Studies of Reaction Mechanisms in Organic Chemistry Using Radiochemical Techniques

Hashish, Zeinab M (1968) Studies of Reaction Mechanisms in Organic Chemistry Using Radiochemical Techniques. PhD thesis, University of Glasgow.

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An investigation of the mechanism of dehydrogenation of a hydroaromatic compound, 1:4,dihydronaphthalene, by a quinone, tetrachloro-1:4,benzoquinone, has been made. The reaction has been studied by kinetic measurements, spectroscopic measurements and by the determination of tritium isotope effects. The measurements were carried out at 80 C and phenetole was used as the solvent for the reaction mixture. In previous work in these types of systems second order kinetics are generally observed and the bulk of the experimental evidence suggests that the reaction proceeds by an ionic rather than a free radical mechanism. Transfer of a hydride ion has been postulated as the rate determining step. The present work does not entirely support this viewpoint; second order kinetics were only observed in the later stages of the reaction and in the initial reaction period the kinetic data suggest the mechanism is more complex than has previously been supposed. In addition, during the time over which the reaction was normally followed, i. e. 120 minutes, the amount of naphthalene formed is equivalent to approximately half of the 1:4,dihydronaphthalene consumed. The isotope effect measurements, using partially tritiated 1.4,dihydronaphthalene, have given a value of kT/k. =1.16+/-0.16. This result does not support the view that fission of the carbon-hydrogen bond is the rate-determining step, i. e. hydride ion transfer. The result is interpreted in terms of the first step in the reaction being the establishment of an equilibrium involving the formation of a charge-transfer complex and the value of k T/k greater than unity results from an inverse secondary isotope effect. The spectroscopic measurements prove the existence of charge-transfer complexes in the reaction mixture and indicate that the formation of these complexes is the initial step in the reaction. The extra consumption of 1:4,dihydronaphthalene in the reaction is ascribed to the formation of a chloro-substituted quinone in which a chlorine is replaced by monohydronaphthalene group. This byreaction would lead to the production of hydrogen chloride and the latter has been detected among the reaction products. It has been observed that significant amounts of hydrogen chloride do have a small catalytic effect on the reaction. The present work suggests that the reaction proceeds by an ionic mechanism but modifications in the earlier schemes are required. The following scheme is suggested for the reaction of the hydroaromatic compound (RH2) with the tetrachloroquinone (QCL4): RH2 + QCL4 ↔ charge-transfer complex (1) CT complex → RH+ + QCL4H- (2) RH+ + QCL4H-→ R + QCL4H2 (3) or CT complex → RHQCL3 + HCl (4) In systems where the quinone has a low redox potential, or where there is substantial steric hindrance, the formation of the charge transfer complex, step (1), can be rate determining, while with quinones of high redox potential step (1) is relatively rapid and step (2) is the rate determining step.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry
Date of Award: 1968
Depositing User: Enlighten Team
Unique ID: glathesis:1968-78480
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:16
Last Modified: 30 Jan 2020 15:16

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