LOSCHMIDT'S BENZENE

While taking group photographs in conferences, seminars, workshops, etc.  Indian chemists and researchers prefer to say ‘benzene’....instead old one ‘cheese’ to make happy faces.

But what do we know about benzene? As a chemist do we really know the struggle faced by the researchers to derive its structure? Who discovered benzene? Who synthesized benzene for the first time?  I hope this blog will be quite useful for those who are willing to know the answers of the above mentioned questions.

Benzene

What we know today about benzene?

Benzene is a colourless organic flammable liquid with characteristic odour which boils and freezes at 80.1 and 5.5 °C respectively. It is immiscible with water and has low density than it (water). The molecular formula of benzene is C₆H₆ where all the C-C and C-H bonds are of equal length. Benzene is a planar aromatic molecule where all carbon atoms are SP² hybridized. The aromatic nature can be evidenced from its NMR spectrum. In UV-visible spectroscopic analysis, it shows primary and secondary bands. Primary bands (two bands) appear at 184 and at 202 nm whereas secondary band has a characteristic fine structure appears at 255 nm (Figure 1).

Figure 1. UV-Vis spectrum of Benzene

Unlike other alkenes, benzene does not show addition reactions like addition of bromine, HBr, etc. To retain its aromatic nature, it prefers electrophilic aromatic substitution where hydrogen from benzene is replaced as a proton by electrophiles. Delocalisation of pi electron cloud in benzene is responsible for its highly stable structure. Thermochemical calculations showed that it is a stable molecule compared to hypothetical molecule 1,3,5-cyclohexatriene (Figure 2) by approx. 150 KJ/mole.

Figure 2. Structure of 1,3,5-cyclohexatriene

Benzene is quite toxic compared to other organic solvents. The long term exposure of it, found to decrease the number RBCs with the enhancement of malfunctioning WBCs in the blood and thus and can cause leukemia. While alkylsubstituted benzenes (methyl or ethyl) e.g. toluene and ethylbenzene are least toxic compared to it because certain enzymes neutralise their effects by oxidizing alkyl groups.

Discovery

On 16^th July 1825 Michael Faraday reported new hydrocarbon (benzene) obtained from the pyrolysis of sperm oil (obtained from sperm whales). His work was appreciated by Berzelius as the most important result of 1825. Many years later in 1890, at the ‘Benzene Festival' in Berlin showed 3 ml of purified benzene to the audience.

But how Faraday could discover benzene? The story started long before when the old historians reported the flames coming out of the earth crust. In 1809, world’s first large gasworks company named as ‘Privileged Gas Lighting Company’ was established to light London and Westminster. Quickly after 6 years in 1915, John Taylor, developed a technique where he produced light by the pyrolysis of the fatty oils and filed a patent. Later, David Gordon patented the production of light by using portable gas for which he used Faraday’s oil source in which Faraday reported three different types of hydrocarbon in 1925 [M. Faraday, Philos. Trans. Roy. SOC. (London) 1825, I, 440].

‘The whole thing a piece of blocked gas pipe’

A. W. Hoffman started his speech in 1890 at ‘Benzene festival’ with above sentence. It means that the blockage was due to benzene which solidifies in winter season below 5.5 °C responsible for blockage.

Bicarburet of hydrogen

Faraday did not give the formulas of discovered molecules in the original articles. So he called benzene as a bicarburet of hydrogen.

Synthesis

In 1833, Eilhard Mitscherlich (University of Berlin) successfully synthesized benzene by the distillation of benzoic acid in presence of lime.

 How the name benzene derived?

Before Mitscherlich’s synthesis of benzene, benzoic acid had been known to world for several hundred years. There are many trees which exude resinous materials known as balsams. Gum benzoin is one the balsams obtained from a tree that typically grows in Sumatra and Java.

From F. Carey, Organic Chemistry Book: Benzoin word is derived from the French equivalent ‘benjoin’ which in turn comes from the Arabic luban jawi , meaning “incense from Java.” Though benzoic acid is odourless but it can be easily isolated from benzoin gum thus named benzin first and finally benzene. Another interesting example: Pleasant smelling resin named ‘tolu balsam’ obtained from tolu trees from south America on distillation gave methyl derivative of benzene (discovered in 1840s) named as ‘toluene’.

In 1845, Charles Mansfield and A. W. Hofmann successfully isolated benzene and its derivatives from coal tar.

Later, in 1870, Berthelot synthesized benzene by passing acetylene gas through red hot tube.

