Benzene

Basic Facts
  • Brief Description: Benzene is most often found in liquid form which has no color and a sweet smell. It's highly flammable, evaporates quickly in air, dissolves slightly in water and is formed both naturally and synthetically.
  • Chemical Formula: C6H6
  • Molecular Weight: 78.11 g/mol
  • Structural Formula:
         H        H
          \      / 
           C -- C
          //    \\
    H -- C        C -- H
          \      /
           C == C
          /      \
         H        H
    
The Structure and History of Benzene

Benzene was first isolated from whale blubber in the 1800s and while its formula was known, its structure was not. It was also noted to be remarkably stable, only undergoing standard reactions like the Diels-Alder or halogenation under very harsh or specific conditions. All of the carbons in the atom appear to be identical. Possibilities for the structure included:

"Dewar" Benzene (bicyclo[2.2.0]hexa-2,5-diene)
________
||___|___||

Benzvalene (tricyclo[3.1.0.02,6]hex-3-ene)
"Ladenburg" Benzene or Prismane (tetracyclo[2.2.0.02,6.03,5]hexane)
3,3'-Bicyclopropenyl
- first made in 1989 by W.E. Billups' research group at Rice University

Friedrich August Kekulé von Stradonitz, a German Chemist, suggested the ring structure that we think of as benzene today in the mid-1800s which is more rigorously known as 1,3,5-cyclohexatriene. One story that's been passed down is that Kekulé was inspired by a dream he had in which a snake was biting its own tail and afterwards he was able to imagine chains of carbon atoms behaving similarly. Other chemists who also deduced ring structures for benzene but don't receive the credit are Alexandr M. Butlerov and Johann Josef Loschmidt.

The problem with the 1,3,5-cyclohexatriene structure is its inability to explain benzene's stability and later when it was shown that all bonds in benzene have equal lengths this plagued the proposed structure even more. It would be expected in 1,3,5-cyclohexatriene that the double bonds would be 1.33 angstroms long and the single bonds 1.47 angstroms long, but all of the bonds are 1.39 angstroms long. In order to explain this you have to take a second look at the structure and think in terms of resonance. You can draw the ring structure in two different resonance forms:

     H        H
      \      / 
       C -- C
      //    \\
H -- C        C -- H
      \      /
       C == C
      /      \
     H        H

     H        H
      \      / 
       C == C
      /      \
H -- C        C -- H
      \\    //
       C -- C
      /      \
     H        H

Combining these two resonance forms lets you think of the bonds between the six 2p orbitals of benzene as two continuous rings of electron density, one above the ring and one below the ring. It is these rings that stabilize benzene and make it so unreactive. This stability is known as aromatic character or aromaticity and to enforce the concept of the ring it is often drawn like this:

     H        H
      \      / 
       C -- C
      /      \
H -- C    O   C -- H
      \      /
       C -- C
      /      \
     H        H
Benzene's Uses and Effects

Benzene is one of the top 20 chemicals produced by volume in the United States today. It's used to synthesize other chemicals and in the making of things like plastics, nylon and other synthetic fibers, rubbers, drugs and pesticides. You can find it in crude oil, cigarette smoke and gasoline. Volcanoes and forest fires naturally produce benzene. Most of the benzene found in the environment is from the industrial sources. It can pass into the air from water and soil, where it reacts with other chemicals and within the next few days it will break down or attaches to water in various states in the air and finds its way back into the soil. While it does not build up in plants or animals, it does build up and break down slowly in soil and can pass through it into ground water.

Benzene exposure can come from both water and air. The most common source of benzene exposure is tobacco smoke but it can also be found outdoors in engine exhaust and industrial emissions, inside from paints, glues and detergents and in water drawn from a well contaminated with industrial waste. When large amounts of benzene are inhaled it can cause drowsiness, headaches, confusion, tremors, dizziness, rapid heart rate and in larger doses unconsciousness and death. Ingesting large amounts of benzene can cause vomiting and stomach irritation, drowsiness, convulsions, rapid heart rate and death. Long term exposure for a year or more harms the bone marrow, decreases the amount of red blood cells and can cause excessive bleeding and weaken the immune system. In pregnant animals is can cause low birth weights, delayed bone formation and bone marrow damage to the fetus. While similar effects haven't been confirmed in humans, menstrual periods become irregular and ovaries decrease in size. Benzene is a known carcinogen and has been confirmed to cause leukemia.

Sources:
ATSDR - ToxFAQs: Benzene. ATSDR. 10 May 2004 <http://www.atsdr.cdc.gov/tfacts3.html>.
Jones, Maitland Jr. Organic Chemistry. New York: W.W. Norton & Company, Inc., 2000.