Here we explore the chemical structure and properties of this "wonder chemical" and uncover some interesting facts about it.
What is benzene?
Benzene, a molecule with the chemical structure of C6H6, has a molar mass of just over 78 grams/mole. At room temperature, Benzene tends to exist in a liquid state. It is colorless and has a very distinctive gasoline-like odor.
Benzene is soluble in water and has a melting point of 5.33 degrees Celsius. It also has a boiling point of 80.1 degrees Celsius.
Benzene is also highly flammable. This chemical is carcinogenic and tends to be used as an additive to gasoline, plastics, synthetic rubbers, dyes, and industrial solvents.
It is also the parent compound for numerous other aromatic compounds.
But its most interesting feature is its characteristic hexagon ring-like structure of six carbon atoms with associated hydrogen bonds at regular intervals. This structure helps to make the molecule incredibly stable.
It is also technically termed an aromatic compound, which are flat (planar) rings of atoms joined by covalent bonds.
Benzene contains delocalized pi-electron clouds instead of discrete alternating single and double bonds. The six pi-electrons occupy a region above and below the plane of the ring. Each electron is shared by all six carbons, which maximizes the force of attraction between the nuclei (positive) and the electrons (negative).
Double (C=C) bonds are generally shorter than single carbon to carbon bonds. However, all six carbon-carbon bonds have the same length in benzene, which is intermediate between that of a single and that of a double bond.
In a cyclic molecule with three alternating double bonds, the bond length of the single bond would be 154 picometers, and that of the double bond would be 134 picometers. X-ray diffraction of Benzene has shown this is not the case. In fact, each bond has a length of roughly 140 picometers (0.14 nm) — which is intermediate between C-C and C=C bond lengths.
"Benzene itself has a molecular shape of Trigonal planar, which means that the molecule exists in a triangular shape across a single plane in space," according to benzeneproject.weebly.com.
How was the structure of benzene discovered?
Benzene, in and of itself, was first isolated by the British scientist Michael Faraday in 1825 when investigating illuminating gases. It was first created artificially by a German scientist Eilhardt Mitscherlich in 1833.
He did this by heating benzoic acid with lime. Benzene was first isolated from coal tar by another German scientist, A W. von Hoffman, in 1845.
Its actual structure would take a little longer to decipher.
"German chemists Joseph Loschmidt (in 1861) and August Kekule von Stradonitz (in 1866) independently proposed a cyclic arrangement of six carbons with alternating single and double bonds. Kekule subsequently modified his structural formula to one in which oscillation of the double bonds gave two equivalent structures in rapid equilibrium.
How is benzene formed?
The benzene that people use or come into contact with has been created artificially. Traditional methods involved light oil distillation produced through the manufacture of coke.
Today, most benzene is formed from the catalytic reforming of naptha, or steam cracking of liquid petroleum and the hydrodealkylation (HDA) of toluene, and toluene disproportionation (TDP).
"In Europe, benzene is obtained primarily from pyrolysis gasoline coproduced in the steam cracking of naphtha, gas-oil or condensates to make olefins. The [number] of aromatics produced can be increased by employing heavier feedstocks (raw materials not edible).
In the US, catalytic reforming is a major source of benzene. Naphtha is mixed with hydrogen and fed into a reactor containing a catalyst and operating at 425-530oC and 7-35 bar. An aromatic-rich fraction is separated from the reformate," according to icis.com.
In either case, any benzene that forms needs to be recovered from the resultant aromatic products. "Methods include solvent extraction, extractive or azeotropic distillation, solid adsorption using molecular sieves, and crystallization," notes icis.com.
"A growing source of benzene is by the selective disproportionation of toluene where benzene is coproduced in the manufacture of a paraxylene-rich xylenes stream. The gasoline pool is also an increasing source as stricter gasoline regulations limit the benzene/aromatics content of gasoline," states icis.com.
Yet another method of creating benzene is the previously mentioned hydrodealkylation of toluene. This uses either catalytic or thermal processes.
HDA plants of this kind are usually used to maintain the balance of benzene supply when other sources are at capacity.
A newer process called Cyclar has also been developed. This process converts butanes and propane into aromatics and was developed by BP and UOP. To date, only one plant in Saudi Arabia has been built using the new technology.
Benzene can actually be formed naturally and can be a byproduct of processes like fires and volcanic eruptions. It is also a common component of cigarette smoke.
Small amounts of benzene can be disposed of in a variety of ways, but this usually involves mixing it with sand or dirt in order to dispose of it safely and cleanly. Other methods include combusting.
For disposal of large quantities, Benzene tends to be atomized in a large combustion chamber. Atomization is the process of converting larger molecules into their constituent atomic parts. This makes disposal easier and safer.
Some more interesting facts about Benzene
We've already covered some interesting facts about benzene above, but here are some more:
1. In 1903, a German coffee merchant, Ludwig Roselius, used benzene to decaffeinate coffee. Since we've come to learn that benzene is carcinogenic, this use for benzene is thankfully no longer the case.
2. In the late 19th and early 20th centuries, benzene was actually used as an ingredient for aftershave because of its aroma. However, much like coffee decaffeination, it is no longer used for this purpose.
3. Benzene has many uses in its pure form, but over 80% of benzene produced is used for making other chemicals. Of these, the vast majority include ethylbenzene, cumene, cyclohexane, nitrobenzene, and alkylbenzene.
4. Benzene is one of the most-used chemicals in the world. In the United States, it ranks in the top 20 of chemicals produced by volume.
5. As benzene tends to be a byproduct of various combustion cycles, you are exposed to a small amount of it every time you go outside. Sources include cigarette smoke, gas stations, car exhausts, and other industrial emissions.
6. Many products inside your home also contain benzene to some degree. Glues, paints, furniture wax and detergents regularly have benzene as an ingredient.
7. Severe exposure to benzene can prevent cells from working properly. It can, for example, cause bone marrow to stop producing enough red blood cells.
8. Benzene readily evaporates at room temperature. Its vapor form is also heavier than air, and so it tends to pool in low-lying areas.
9. While Benzene is soluble in water, it is less dense and so tends to form a layer on top of the water.
10. Very low levels of benzene have been detected in foodstuffs like fruit, vegetables, nuts, dairy products, eggs, and fish. However, this tends to be pretty safe. Somewhat higher levels have been found in some processed foods.
11. Long-term exposure to benzene can lead to blood-related cancers like Leukemia. It can enter the body either through direct absorption via the skin, through inhalation, or consumption.