Aviva Litovitz - McMaster University
Placement: Dalhousie University
Supervisor: Dr. Fran Cozens
Reactivity of substituted cumyl cations in alkali-metal-exchanged Y-zeolites
Zeolites are intriguing media which effect and catalyze a wide variety of chemical reactions. These solids are known to consist of an aluminosilicate framework with channels and cavities into which organic compounds can be absorbed. Yet the mechanisms through which zeolites participate in reactions is not fully understood. In the series of experiments undertaken here, the behaviour of intrazeolite cumyl carbocations are studied using nanosecond laser flash photolysis. This permits a comparison of the behaviour of various carbocations within the zeolite and an examination of the mechanism by which zeolites interact with carbocations.
The cumyl cations are produced by irradiating bicumenes, to generate bicumene radical cations which then fragments into cumyl cations and cumyl radicals.
A series of bicumenes were irradiated to explore the effect of aromatic substituents on the intrazeolite lifetimes of the carbocations. Substituents such as p-fluoro, p-methoxy and p-methyl groups should stabilize the carbocations produced through resonance and/or electron-donating properties and so lengthen the lifetime of the carbocation. In contrast, substituents such as m-methoxy with electron-withdrawing properties should destabilize the carbocation reducing its lifetime.
The substituted carbocations were examined in alkali-cation exchanged zeolites to help ascertain the role of the zeolite frameworks negative charges on carbocation stability. Each time an aluminum atom replaces a silicon in the zeolite framework, a negative charge is introduced which must be associated with a positive counterion. Here these counterions range from in size from Li+ to Cs+. Larger cations with a more diffuse charge are expected to be less strongly attracted to the negative charges on the zeolite framework. This could make these negative charges more available to interact with the encapsulated cumyl cations, resulting in shorter carbocation lifetimes. By varying both the substituents and the alkali counterions, the influence of these counterions on substituent effects can be determined.
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