The Synthesis and Study of Polycyclic Aromatic Hydrocarbons

Mullen, Alexander (1971) The Synthesis and Study of Polycyclic Aromatic Hydrocarbons. PhD thesis, University of Glasgow.

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The work in this Thesis mainly concerns the syntheses of polycyclic aromatic hydrocarbons and study of their u.v. spectra with special regard to the Annellation Principle and strict application of Robinson's aromatic sextet. We have proposed the name 'starphenes' for benzologues of tripheny. lene, and the numbering system follows a principle introduced recently. The figures give the number of rings in each of the three branches annellated linearly and radiating from the central ring. The total number of rings is the sum of the three figures plus one, as indicated by the name of the starphene. Decastarphene-(3,3,3) (i) and nonastarphene-(2,3,3) (ii) were synthesised. It was found that in any one moment of time conjugation only took place in two branches of the molecule. The third branch was effectively cut off from conjugation. The spectral results could be explained by postulating that in any aromatic ring the pair of pi electrons on the lowest level belongs to a double bond. This party cuts off conjugation as in the above crossconjugated case. In the case of benzene, the double bond is freely mobile, The remaining four pi electrons are divided into two levels, where one pair remains delocalised within the ring and a third pair is delocal-ised outside the ring and capable of migrating from one ring to the next. It is this electron pair which accounts for the conjugation 4. 2,3:12,13-Dibenzoterrylene(iii), 2,3:12,13:15,16-tribenzoterrylene(iv), 1,16:2,3:12,13:14,15-tetrabenzoterrylene(v) and 2,3:6,7:8,9:12,13:15,16-pentabenzoterrylene(vi) were synthesised. Their u.v. spectra exhibited the expected absorption bands applicable to the formulae derived by using aromatic sextets. Tetrabenzoterrylene (v) is a fully benzenoid hydrocarbon with seven aromatic sextets and it possesses the typical properties of a condensed polyphenyl type hydrocarbon. The terrylene benzologues were compared with their perylene analogues and a remarkable similarity in both their reactions with maleic anhydride and their u.v. absorption spectra was found, Dianthraceno(2':3',3:4);(2":3",9:10)pyrene (vii) was synthesised and its u.v. absorption spectra compared with other lower benzologues of pyrene. It was found that, although two types of conjugation, a phene and a picene type,were obviously present in the lower benzologues only a picene type was present in dianthracenopyrene (vii). This is readily explained by the fact that the picene arrangement has three sextets whereas the phene arrangement only has two sextets. As conjugation is extended, the two sextets in the phene arrangement become more diluted relative to the picene. Thus the latter arrangement is favoured, as the molecule is less reactive and would retain its pi electrons in a more aromatic distribution. 2,9-Diphenyl-coronene (viii) was synthesised and its spectral properties examined in order to ascertain whether there was any extension of conjugation. This would be expected if the substitution affected the superaromat-icity of coronene and caused it to behave like ? and ? substituted naphthalene fused units. No extension of conjugation was found indicating that the superaromatic nature of coronene is still evident in this derivative, and is not affected by the substitution of two phenyl groups, 3,8-Diphenyl-5,10-dimethylpyrene (ix) could not be obtained in a crystalline form, despite the fact that a correct microanalysis of the crystalline picrate was obtained. Naphtho (2':3',2:3) fluoranthene (x) was synthesised and its u.v. absorption spectrum compared with those of 2,3-henzofluoranthene 9 and fluoronthene, indicating a gradual dominance of phene character in the series, Anthraceno (2':3',2:3) fluoranthene (xi) defied 'synthesis, attempted decarboxylation of the hydrolysed 1',4'-dicyano-anthraceno (2':3'2:3) fluoranthene with soda lime, resulted in the decomposition of the starting material. 1-Methylfluoranthene (xiii) and 8,9-dimethylflucranthene (xii) were synthesised using modifications of established techniques, Together with other methylfluoranthene derivatives prepared by other workers 10, a comprehensive study was made of the nature of the electronic arrangements in fluoranthene. Using methyl signal splitting in the N. M. R. as an indication of the degree of double bond character in a polycyclic hydrocarbon, fixed double bond character was found at the 2,3,4,5 and 8,9 positions in fluoranthene. 2,3:4,5:8,9:10,11-Tetrabenzoperylene (xiv) and dianthraceno(1':8', 2:5);(1":8",8:11)perylene (xv) both defied synthesis, due mainly to difficulty experienced in attempting to open the lactone rings of the intermediates. This snag coupled with the time factor involved prohibited a satisfactory completion of the project.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry
Date of Award: 1971
Depositing User: Enlighten Team
Unique ID: glathesis:1971-78579
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:09
Last Modified: 30 Jan 2020 15:09

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