The History of Aromaticity

Pasteur in Paris and was one of the pioneers of modern atomic theory, proposed the name phene . The word phene (from the Greek phainen , to shine) was proposed because benzene burns with a bright flame. Although it was not adopted for benzene, this word is now used in a number of names of arenes, such as phenol, phenanthrene, etc. 

The X-ray crystal structure of benzene, proving the equivalence of the six C-C bonds, appeared in 1929 and 1932, and Pauling reported its electron-diffraction data in 1931. Note that several of the structures proposed in the 19th century, such as Dewar benzene (non-planar) and Ladenburg s prismane, which are valence isomers of benzene, have now actually been prepared from benzene derivatives photochemically. They are kinetically stabilized, since they do not spontaneously revert to benzene or its derivatives [17-20]. 

Canonical valence-bond structures of benzene according to the theory of resonance by Slater (1929) 

Anti-aromaticity was predicted by the Hiickel approach for conjugated cyclic planar structures with 4n 7i electrons due to the presence of two electrons in antibonding orbitals, such as in the cydopropenyl anion, cydobutadiene, and the cydopentadienyl cation (n = 1), and in the cydoheptatrienyl anion and cydooctatetraene (n = 2). It has been argued that a simple definition of an anti-aromatic molecule is one for which the 1H NMR shifts reveal a paramagnetic ring current, but the subject is controversial. The power of the Hiickel theory indeed resides not only in the aromatic stabilization of cydic 4n + 2 electron systems, but also in the destabilization of those with An electrons [22, 27, 42]. 

The term homoaromatidty has been coined by Winstein [43, 44]. The rupture of a cydic conjugation due to the insertion of a saturated fragment such as CH2 partly preserves the aromatic stabilization of the original aromatic molecule or ion. Winstein suggested that homoaromaticity, a type of aromaticity, is found for cations that have neither the u-electron 

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Table 1 Significant moments in the history of aromaticity till 1970...

The history of aromatic chemistry was, at the outset, closely linked to the development of coal carbonization to produce coke, gas and tar. 

Three phases are discernible in the history of aromatic theory. From Kekule s perception of the cyclic nature of benzene until 1922, classical valency theory was exploited to its limits. The decade 1920-30 saw the emergence of electronic theories, and the confusion of electronic and classical ideas presented by the chemical literature of that time is difficult to resolve in retrospect. From about 1930 onwards, the significant ideas of the previous decade assumed clearer outlines and crystallized in a quantum theory of valency which to a great extent has solved the problem of aromatic stability and reactivity. 

This reaction was one of the earliest electrophilic substitutions to be developed in the history of aromatic chemistry and dates back to the mid-nineteenth century. The reagent used then (and now) was nitrating mixture, formed by mixing 1 mol of cone, nitric acid with 2 mol of cone, sulfuric acid. Cryoscopy (depression of freezing point, check back to your general chemistry text on colligative properties) shows that this results in the formation of four ions (Equation 12.2) ... 

The development of theoretical organic chemistry was intimately entwined with the development of that particular aspect of it concerned with aromatic substitution the history of this twin growth has been authoritatively traced. Only the main developments, particularly as they affect nitration, will be noted here. 

Let s begin by tracing the history of benzene its origin and its structure Many of the terms we use including aromaticity itself are of historical origin We 11 begin with the discovery of benzene... 

Influence of the mode of operation on process performance. The mode of operation of stirred-tank reactors can also significantly affect reactor performance. The history of concentrations will be changed by the time policy of reactant(s) addition to the reaction mixture. In view of our very limited possibility of controlling of temperature in stirred-tank reactors, the temperature-time dependencies for different policies of dosing will also be different. For example, the result of nitration depends upon the method of addition of nitric acid to aromatics, and the choice which phase is dispersed and which is continuous. Consequently, if the reaction is concentration- or temperature-sensitive the result will be dependent on the mode of operation (see Example 5.3.1.5). 

The bible , historically important, and a stimulating read, is R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry, Verlag Chemie, Weinheim, 1970. A little more of the history of the development of the ideas is in R. B. Woodward, in Aromaticity, Special Publication of the Chemical Society, No. 21, 1967, p. 217. 

During his researches into the history of the theory of the relationship between colour and constitution of organic compounds,167 168 Dahne has examined the contributions of W. A. Ismailsky (1885-1973), whose career (mainly in Moscow) spanned both the closing years of pre-revolutionary Russia and the Soviet period.169170 Ismailsky appears to have been one of those who anticipated the theory of resonance in connection with the structures of aromatic molecules. This was in his thesis at the Technical University of Dresden in 1913, where he had worked under the direction of Walter Koenig. 

As indicated above, the antiknock effect is shown by many compounds. Outstanding among these are the organometallic compounds of many metals. Effective derivatives of many metals include alkyl compounds, aryls, carbonyls, nitrosyls, phosphines, cyclopentadienyls, and many mixed compounds. The aromatic amines are also good antiknock agents, but far less effective than the organometallic compounds. The reason why commercialization efforts were concentrated on tetraethyllead early in the history of antiknock investigations is evident from Table 1, which is a composite of early data 6 8,216). 

One of the earliest perfumery materials to be produced industrially was benzaldehyde, prepared from toluene in 1866. In 1868 coumarin was first synthesized, soon to be followed by heliotropin, ionone, and vanillin. The first nitro musks appeared in 1888, and amyl salicylate in 1898. Since then, for the last hundred years, the history of perfumery has been dominated by the creation of new aromatic chemicals. 

The history of benzene is one of the most intriguing in science. It started in 1825 with the isolation of benzene by Michael Faraday from the condensed phase of pyrolyzed whale oil. Its planar cyclic structure was first proposed in 1861 by the Austrian physicist and physical chemist Johann Josef Loschmidt [1—5]. However, it was only fully understood some 70 years later, around 1930, with the advent of the modem theories of aromaticity, i.e. the theory of molecular orbitals (Hiickel s theory) [6-8] and the theory of resonance [9-12]. 

As pointed out in the preceding section, a second route for developing fibers having properties approaching the ultimate is the use of polymer chains that have high intrinsic stiffness and will remain extended in solution or melt. The development of aramid organic fibers based on aromatic polyamides met these requirements and added another chapter to the history of the development of synthetic fibers. Nomex aramid, a thermally resistant fiber based on a meta-oriented structure, was commercialized by the DuPont company in 1962. 

Although these liquid-phase and vapor-phase alkylations serve well to attach aliphatic groups to silicon, they are not so satisfactory for the substitution of aromatic groups. Very early in the history of organosilicon chemistry, Ladenburg found that the aryl compounds of mercury were more effective reagents than those of zinc. For example, mercury diphenyl reacted with silicon tetrachloride in a sealed tube at 300° to form phenyltrichlorosilane ... 

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