SN1 - First-order Nucleophilic Substitution
Influence of the Solvent on Reactions
The solvent plays a great role in reactions. However, in contrast to reactions, it is not the stabilization of the nucleophile, but the stabilization of the first reaction step's transition state and the stability of the intermediate that influence the reaction rate, as the nucleophile does not participate in the rate-determining step. Due to its ionic structure orcharacter, the intermediate and transition state can be effectively stabilized by polar and solvents. As a result of the transition state's stabilization by a polar (or protic) solvent, the activation energy is decreased, as the non-ionic and comparatively less polar starting product is stabilized by the solvent to a lesser degree.
The ability of solvents to stabilize ions through solvation is directly associated with their polarity. Polar solvents, such as water, methanol, and dimethyl sulphoxide, can effectively stabilize ions through solvation, while non-polar solvents, such as ether and hydrocarbons, cannot. The dielectric constant is a measure of a solvent's polarity. The higher the dielectric constant of a solvent is, the higher its polarity also is and, thus, cations and anions may be separated by the solvent molecules more efficiently.
Protic (organic) solvents possess acidic hydrogens. These are usually bound to oxygen or nitrogen. In hydrogen bridge bonds, the acidic hydrogen acts as an acceptor of electron density, while the heteroatom (usually oxygen or nitrogen) acts as a donator, as the hydrogen-heteroatom bond is polarized due to the electronegativity difference of the atoms in question. Thus, this type of solvent is able to stabilize not only positively charged, but also negatively charged, ions and molecules.
|Aprotic solvents||ε||Protic solvents||ε|
|Hexamethylphosphoramide (HMPT)||30||Formic acid||58.0|
|Dimethyl formamide (DMF)||38||Water||80.4|
|Dimethyl sulfoxide (DMSO)||48|
Due to the points made above, reactions proceed much more rapidly in polar than in non-polar solvents. The nucleophilic substitution of 2-chloro-2-methylpropane in water, for instance, runs 100,000 times faster than that in ethanol.
reactions proceed more rapidly in protic, polar solvents because such solvent molecules stabilize the (rate-determining) transition state and the intermediate carbocation. That is, the transition state is lower in energy than in aprotic, non-polar solvents, while the energy of the starting product is hardly influenced by the solvent. As a result, the activation energy in protic, polar solvents is lower than that in aprotic, non-polar solvents.