Abstract
Nucleofugalites of tetrahydrothiophene, dimethyl sulfide and differently substituted pyridines in 80 % aqueous acetonitrile have been derived from the SN1 solvolysis rate constants of the corresponding X,Y-substituted benzhydryl derivatives (1–10). In sol-volysis of sulfonium ions in 80 % aqueous acetonitrile, where acetonitrile is a good cation solvator, the solvation of the reactant ground state is an important rate determining variable since the positive charge is almost entirely located on the leaving group. As a consequence, reaction rates of sulfonium ions are more sensitive to the substrate structure in 80 % aqueous acetonitrile than in pure and aqueous alcohols, which are less efficient as cation solvators. In solvolysis of pyridinium ions the solvation of the reactant ground state is less important, since the positive charge is considerably distributed between the carbon at the reaction center and the leaving group. In such cases the important rate determining variable is solvation of the transition state. Slower reactions of pyridinium sub-strates progress over later, carbocation-like transition states in which the solvation is more important, so those substrates solvolyze slightly faster in aqueous acetonitrile than in methanol (k80AN > kM). Faster reactions proceed over earlier TS in which the solvation is diminished, so those substrates solvolyze somewhat faster in methanol than in aqueous acetonitrile (k80AN < kM). This work is licensed under a Creative Commons Attribution 4.0 International License.
Highlights
I T IS well known that solvents may have a strong influence on the rates of chemical reactions and chemical equilibria, and that the rate of a chemical reaction can be changed by several orders of magnitude only by changing the reaction medium.[1,2,3] through judicious choice of solvent, decisive acceleration or deceleration of a chemical reaction can be achieved.Kinetic solvent effects on chemical reactions in different media are usually correlated in terms of "solvent polarity", which sums up all the specific and nonspecific interactions of the media with initial the reactant ground and transition state
Nucleofugalites of tetrahydrothiophene, dimethyl sulfide and differently substituted pyridines in 80 % aqueous acetonitrile have been derived from the SN1 solvolysis rate constants of the corresponding X,Y-substituted benzhydryl derivatives (1–10)
In solvolysis of sulfonium ions in 80 % aqueous acetonitrile, where acetonitrile is a good cation solvator, the solvation of the reactant ground state is an important rate determining variable since the positive charge is almost entirely located on the leaving group
Summary
Kinetic solvent effects on chemical reactions in different media are usually correlated in terms of "solvent polarity", which sums up all the specific and nonspecific interactions of the media with initial the reactant ground and transition state. Using hydrogen bonding as intermolecular interaction between the solvent molecules and solutes, the solvents used in SN1 displacement reactions are usually classified as polar protic and polar aprotic (apolar aprotic solvents are not useful).[4] Polar protic solvents contain hydrogen atoms bound to electronegative elements as oxygen or nitrogen and they are usually good anion solvators. Solvents without the ability to form hydrogen bonds with the solute molecules are aprotic solvents. These solvents usually have dielectric constants larger than 30 and their molecules exhibit dipole moments larger than 2.00 D. Typical polar aprotic solvents are nitromethane, nitrobenzene, N,N-dimethylformamide, dimethyl sulfoxide, acetone, tetrahydrothiofen dioxide (sulfolane), cyclic ureas, etc
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