Kinetic and Mechanistic Studies on Compounds With Strained Rings

Hanlon, Peter (1977) Kinetic and Mechanistic Studies on Compounds With Strained Rings. MSc(R) thesis, University of Glasgow.

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A theory explaining the stabilisation of carbonium ion intermediates, termed "vertical stabilisation", has been put forward by T. G. Traylor and co-workers. Vertical stabilisation has been defined as that stabilisation in Y-CH2+ or Y-CH2- which is afforded while the internal geometry of Y and the Y-C bond length remain essentially as they were in the reactant. Traylor suggests that vertical stabilisation is a form of sigma-pi conjugation (hyperconjugation) and that it may be determined whether a group acts to stabilise a positive charge by internal nucleophilic displacement or by sigma-pi conjugation by studying the effect of that group in a vertical process. The reaction studied by Traylor was the charge transfer between substituted benzenes and a standard acceptor, tetracyanoethylene (T. C. N. E.). A relationship has been shown by various workers to exist between ionisation potentials and reactivity, i. e. sigma+, in another vertical process, gas-phase removal of an electron. Traylor believes that as the absorption frequencies of the charge transfer complexes are directly related to ionisation potentials then a relationship should be apparent between charge transfer frequency and sigma+. Evidence is given by Traylor for a linear correlation between absorption frequency and reactivity and it is postulated that a method of directly estimating the sigma+ constant for a particular substituent is possible by determining the charge transfer absorption frequency for the respective substituted benzene - T. C. N. E. complex. In particular, Traylor has suggested that strained cyclic substituents should exhibit vertical stabilisation in reactions involving carbonium ion formation. Traylor has attempted to demonstrate this theory of stabilisation by comparing the rates of solvolysis of various cyclic substituted carbinyl derivatives relative to the neopentyl derivative with the charge transfer frequencies of their respective substituted benzene - T. C. N. E. complexes. A plot of log relative rate of solvolysis against charge transfer frequency obtained by Traylor is shown. Using this plot Traylor has predicted the rate of solvolysis of the 1-norcaryl-and homocubyl-carbinyl systems. The scope of the present work has been to test this theory by determining the kinetics of electrophilic aromatic substitution of these cyclic substituted benzenes to see if, in fact, the rates predicted by Traylor are correct. Also, the homocubyl carbinyl compound was synthesised and the kinetics of solvolysis of the 3,5-dinitrobenzoate derivative in aqueous acetone were determined. Finally, the charge transfer absorption spectra of the cyclic substituted benzenes were recorded to see how accurately the absorption frequency could be determined for a particular substituent and to corroborate Traylor's results. The results obtained in this work suggest that Traylor's hypothesis of a method of estimating reaction rates by measuring the absorption maxima of the charge transfer complexes with T. C. N. E. needs to be taken with a great deal of reservation for several reasons. Firstly the inability to estimate accurately enough the exact position of the charge transfer absorption maxima. Secondly, the fact that the results of the electrophilic aromatic substitution reaction studied in this work, i. e. nitration, do not corroborate Traylor's theory. Finally, the rate of solvolysis of the homocubyl carbinyl system is much slower than that predicted by Traylor.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Keywords: Organic chemistry
Date of Award: 1977
Depositing User: Enlighten Team
Unique ID: glathesis:1977-78775
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 14:55
Last Modified: 30 Jan 2020 14:55

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