Carbenium ions hydride affinity

All alkyl ions tested demonstrate a comparable behaviour independent of the sign of their charges. The decrease of the reaction enthalpies AH (11) with the change from the methyl to the ethyl cation (AAH (ll) = 165 kJ mol-1) and from the ethyl to the but-2-enyl cation (AAH°(11) = 117 kJ mol-1) corresponds to the increase of stability of these carbenium ions, which are expressed by the difference of their heats of formation (AAH f = —118 and AAHj = —42 kJ mol-1 90)) and of their hydride ion affinity (AHIA = 176 and 126 kJ mol-1 91)), respectively. 

The lifetime of the carbenium ion formed will be limited by transferring a proton back to the zeolite, thus completing the dehydrogenation ofthe hydrocarbon. Hydride abstraction from xylene is assumed to be the initial step in its disproportionation into toluene and trimethylbenzene [9]. The parent compound (7, Fig. 22.9) ofthe carbenium ion formed (6) has such a high proton affinity (1031 kj mohh Table 22.1) that proton transfer back to the zeolite does not occur at all. However, the lifetime of carbenium ions in zeolites is not only limited by proton transfer, but also formation of a C-O bond between the carbenium ion and a framework oxygen atom, yielding an alk-oxide, needs to be considered. In ferrierite (FER) the alkoxide of 6 is found to be 50 to 60 kJ mofi more stable than the carbenium ion [9]. 

Stability of Carbenium Ions. In terms of thermodynamic stability, carbenium ions can be ranked on the basis of gas-phase hydride affinity, heats of ionization, ionization equilibrium in solution, and Sn 1-solvolysis rate (21). The stability scales obtained by these different methods correlate reasonably well... 

Table 2 lists selected carbenium ions in order of increasing stability, along with their hydride affinity and the solvolysis rate constants of the corresponding alkyl halides. 

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