Structure of benzene

The structure of benzene was postulated from the number of substituted isomers that could be identified. Kekule is considered still today as the inventor of the structure of benzene (1865), although some people consider Loschmidt as the originator. Assuming a certain structure of the benzene, only a certain number of dichloroben-benzenes, trichlorobenbenzenes, etc., can be formed. 

By a careful compilation of the number of isomers of the known isomers of various aromatic compounds at that time, Kekuld postulated the benzene ring. Several other structures of benzene have been proposed, e.g., the Dewar benzene structure, theLadenburg benzene structure. 

The formula of benzene is C Hg. The ring structure has alternate double and single bonds as shown in the diagrams below. According to the latest theories, benzene is in resonance between the two structural entities as shown below. Benzene has a structure that is a hybridized form of these two structures. 

The hybridized structure is usually represented as shown below  

Many derivatives of benzene are substituted derivative compounds. Some examples of such compounds are shown below  

In benzene, there are six pi electrons in the ring with overlapping p orbitals. 

With respect to the substituent (the methyl group) arrow (1) indicates the ortho position, arrow (2) indicates the meta position, arrow (3) indicates the para position. 

Note that these structures were proposed before the electron had been discovered and before the Idea of the covalent bond had been developed, 

Based on elemental composition and relative molecular mass determinations, the formula of benzene was found to be The saturated 

In 1867, Dewar proposed several possible structures for benzene, one of which was 5. However, in 1874, Ladenburg proved experimentally that all the hydrogen atoms of benzene were equivalent and suggested the prismatic structure 6. 

Kekule s proposed structure 4 looks more in keeping with our current knowledge of benzene, although it does not explain how the double bonds differ from the aliphatic type. Furthermore, although the two structures 7 and 8 can be drawn for a 1,2-disubstituted benzene, only one such compound exists. Kekule proposed that the equivalent structures 7 and 8 oscillated between each other, averaging out the single and double bonds so that the compounds were indistinguishable. 

Based on elemental composition and relative molecular mass determinations, the formula of benzene was found to be C6H6. The saturated hydrocarbon hexane has the molecular formula C6H14 and therefore it was concluded that benzene was unsaturated. Kekule in 1865 proposed the cyclic structure 4 for benzene in which the carbon atoms were joined by alternate single and double bonds. Certain reactions of benzene, such as the catalytic hydrogenation to cyclohexane, which involves the addition of six hydrogen atoms, confirmed that benzene was a ring compound and that it contained three double bonds. However, since benzene did not undergo addition reactions with HC1 and HBr, it was concluded that these double bonds were different from those in ethene and other unsaturated aliphatic compounds. 

Nole that ttieHe slrut lurea were t,iroj iosstt before Ifie electron hac boc [i ijjscovereci irid before the idea oi I he i ovalenl bond had tieeri rievelopeil. 

---

If has long been known that the enthalpy of hydrogenalion of benzene (49.8 kcal moU Conant and Kistiakowsky, 1937) is not the same as three times the enthalpy of hydrogenation of cyclohexene (3 x 28.6 kcal moU ). Evidently, the double bonds that w e write in the Kekule structure of benzene... 

Another of Kelule s revelations that supposedly came to him in a dream was his famous structure of benzene. This related to how a carbon chain can close into a ring. To satisfy the four valenee of carbon, this, of course, raised the need to involve alternating double bonds. 

In 1866 only a few years after publishing his ideas concerning what we now rec ognize as the structural theory of organic chemistry August Kekule applied it to the structure of benzene He based his reasoning on three premises... 

Figure 17. The structure of benzene (A) conventional illustration of double bonds, (B) illustration implying resonance.

Figure 18. The structures of benzene and some of its common derivatives.

A 1968 German stamp combines a drawing of the structure of benzene with a portrait of Kekule. 

At ceremonies in Berlin in 1890 celebrating the twenty-fifth anniversary of his proposed structure of benzene, August Kekule recalled the origins of his view of the benzene structure. 

F, A. Kekule s structural formulae for organic compotmds ring structure of benzene 1865. 

Benzocyclopropene is an intriguing example in which the electronic structure of benzene is greatly perturbed by the fusion of the smallest alicyclic ring, cyclopropene, to the aromatic system. Benzocyclopropene thus arouses theoretical interest and the high strain energy (approximately 68 kcal./mole)3 associated with the compound suggests unusual chemical reactivity. A review article has recently appeared.4... 

The molecular structure of benzene, C(,H(, is planar. Is the molecular structure of cyclohexane, C6HI2, planar as well ... 

Benzene is more stable and less reactive than would be predicted from its Kekule structures. Use the mean bond enthalpies in Table 6.8 to calculate the lowering in molar energy when resonance is allowed between the Kekule structures of benzene. 

Kekule structures Two Lewis structures of benzene, consisting of alternating single and double bonds, kelvin (K) The SI unit of temperature. See also Appendix IB. 

The Electron Diffraction Investigation of the Structure of Benzene, Pyridine, Pyrazine, Butadiene-1,3, Cyclopentadiene, Furan, Pyrrole, and Thiophene... 

Figure 2.7 Valence-bond (resonance) approach to the structure of benzene...

One of the major problems of elementary organic chemistry is the detailed structure of benzene. The known planar structure of the molecule implies sp2 hybridisation with p atomic orbitals, at right angles to the plane of the nucleus, on each of the six carbon atoms (4) ... 

Reference has already been made to the structure of benzene and, in particular, to its delocalised -ir orbitals (p. 15) the concentration of negative charge above and below the plane of the ring-carbon atoms is thus benzene s most accessible feature ... 

The second mechanism, due to the permutational properties of the electronic wave function is referred to as the permutational mechanism. It was introduced in Section I for the H4 system, and above for pericyclic reactions and is closely related to the aromaticity of the reaction. Following Evans principle, an aromatic transition state is defined in analogy with the hybrid of the two Kekule structures of benzene. A cyclic transition state in pericyclic reactions is defined as aromatic or antiaromatic according to whether it is more stable or less stable than the open chain analogue, respectively. In [32], it was assumed that the in-phase combination in Eq. (14) lies always the on the ground state potential. As discussed above, it can be shown that the ground state of aromatic systems is always represented by the in-phase combination of Eq. (14), and antiaromatic ones—by the out-of-phase combination. 

Boron also forms many other compounds with nitrogen. One of the most interesting of these is bora-zine, B3N3HS (m.p. -58 °C, b.p. 54.5 °C). Shown here is the structure ofborazine, which is similar to the structure of benzene. In fact, borazine has sometimes been referred to as "inorganic benzene."... 

Best, A. P., and C. L. Wilson Structure of Benzene. Part XIII. Hydrogen... 

Since its initial discovery by Michael Faraday in 1825,58 benzene (C6H6) has been recognized as an extraordinary substance. The spectacular properties of benzene and its derivatives (particularly the aniline dyes discovered by W. H. Perkin) initiated dramatic growth of the pharmaceutical, dyestuff, and munitions industries in the mid nineteenth century. The famous puzzle of the chemical structure of benzene was solved in 1865 by August Kekule in terms of two alternative six-membered-ring formulas ... 

---