Hydrogen-bridged cations

A mercurinium ion has both similarities and differences as compared with the intermediates that have been described for other electrophilic additions. The proton that initiates acid-catalyzed addition processes is a hard acid and has no imshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron-deficient and highly reactive. 

Model calculations of the dimethyl vinyl cation system at the MP2/6-31G level show that the hydrogen-bridged cation is more stable than the secondary cation and that the barrier for the degenerate hydride transfer is very low (< 1 kcalmol Fig. 11). The isomeric allylic cation, which could be generated by a 1,2-hydride shift across the single bond of the secondary vinyl cation, is much more stable than the vinyl cation, but the barrier for this 1,2-hydride shift is very high since allylic conjugation cannot contribute to the stabilization of the transition state for the hydride shift. 

For tiie more stable of the two isomers of each type (1,5 and 1,6-p-H-bridges), cations 16 and 18, one can locate a transition-state for dehydrocyclization, leading to product ions 20 and 21 respectively, plus molecular hydrogen. 

Adsorption of nonionic compounds on subsurface solid phases is subject to a series of mechanisms such as protonation, water bridging, cation bridging, ligand exchange, hydrogen bonding, and van der Waals interactions. Hasset and Banwart (1989) consider that the sorption of nonpolar organics by soils is due to enthalpy-related and entropy-related adsorption forces. 

Other alkenes, however, may not react selectively. Product distribution in the transformation of 32 [Eq. (11.70)] is consistent with the equilibrium of the tertiary and secondary cations 33 and 34, or the presence of hydrogen-bridged ion 35 221... 

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