Benzene problem

A theoretical understanding of furan and its congeners followed in the wake of the progress with the benzene problem, but for the five-membered heterocyclic compounds Bamberger s centric formula was always the most reasonable one, as Kekule structures could not be written at all. It is not now clear to what extent the heterocycles were really regarded as benzene analogues, but a remark made by Hantzsch, that coumarone is the furfurane of the naphthalene series was certainly percipient. Furan is variously said to be aromatic, superaromatic or not aromatic at all, for the debate continues. 

For details on the benzene problem the reader is referred to the paper of S.G. Brush, Dynamics of theory change in chemistry Part 1. The benzene problem 1865-1945, Studies in History and Philosophy of Science 30 (1999) 21-79. 

S. G. Brush, Stud. Hist. Philos. Sci. 30, 21 (1999). Dynamics of Theory Change in Chemistry Part 1. The Benzene Problem 1865-1945. 

The first step led soon afterwards to a fundamentally new insight into the constitution of benzene and other aromatic molecules. While in general a correct conception regarding the molecular constitution had been obtained quite early in organic chemistry by inductive intuitive method this was not the case with the aromatic molecules. It was the quantum mechanical theory which first gave the solution of the benzene problem. The same ideas also laid the foundation for a better understanding of the very important problem... 

Huckel, E. Quantum-theoretical contributions to the benzene problem. I. The Electron configuration of benzene and related compounds, Z. Physik 1931, 70,204-288. 

The application of Werner s concept of affinity and valence to the phenomena mentioned above is difficult to summarize without considering numerous figures in detail. The following shows in some detail his explanation of the perennially popular benzene problem. ... 

Fig. 28. The three most interesting splits for the acetone/chloroform/benzene problem.

Brush, Stephen. 1999. "Dynamics of Theory Change in Chemistry Part I. The Benzene Problem 1865-1945." Studies in the History and Philosophy of Science, 30 21-79. 

Figure 4.13 Light-scattering determination of the molecular weight of polystyrene in benzene (Problem 4.12).

Now the main problem commences benzene has four exg vibrations which of them actually gives a large enough value of V,y For answers to this problem the reader is referred to the articles cited earlier in the text, and to a paper by Craig (10) that deals with the benzene problem in an interesting manner. 

The paper about benzene is part 1 of a series with the translated title Quantum chemical contributions to the benzene problem. I. The electron configuration of benzene and related compounds. Three other papers followed. The second paper [4] has the same series title and the translated subtitle IT. Quantum theory of induced polarizabilities. The study is an attempt to correlate chemical behaviour of substituted benzenes with the charge distribution in the ring which becomes disturbed by the substituents. The third paper [5] has a slightly different series title with no further subtitle Quantum theoretical contributions to the problem of aromatic and unsaturated compounds. ITT. The final paper in the series [6] has the subtitle Free radicals in organic chemistry ... 

Hflckel E (1931) Quantum contributions to the benzene problem. Z Physik 70 204-286 Wiberg KB (2001) Aromaticity in monocyclic conjugated carbon rings. Chem Rev 101 1317-1332... 

Every gain in our understanding of benzene has enriched our insight into the nature of heteroaromatic compounds such as pyridine and pyrrole, prototypical of all those to be discussed in this book. It is interesting, therefore, to consider the development of structures for pyridine and pyrrole against the background of the benzene problem in the first phase of its growth and as it was affected by some of the ideas from the second phase. 

Sketch potential-energy/reaction-coordinate diagrams for the two propagation steps of the monobromination of benzene (Problem 24). 

In 1931 Hiickel published the first in a series of four papers in which he applied the new quantum mechanics to the benzene problem using for the first time the VB method for aromatic compounds. Little reference is now made to this part of Hiickel s paper. In the second part of this same paper, he applied to aromatics a method that Bloch had used for crystal lattices. In this method electrons are placed into orbitals that extend over the entire molecule instead of being localized on a single atom. These molecular orbitals may in turn be constructed as linear combinations of atomic orbitals, and such an approach is usually called the Hund—Mulliken—Huckel (HMH) or molecular orbital (MO) method. Huckel s work in this second part of the paper leads to the famous 4/7 -f 2 rule for monocyclic conjugated hydrocarbons, though we do not find it explicitly stated here. 

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