In: Chemistry
Question 1) What affect would switching from a lipophilic to a hydrophilic solvent have on the overall rate of an SN1 reaction mechanism? Explain your answer.
Question 2) What affect would switching from a hydrophilic to a lipophilic solvent have on the overall rate of an SN2 reaction mechanism? Explain your answer.
3 MARK QUESTIONS EACH
Hi,
A) Influence of the Solvent on SN1 Reactions
The solvent plays a great role in SN1 reactions. However, in contrast to SN2 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 orpolar character, the intermediate and transition state can be effectively stabilized by polar and protic 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.
Table Dielectric constants (?, at 25