Ethane protonation states

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

Study of the C-C protonation of ethane has revealed that from the minimum reaction path, which corresponds to the approach of a proton nearly perpendicular to the C-C bond, the system can easily protonate at C-H, affording a transition state which is H-ethonium like. 

Figure 27. H-carbonium ion-like transition states for methane, ethane, propane and isobutane protonation by HF/SbFs.

The largest number of hydrogen bonds in crystal structures of alkyl hydroperoxides refer to intermolecular bonds between the hydroperoxide proton and functionalities of the type 0=X, where X denotes a sulfur (e.g. 27), carbon (e.g. 30) or a phosphorous atom (e.g. 32, Figure 14, Table 7)93,108,115 geometry of [l,2-bis(diphenylphosphinoyl)ethane] bis(2,2-dihydroperoxypropane) (32) in the solid state is a rare example of a bifurcated hydrogen bond between an OOH donor and an 0=X proton acceptor. 

The clearest example of the danger in using a as a measure of transition state structure is illustrated in the work of Bordwell et al. (1969, 1970, 1975). In the rate-equilibrium relationship for the deprotonation of a series of nitroalkanes the unprecedented Br nsted slopes of 1 61 for l-aryl-2-nitropropanes and 1-37 for 1-arylnitro-ethanes were obtained. The simple exposition of the mechanistic significance of a disallows values greater than 1. This, coupled with the fact that the transition state for the proton transfer is not product-like (as established by alternative criteria) indicates at best that, in at least some cases, a does not reflect the selectivity of a particular reaction. Several attempts to rationalize these anomalous results have been made. 

A turning point in the revival of interest in strong acid chemistry was a publication in 1968 in which ionization of the C-H bonds of the extraordi-norily unreactive "lower poroffins" methane and ethane In HS03F-Sbp5 at 50°C was reported (14). The propiosed mechanism (Scheme 2) proposes the existence, in super acid solution, of protonated alkanes or pentacoordinoted ions, at least as possible transition states, and attempts quite logically to draw a parallelism between the presence of such species in solution chemistry... 

Ab initio molecular dynamics simulations for the protonation of simple alkanes in HF-SbFs solution showed that the presence of extremely reactive protons at high SbFs concentrations open reaction channels for the C-H protonation with a considerably decreased barrier with respect to dilute solutions and the gas phase. This and the progressive presence of neutral SbFs were suggested to be responsible for the experimentally observed increased reactivity with increasing concentration of SbFs. Furthermore, an electrostatic stabilization of the transition state in solution also contributes to the lowering of the reaction barrier and, as a result, C-H protonation competes with C-C protonation in the case of ethane. 

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