Alkyl cation affinity

Methyl cation affinities of benzene and some substituted benzenes have been calculated. These follow a simple additivity rule and the value for benzene shows good agreement with the experimental estimate. Conclusive evidence is presented that these values are linearly related to the corresponding proton affinities. The competition between deuteriation and alkylation in the reaction of radiolytically formed perdeuterio ethyl cations with iV-methylpyrrole and with thiophene has been studied. Deuteriation, the Brpnsted acid pathway, predominates and intramolecular selectivities have been determined for each reaction. ... 

A catalytic cycle is composed of a series of elementary processes involving either ionic or nonionic intermediates. Formation of covalently bound species in the reaction with surface atoms may be a demanding process. In contrast to this, the formation of ionic species on the surface is a facile process. In fact, the isomerization reaction, the hydrogenation reaction, and the H2-D2 equilibration reaction via ionic intermediates such as alkyl cation, alkylallyl anion, and (H2D)+ or (HD2)+ are structure-nonrequirement type reactions, while these reactions via covalently bound intermediates are catalyzed by specific sites that fulfill the prerequisites for the formation of covalently bound species. Accordingly, the reactions via ionic intermediates are controlled by the thermodynamic activity of the protons on the surface and the proton affinity of the reactant molecules. On the other hand, the reactions via covalently bound intermediates are regulated by the structures of active sites. 

The group contributions apply only to alkyl cations and are of limited practical value. However, apart from illustrating the application of group additivity contributions to energies of formation of carbocations, they offer a significant insight into comparisons of stability based on hydride ion affinities (HIAs) and pAlR values. 

Fig. 2 Plot of hydride affinities (HIA) in the gas phase against values in aqueous solution at 25°C. Filled circles, alkyl cations open circles, methoxyalkyl cations triangles, vinyl cations.

Does the alkyl group effect on proton affinity depend on the position of substitution Is the proton affinity oftrans-2-butene (leading to 2-butyl cation) larger, smaller or about the same as that of its isomer, 2-methylpropene Rationalize what you observe. 

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. 

If P is a substance of electron affinity lower than that of the solvent (e.g., AlkHal in SFg), the process leads to the formation of the cation-radical. If P is a substance of electron affinity higher than that of a solvent (e.g., AlkHal in CH3OH), the process leads to the formation of the anion-radical. These possibilities are depicted in the following equations, based on alkyl halides ... 

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