For decades organic chemists have been fascinated by carbocations arising in myriads of organic reactions.  Hydrolysis and other solvolytic reactions have been used to gauge the role and stability of carbocations in these solvolysis reactions with extensions to other reactions.  Several mechanisms have been proposed involving free carbocations and cations with various degrees of participation by solvent.  In this study we have attempted to relate various theoretical measures of carbocation stability and related parameters to published rates of solvolysis.  Experimentally alkyl methanesulfonate esters were chosen because of their prototypic behavior as electrophiles.  The methanesulfonate anion is a particularly good leaving group, which leaves behind a cationic species whose stability and alkylating ability is dependent on the electronic and structural characteristic of the cation resulting from the ionization.  Kinetic rate constants for hydrolysis and for solvolysis by trifluoroacetic acid for a series of alkyl methanesulfonates were compared with the free energy of dissociation of these compounds to alkyl carbocations and methanesulfonate anion in the gas and aqueous phases calculated at the B3LYP/6-31(d,p) level.  Both classical and non-classical carbocations were investigated.  A good correlation was found for hydrolysis rates with the free energy of dissociation in the gas phase, which relates to the intrinsic relative stability of the corresponding carbocation.  The results in the solution phase with the SM8 Model also show a modest correlation.  Other correlations with computed parameters (LUMO energy of the cation, charge distribution in the carbocation, etc.) were investigated.  The relationship of these computed parameters to the alkylating activity and mutagenicity of these methanesulfonate esters was also considered.

Toward understanding the role of carbocations in solvolysis

248th National Meeting (2014)

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