Question

How does changing the solvent polarity affect reactant vs product stability? Can you justify these observations based on your underwear of organic chemistry and bonding behavior?

Answer 1

Solvent is key ingredient of the reaction which affects many factors like reaction rate, stability.

Solvent stabilize product as well as reactant depends on the nature of solvent.

Different solvents can affect the equilibrium constant of a reaction by differential stabilization of the reactant or product. The equilibrium is shifted in the direction of the substance that is preferentially stabilized. Stabilization of the reactant or product can occur through any of the different non-covalent interactions with the solvent such as H-bonding, dipole-dipole interactions, van der waals interactions etc.

The cis enol form predominates in solvents of low polarity, whereas the diketo form predominates in solvents of high polarity. The intramolecular H bond formed in the cis enol form is more pronounced when there is no competition for intermolecular H bonding with the solvent. As a result, solvents of low polarity that do not readily form H bonds allow cis enolic stabilization by intramolecular H bonding.

Let’s see example of famous SN1 reaction and role of solvent-

The solvent used in substitution reactions inherently determines the nucleophilicity of the nucleophile; this fact has become increasingly more apparent as more reactions are performed in the gas phase.^[12] As such, solvent conditions significantly affect the performance of a reaction with certain solvent conditions favoring one reaction mechanism over another. For S_N1 reactions the solvent's ability to stabilize the intermediate carbocation is of direct importance to its viability as a suitable solvent. The ability of polar solvents to increase the rate of S_N1 reactions is a result of the polar solvent's solvating the reactant intermediate species, i.e., the carbocation, thereby decreasing the intermediate energy relative to the starting material. The following table shows the relative solvolysis rates of tert-butyl chloride with acetic acid (CH₃CO₂H), methanol (CH₃OH), and water (H₂O).

Solvent	Dielectric Constant, ε	Relative Rate
CH3CO2H	6	1
CH3OH	33	4
H2O	78	150,000