kekule structures.(Ref.1)
Benzene is a colorless volatile liquid with a characteristic strong odor and is the starting point for the manufacture of many aromatic compounds including aniline, phenols.(Ref.1)
Modern bonding models (valence bonding and molecular orbital theory) explain the stability of the benzene due to the delocalization of the six electrons. This delocalization refers to the attraction of an electron by all six carbon atoms instead of one or two, making benzene very stable. Unlike other unsaturated hydrocarbons, the hydrogenation and oxidation of benzene is a slow process. Most reactions of benzene are electrophilic aromatic substitution where the ring is intact and replacement of hydrogen occurs. Such reactions are important in forming derivatives of benzene.(Ref.1)
Benzene is present/manufactured in oil refineries, rubber and shoe manufacturing as well as in gas stations and firefighting.(Ref.2). Benzene has been documented as a cancer-causing agent and the results of exposure are mentioned in Nuggets.
The next aromatic compound we will study is toluene, also known as methyl benzene where one of the hydrogen atoms is replaced by a methyl(CH3)group. The odor is similar to benzene and it is usually a stable compound unless subjected to high temperatures. It is one of the by-products of gasoline production, but most of the benzene is manufactured starting with toluene. Toluene is used in the manufacture of adhesives, rubber, laboratory paints, pesticides and pharmaceuticals.(Ref.3)
The third set of aromatic compounds are called phenols. When a hydroxyl group, -OH group is attached to a carbon atom of a benzenoid ring, the compound is called a phenol. Phenols are similar to alcohols; however the hydrogen bond is stronger so they are more soluble in water than alcohols and have a higher boiling point. They are usually colorless liquids or white solids and are toxic and caustic. Household products that contain phenols include disinfectants and mouthwash. In industry, phenol is the starting point for plastics, explosives,photography, dyes and aspirin.(Ref.4)
Finally we will look at the chemical composition of moth balls, another benzenoid compound. Today we use dichloro-benzene , but years ago naphthalene was used in the manufacture of moth balls. 2 fused rings of benzene comprise naphthalene with the formula C10H8. Naphthalene is a white, solid crystal and has a strong odor of coal tar. It can undergo electrophilic aromatic substitution. It can be hydrogenated partially to form 1,2,3,4 tetra hydro-naphthalene or completely to form decalin, C10H18. Both are used as low volatile solvents. Naphthalene is used as a fumigant/ pesticide mainly as well as in the production of dyes.(Ref.5)
We have looked at some of the benzenoid compounds; we use them in our everyday lives. Look at Nuggets to realize that some of these compounds are carcinogenic, some are explosives and some are also medicines. However, all are essential in our lives today.
Activities for Middle School Teachers:
Benzene has 3 different positions on the 6 carbon atoms: Immediately adjacent to the first carbon atom is called the ortho position, next to it is the meta position and directly opposite to it is the para position. Create the basic structure of benzene using kits available and then substitute different methyl groups in the different positions. Name them. Then change the hydrogen atom in the first position and insert an amine or hydroxyl group and repeat the process. Find out if such benzenoid compounds exist and what their names and uses are.
Also create structure of aspirin and salicylic acid. What is the latter used for?
Build structures for other pain relievers like ibuprofin and tylenol; find out their chemical/generic names as well.
Nuggets of Information:
Most of the benzene was originally obtained from coal tar, but from 1950, it has been derived from petroleum-based processes. More than half the benzene is converted to polystyrene, a common plastic. The steps to the conversion as as follows: benzene to ethyl benzene to styrene and then the polymer polystyrene.(Ref.1)
Benzene is one of the twenty most widely used chemicals in the United States. It is used mainly to manufacture plastics, lubricants, rubbers, dyes, detergents, drugs, and pesticides. You could breathe or ingest this aromatic compound easily in such environments.(Ref.2)
Benzene is present in cigarette smoke; in the presence of smokers we inhale benzene vapor. In fact, cigarette smoking causes half the exposure to benzene in the U.S.(Ref.2)
Benzene is a known carcinogen and workers who are constantly exposed to it are at a higher risk in developing leukemia and related cancers.(Ref.2)
Toluene derived its name from a tree in Colombia which has an aromatic extract called tolu balsam.(Ref.3)
TNT or tri-nitro-toluene is a known explosive and is made from nitric acid and toluene.(Ref.3)
Toluene is also used as an octane booster in gasoline. In biochemistry experiments toluene is used to rupture red blood cells for hemoglobin extraction.(Ref.3)
Phenols were the first surgical antiseptic used in 1865. Surgical amputations fell from 45% to 15% after using phenols. However, they were toxic as well.
A less toxic phenol like resorcinol, that contains two hydroxyl groups is often found in cough drops.(Ref.4)
Picric acid, which is trinitro phenol is used as a yellow dye as well as an explosive like TNT.(Ref.6)
Trace amounts of naphthalene are found in magnolias and certain kinds of deer.(Ref.5)
Naphthalene is considered to be a carcinogen; excessive inhalation or ingestion could cause hemolytic anemia.(Ref.7) Hemolytic anemia is caused by the destruction of red blood cells in the body.(Ref.8)
Aspirin, which is also known as methyl salicylate or 2 acetoxy-benzoic acid is an analgesic and is a non-steroidal anti-inflammatory agent. It is used usually for minor headaches and pains. Aspirin exhibits anti-coagulant properties(Ref.9) and therefore small doses are recommended every day to prevent heart attacks for some patients. It is an aromatic benzenoid compound.
Aniline, which is benzene with a hydrogen atom replaced by an amine _NH2 group is essential in the manufacture of dyes. It is responsible for the indigo color in blue jeans. Aniline is also used to make polymers like polyurethane, fungicides and herbicides. It is also used to make additives in rubber.(Ref.10)


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