Activation energy cyclic ether reactions

HPA is very soluble in oxygen-containing polar solvents such as water, alcohol, ether, and ketone. In these polar solvents, HPA works as an efficient acid catalyst for alkene hydration, the Prins reaction, nucleophilic cleavage of alicyclic and cyclic ethers, esterification, hydrolysis, transesterification, and acetalization. The catalytic activity of HPA in these reactions is much higher than that of ordinary protonic acids such as sulfuric acid and p-toluenesulfonic acid, and the activation energies of the HPA-catalyzed reactions are remarkably reduced, owing to stabilization of the cationic reaction intermediates by the heteropoly anion [1,4]. 

Equation 8.28 shows only the anionic nucleophile explicitly, since the counterion does not appear to take part in the reaction. Nevertheless, the counterion affects the solubility of a nucleophilic salt, which therefore can influence the polarity of the solvent needed for the reaction. An alternative to the use of a more polar solvent to dissolve a salt for nucleophilic substitution is to use crown ether additives. Crown ethers are cyclic polyethers that can coordinate with cations and therefore increase their solubility in organic solvents. The nomenclature provides the total number of atoms and the number of oxygen atoms in the ring. Compoimd 51 is 12-crown-4, and 52 is 18-crown-6 (Figure 8.32). Coordination of a crown ether with a cation helps to dissolve the salt in a less polar solvent and leaves the anion relatively unsolvated. The activation energy for substitution therefore does not include a large term for desolvation of the nucleophilic anion, and the reactions are fast. For example, adding dicyclohexano-18-crown-6 (53) to a solution of 1-bromobutane in dioxane was found to increase its reactivity with potassium phenoxide by a factor of 1.5 x 10. Moreover, Liotta and Harris were able to use KF solubilized with 18-crown-6 (52) to carry out Sn2 reactions on 1-bromooctane in benzene. ... 

In addition to ethylene and propylene oxide, a variety of other cyclic ethers have also been copolymerized with MA. Monomers such as cyclohexene oxide,piperylene dimer mono and diepoxide, epichlorohydrin, " " 3,3,3-trichloropropylene oxide, " tetrahydrofuran, " " and ethylene carbonate or ethylene sulfite " have received attention. Condensation reactions between allyl glycidyl ether and MA are reported to be highly useful for preparing plastics with remarkable hardness, high heat distortion, and brilliant clarity.The cyclohexene oxide copolymerizations were second order in MA, with an activation energy of 13.8 kcal/mol. For the epichlorohydrin system the rate was dependent on the temperature and proportional to the catalyst concentration, with an activation energy of 14.5 kcal/mol. 

Owing to the rapid decomposition of the intermediate precursor of the unstable cycloalkyne, such as 26 or 27, kinetic investigations to confirm the intermediacy of cyclic acetylenes could not be performed. However, l-Iithio-2-bromocyclopentene (48) was found to be fairly stable at room temperature. The kinetic measurements indicate that 48 loses lithium bromide in a first-order reaction (k = 2 x lO" s at 20 °C in ether), and the Arrhenius energy of activation for this reaction was estimated... 

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