Synthesis of Polycyclic Aromatic Hydrocarbons

Stewart, David G (1953) Synthesis of Polycyclic Aromatic Hydrocarbons. PhD thesis, University of Glasgow.

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The Kekule structures of benzenoid hydrocarbons have, according to the valency-bond treatment of L. Pauling, a statistical significance only, while the molecular-orbital theory dispenses with them entirely. The synthesis of a hydrocarbon with no Kekule structure, named for simplicity triangulene, was attempted to find out how far these assumptions are justified. Dehydrogenation of dodecahydrotriangulene and hexahydro-triangulene, and a study of the reduction of triangulene-quinone indicated that triangulene is an unstable diradical which polymerises as soon as it is formed. It can therefore be concluded that a hydrocarbon which lacks a Kekule structure possesses no aromatic character. While investigating triangulene, a new synthesis of pentaphene was discovered, which is superior to the earlier synthesis in that no isomeric hydrocarbons are formed. As coeranthrene and naphthanthrene have skeletons closely related to that of triangulene, these compounds and their derivatives were prepared and used for comparison purposes. To arrive at a fuller understanding of the aromatic state it is necessary to ascertain if a single Kekule structure is sufficient to lend aromatic character to a benzenoid ring system. The hydrocarbon, zethrene in which the two central double bonds are fixed when formulated in terms of classical structures, has been synthesised. Its properties are so extraordinary that it must be concluded that zethrene is not a true aromatic hydrocarbon and that it has a pronounced tendency to assume a structure in which two localised electrons are surrounded by a closed system of conjugated double bonds. Prom these experimental facts the following definition is proposed: aromatic resonance demands the participation of at least two Kekule structures in which no double bond is isolated and which are of similar energy. The unusual properties of perinaphthenone, e.g., its solubility in hydrochloric acid, have been attributed to its assuming a structure in which the external carbon atoms are joined by a system of alternating double bonds. Further investigation of this phenomenon has resulted in the production of stable free radicals derived from perinaphthenone, in which a lone electron is localised at the central carbon atom. Symmetrical tribenzoperinaphthene has been synthesised, and although attempts to form the corresponding free radical were unsuccessful, the isolation of tribenzo-perinaphthylium salts was accomplished. A hydrocarbon obtained as a by-product in a preparation of hexahydrotriangulene was identified as 2:3-benzofluoranthene and, from a study of its absorption spectrum and the application of the Anellation Principle of E. Clar, it was predicted that the red hydrocarbon, obtained in the ring-closure of 9-phenyl-1:2-benzanthracene to 1:2-3:4-dibenzopyrene, was 2 :3-6:7-dibenzofluoranthene. This hydrocarbon was therefore synthesised from 5-phenyl-naphthacene and the prediction proved to be correct. 5-Benzylnaphthacene was prepared to demonstrate further the tautomerism between the methylacenes and the methylene-dihydroacenes. From a study of the absorption spectrum of this hydrocarbon it appears that there is present an appreciable proportion of the methylene form, 9-benzylidene-9:10-dihydronaphthacene. Attempts to separate the two tautomers, however, were not successful. Finally, the initial steps of an unambiguous synthesis of 1:9-5:10-diperinaphthyleneanthracene were carried out. Although the final reduction to the hydrocarbon proved to be difficult, a product, which is probably the hydrocarbon, was obtained. Throughout these investigations, the absorption spectra of many of the compounds were studied, and they were invaluable in the assigning of correct structures. The Anellation Principle of E. Clar was extended and new anellation series were found.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry
Date of Award: 1953
Depositing User: Enlighten Team
Unique ID: glathesis:1953-79101
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 05 Mar 2020 11:43
Last Modified: 05 Mar 2020 11:43

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