We present quantum mechanical calculations designed to disentangle the influences of solvent effects and substituent effects on ionic nucleophilic substitution reactions. In particular, we compare the SN2 reactions of Cl- with CH3CH(X)Cl and (CH3)3CCH(X)Cl for X = H and CN in the gas phase and aqueous solution. We find that, for all of these reactions, transition state distortion and dielectric descreening effects are quantitatively larger in magnitude than hydrophobic effects or exchange repulsion, but they also roughly cancel one another so that differential solvation contributes little to differences in the free energies of activation associated with a CH3 versus a (CH3)3C group as a substituent at the reacting position. Differential solvation of the transition-state structures relative to the reactants is less unfavorable for X = H than for X = CN because of the greater charge separations in the X = H case, and this separation places more positive charge on the reacting carbon center. The smaller deceleration associated with aqueous solvation for X = H roughly balances the gas-phase acceleration predicted for X = CN so that the aqueous activation free energies for the substrates are predicted to be similar for these two substituents.

removing the nucleophilic nature of the nucleophile. The following table shows the effect of solvent polarity on the relative reaction rates of the SN2 In chemistry, solvent effects are the influence of a solvent on chemical reactivity or molecular associations. Solvents can have an effect on solubility, stability and reaction rates and choosing the appropriate solvent allows for thermodynamic and kinetic control over a chemical reaction. A solute dissolves in a solvent when solvent-solute interactions are more favorable than solute-solute intera A… The case for SN2 reactions is quite different, as the lack of solvation on the nucleophile increases the rate of an SN2 reaction. In either case (SN1 or SN2), the ability to either stabilize the transition state (SN1) or destabilize the reactant starting material (SN2) acts to decrease the ΔG‡activation and thereby increase the rate of the reaction. This relationship is according to the equation ΔG = –RT ln K (Gibbs free energy). The rate equation for SN2 reactions are bimolecular being first order in Nucleophile and first order in Reagent. The determining factor when both SN2 and SN1 reaction mechanisms are viable is the strength of the Nucleophile. Nuclephilicity and basicity are linked and the more nucleophilic a molecule becomes the greater said nucleophile's basicity. This increase in basicity causes problems for SN2 reaction mechanisms when the solvent of choice is protic. Protic solvents react with strong nucleophiles with good basic character in an acid/base fashion, thus decreasing or removing the nucleophilic nature of the nucleophile. The following table shows the effect of solvent polarity on the relative reaction rates of the SN2 reaction of 1-bromobutane with azide (N3–). There is a noticeable increase in reaction rate when changing from a protic solvent to an aprotic solvent. This difference arises from acid/base reactions between protic solvents (not aprotic solvents) and strong nucleophiles. While it is true that steric effects also affect the relative reaction rates, however, for demonstration of principle for solvent polarity on SN2 reaction rates, steric effects may be neglected.

anionic nucleophile which would negatively affect an SN2, but it does not affect an SN1 reaction because the nucleophile is not a part of the rate-determining step (See SN2 Nucleophile). Polar protic solvents actually speed up the rate of the unimolecular substitution reaction because the large dipole moment of the solvent helps to stabilize the transition state. The highly positive and highly negative parts interact with the substrate to lower the energy of the transition state. Since the carbocation is unstable, anything that can stabilize this even a little will speed up the reaction. Sometimes in an SN1 reaction the solvent acts as the nucleophile. This is called a solvolysis reaction (see example below). The polarity and the ability of the solvent to stabilize the intermediate carbocation is very important as shown by the relative rate data for the solvolysis (see table below). The dielectric constant of a solvent roughly provides a measure of the solvent's polarity. A dielectric constant below 15 is usually considered non-polar. Basically, the dielectric constant can be thought of as the solvent's ability to reduce the internal charge of the solvent.

starting material then increasing the polarity of the solvent will increase the rate of the reaction since a more polar solvent be more effective at stabilizing In theoretical chemistry, an energy profile is a theoretical representation of a chemical reaction or process as a single energetic pathway as the reactants are transformed into products. This pathway runs along the reaction coordinate, which is a parametric curve that follows the pathway of the reaction and indicates its progress; thus, energy profiles are also called reaction coordinate diagrams Solvent… SN1: Figure 10 shows the rate determining step for an SN1 mechanism, formation of the carbocation intermediate, and the corresponding reaction coordinate diagram. For an SN1 mechanism the transition state structure shows a partial charge density relative to the neutral ground state structure. Therefore, increasing the solvent polarity, for example from hexanes (shown as blue) to ether (shown in red), would decrease the rate of the reaction. As shown in figure 9, the starting material has approximately the same stability in both solvents (therefore…

In fact, it's the exact opposite: when basicity decreases, nucleophilicity increases and when basicity increases, nucleophilicity decreases. Aprotic Solvents An aprotic solvent is a solvent that lacks a positively polarized hydrogen. The next diagram illustrates several polar aprotic solvents that you should become familiar with. Aprotic solvents, like protic solvents, are polar but, because they lack a positively polarized hydrogen, they do not form hydrogen bonds with the anionic nucleophile. The result, with respect to solvation, is a relatively weak interaction between the aprotic solvent and the nucleophile. The consequence of this weakened interaction is two-fold. First, by using an aprotic solvent we can raise the reactivity of the nucleophile. This can sometimes have dramatic effects on the rate at which a nucleophilic substitution reaction can occur. For example, if we consider the reaction between bromoethane and potassium iodide, the reaction occurs 500 times faster in acetone than in methanol.