Structure

In 1865, August Kekule proposed the structure of benzene but he was unable to prove it practically. He claimed that benzene contains three pi bonds and all hydrogens in structure are equivalent. In 1869 Ladenburg showed his disagreement for Kekule’s structure on the ground that disubstituted benzene should give four derivatives (gave prism formula), which is obviously not the case. In 1870, Meyer showed that 1,2 and 1,4 disubstituted benzene structures are same, only differ in the position and thus supported Kekule. Later, Ladenburg himself experimentally proved the Kekule’s claim about equivalency of hydrogens in the benzene structure in 1874. Before Ladenburg’s proof, Claus in 1867 proposed a diagonal structure which he further modified in 1882.

One of the proposed interesting structures of benzenes is Dewar Benzene. Dewar in 1867 suggested number of formulae for benzene. Three structures are possible in Dewar benzene.

Various possible reported structures of benzene ring are well depicted 

below (Figure 3).

 Figure 3. Various possible structures of benzene

Preparation of Dewar Benzene

IUPAC name of Dewar Benzene is Bicyclo [2.2.0]hexa-2,5-diene. It is a non-planar molecule. The synthetic route for the same is well depicted below in scheme 1.

Scheme 1. Synthesis of Dewar Benzene

Controversy and unsung hero

Snake dream

Kekule in 1890, was addressing German Chemical Society where he described the idea that lead to the structure of the benzene ring. His speech was translated in English by F. R. Japp and published in Journal of Chemical Society England. Some part of his translated speech is mentioned below:

Translated by Japp  [taken from   The American Journal of Psychology, Vol. 108, No. 3 (Autumn, 1995), pp. 419-438]

....... I was sitting, writing at 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 gambolling before my eyes. This time the smaller groups kept modestly in the background. My mental eye, rendered more acute by 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 snakelike 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 I also spent the rest of the night in working out the consequences of the hypothesis.

Rudolfsky and Wotiz in 1984 claimed that actual structure of benzene given thirty years before and they were suspicious about Kekule.

Many researchers working in the areas of psychology, science and believe that it is quite difficult to develop modern science and scientific discoveries on the basis of dreams. Even they believe Newton’s story of falling apple is very likely not true. Few people claim that the story narrated by Kekule regarding his dream was not same as previous one.

Either stenographer or phonographer didn’t do well or something else.

Unsung Hero

It was Johann Josef Loschmidt who gave the structure of benzene for the first time before Kekule in 1861. Loschmidt was born to poor peasant family in 1821 in Bohemia. He was doing work in his farm with parents who were thinking him to be useless for work other than studies. He was also a good friend of Ludwing Boltzmann. He did his studies in philosophy, mathematics later in naturals sciences, physics and chemistry. He was involved in several industrial projects like synthesis of oxalic acid, KNO₃,etc. but always received economic failure. Apart from industry, he is well known for the his Loschmidt’s number (calculation for the number of moles in one millilitre of an ideal gas).

Johann Josef Loschmidt

Loschmidt proposed first cyclic structure of benzene, first representation of allyl moiety, first representation of vinyl moiety, first representation of cyclopropane and many more.  

Conclusion:

Today we know the importance of benzene nucleus in variety of molecules and its applications in drugs, dyes, and other materials. Though Kekule was highly praised for his dream but we should not forget the first one i.e. Johann Josef Loschmidt for his great and unsung, less discussed contribution.

I am dedicating this blog to unsung hero Johann Josef Loschmidt on Indian National Science Day which we celebrate every year to commemorate contribution of Sir. C. V. Raman for Raman Effect.

Acknowledgement: 
Herewith I acknowledge my S.Y.B.Sc. students Mr. Ajit Payer and Mr. Shubham Mishra who helped me in gathering some of the information.

References:

1. Aldrichimica Acta, 22 (1) 1889, 17-19.

2. Angew. Chem, 7, 1968, 345-350.

3. J. Chem. Ed. 74(2) 1997, 194-201.

4. The American J. Psychol. 108 (3) 1995, 419-438.

5. Organic Chemistry/Chemistry Books by

Cohen and Rothman;

Zumdahl;

Siebert;

Humiston; S

mith and Cristol;

Normon Coxon;

Snyder;

Morisson and Boyd;

Finar;

Breck, Brown and McCowan;

McQuarrie and Rock;

 F. Carey;

 J. March;

S.K.Ghosh;

 Wade and Singh