Carbonium ions reactivity

Chemical reactivity differences may be calculated if for the transition state of a rate-determining step of a reaction a structural model can be given which is describable by a force field with known constants. We give only two examples. Schleyer and coworkers were able to interpret quantitatively a multitude of carbonium-ion reactivities (63, 111) in this way. Adams and Kovacic studied the pyrolysis of 3-homoadamantylacetate (I) at 550 °C and considered as transition state models the two bridgehead olefins II and III (112). From kinetic data they estimated II to be about 2 kcal mole-1 more favourable than III. 

Carbonium ions, reactivity towards carbon monoxide, 10, 29... 

Following the extensive investigation of bridgehead carbonium ion reactivities, which provides the most conclusive experimental evidence available for the preferred planarity of carbonium ions 187), similar studies of bridgehead free radical reactivity have been initiated. The results are equally instructive. 

It is not apparent whether the initial abstractions in oxidation involve hydrogen atoms or hydride ions. Simplicity favors a radical mechanism, with similar abstractions for the initial and final hydrogen removals. The good correlation with reactivities in known radical reactions is also suggestive. However, initial carbonium ion formation would also involve the same qualitative dependence on structure, although structural effects are usually more pronounced for carbonium ions than for radicals. Qualitatively, the oxidation results correlate well with carbonium ion reactivities found in the solvolysis of substituted aUyl halides (142), again with internal olefins appearing slow in oxidation. 

The reactivity of cationic acetylenic platinum complexes formed from PtX(Me)L2, either in the presence of silver hexafluorophosphate (particularly for alkylacetylenes) or in its absence (particularly for acetylenes containing CFa or other electron-withdrawing groups) has been discussed in terms of carbonium ion reactivity, and a wide range of reactions reviewed. ... 

Carbonium ions, gaseous, from the decay of tritiated molecules, 8, 79 Carbonium ions, photochemistry of, 10, 129 Carbonium ions, reactivity towards carbon monoxide, 10, 29... 

The relative /-values for the alkyl radicals discussed so far show a strong parallel with relative carbonium ion reactivities in solution. 

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