Benzene is normally drawn as a hexagon with alternate single and double bonds. This while useful for understanding its chemical reactions it does not properly represent the reality.

Investigation shows that all 6 carbon-carbon bonds in benzene are identical in length and properties. This is due to the aromatic nature of Benzene where the electrons for the second bonds are delocalised into hybrid orbitals above and below the level of the flat benzene ring.


Benzene is also very useful for throwing over things and setting fire to them in Call of Cthulhu games. As in:

"Is that a bottle of benzene in your pocket or are you just glad to see me?"

This is how my character Benzene Bob the Mad Machete Missionary got part of his nickname.

I think the advice originally suggesting this as an essential CoC accessory came from "The Handbook of the Theron Marks Society".

Benzene's ring structure was discovered in 1864 by German chemist Friedrich August Kekule. He was inspired by a dream he had, in which he envisioned organic molecules as snakes, and he saw one of the snakes biting its own tail. In his own words, from a speech he gave twenty-five years later:

During my stay in Ghent, I lived in elegant bachelor quarters in the main thoroughfare. My study, however, faced a narrow side-alley and no daylight penetrated it ... I was sitting writing on my textbook, but the work did not progress; my thoughts were elsewhere. I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly in the background.

My mental eye, rendered more acute by the repeated visions of the kind, could now distinguish larger structures of manifold conformation; long rows sometimes more closely fitted together all twining and twisting in snake-like motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke; and this time also I spent the rest of the night in working out the consequences of the hypothesis.

He published his paper concerning benzene's structure less than a year later to the Chemical Society of Paris, solving a problem that had concerned modern chemistry for years. His findings weren't only of great theoretical importance, but were useful in the dye industry of the time, and Kekule was offered an appointment to the University of Munich based on them alone.

Never stop dreaming.

Benzene is a form of hydrocarbon of the arene group which consist of rings of Carbon atoms each with a single Hydrogen atom bonded to it. The atomic properties of carbon mean that each Carbon atom in these rings is missing one bond. This bond is exhibited in arene rings as a delocalised electron cloud around the whole ring, or as transient pi bonds which switch from atom to atom very quickly. Benzene rings do not (as the above writeups would mostly have you believe) always have a totally symetrical pattern to their rings, there are several kinds of Benzene which all act in a small and barely noticably different way when Halogenated or Sulphonated. The differences arise from differences in the π bonds created around the ring by the delocalised electron clouds, which can form several different shapes, the full ring being only one of them.

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.
Some of the evidence for benzene having the structure detailed in other writeups (with a delocalised ring of electrons at the centre) as opposed to the Kekulé structure includes thermodynamic considerations.

Let's look at the enthalpy change for the complete hydrogenation of cyclohexatriene (essentially the Kekulée structure) to cyclohexane. The problem is, cyclohexatriene doesn't exist due to its instability. So we use our imaginations, and think as follows:
The enthalpy change for the hydrogenation of cyclohexene (like cyclohexane but with a double bond replacing one of the single bonds) is -120 kJ/mol.
The enthalpy change for the hydrogenation of cyclohex-1,3-diene (like cyclohexane but with two double bonds) is -233 kJ/mol.
This is roughly twice the amount for the same reaction for cyclohexene; we could therefore say that the enthalpy of hydrogenation of cyclohexatriene will be roughly three times that for cyclohexene: -360 kJ/mol.

But when real benzene is hydrogenated, the experimental value for the enthalpy change is -208 kJ/mol. Hence, less energy is given out when 1 mole of benzene is hydrogenated than when 1 mole of cyclohexatriene is hydrogenated. This means that benzene is more stable than cyclohexatriene.

That is the thermodynamic evidence for the structure of benzene. However, there are more hints that the delocalised-ring structure is the true picture, and some of these come from the reactions that benzene undergoes. If benzene was as Kekulé proposed, it would have discrete double bonds, and so would undergo electrophilic addition reactions. This is not the case. Instead, benzene undergoes electrophilic substitution reactions, at the end of which the delocalised ring of electrons is unchanged (although it may be disrupted during the reaction, for instance in the halogenation of benzene). This kind of reaction shows us that benzene must have this structure.

Ben"zene (?), n. [From Benzoin.] Chem.

A volatile, very inflammable liquid, C6H6, contained in the naphtha produced by the destructive distillation of coal, from which it is separated by fractional distillation. The name is sometimes applied also to the impure commercial product or benzole, and also, but rarely, to a similar mixed product of petroleum.

Benzene nucleus, Benzene ring Chem., a closed chain or ring, consisting of six carbon atoms, each with one hydrogen atom attached, regarded as the type from which the aromatic compounds are derived. This ring formula is provisionally accepted as representing the probable constitution of the benzene molecule, C6H6, and as the type on which its derivatives are formed.

 

© Webster 1913.

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