Rearrangement of Cyclopropylcarbinyl Chloride Over Protonic Zeolites: Formation of Carbocations and Behavior as Solid Solvents
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The rearrangement of cyclopropylcarbinyl chloride was studied over protonic zeolites and K-10 Montmorillonite. The energy of activation is lower on zeolites, with K-10 showing almost the same value for the rearrangement in 80% aqueous ethanol solution. HUSY showed the lowest energy of activation, whereas HZSM-5 and HYD [dealuminated with (NH4)2SiF6] presented similar energy of activation. This difference may be due to the presence of extra-framework aluminum species. On the other hand, the entropy of activation is significantly less negative on ZSM-5 and may be associated with the narrower pore structure, providing ionization of the substrate without losing many degrees of freedom. Kinetic isotope effects indicated that ionization is assisted by hydrogen bonding of the zeolite OH groups with the leaving halide, similar to the push–pull mechanism proposed for solution chemistry. Hence, zeolites behave as solid solvents, providing a polar microscopic environment for ionic reactions to take place, and solvating the transition state and the ions formed.
KeywordsZeolites Carbocation Acid catalysis Rearrangement
Authors thank financial support from CNPq and FAPERJ.
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