Solvent effects on chemical reaction

A solvent can be any substance, that turns into a solution by dissolving a solid, liquid, or gaseous solute. The solvent is usually a liquid, but can also be a solid or gas.

In our daily life, we will find the best example of solvent, that is none other than water.

Solvent’s common uses ranges from dry cleaning agents, paint thinners, nail polish removers, glues, spot removers, detergents and in personal care products like perfumes

Examples of solvents

Toluene

Acetone

Methyl acetate

Hexane

Ethanol

Solvents find various applications in chemical, pharmaceutical, oil, and gas industries, including in chemical syntheses and purification processes.

Most other commonly-used solvents are carbon-containing chemicals. These are termed as organic solvents.Solvents usually have a low boiling point and as a result, they will evaporate easily or can be removed by a various simple process called distillation, thereby leaving the dissolved substance behind.Solvents are inert nature, as they will not react chemically with the dissolved compounds. These can also be used to extract soluble compounds from a mixture, the most common example is the brewing of coffee or tea with hot water.

Solvent classifications

The solvents are basically grouped into non-polar, polar aprotic, and polar protic solvents.

Solvent effects on chemical reaction

Solvents can have an effect on various properties of substances like solubility, stability and reaction rates

Solvents effects on solubility

A solute dissolves in a solvent only when it forms favourable interactions with the solvent. This dissolving process all depends upon the free energy change of both solute and solvent. This in turn free energy of solvation is again depended upon several factors.

Solvents effects on stability

Different solvents can affect the equilibrium constant of a reaction by differential stabilisation of the reactant or product. The equilibrium is shifted in the direction of the substance that is preferentially stabilised. Stabilisation 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.

In another instance, the ionisation equilibrium of an acid or a base is affected by a solvent change. The effect of the solvent is not only because of its acidity or basicity but also because of its dielectric constant and its ability to preferentially solvate and thus stabilise certain species in acid-base equilibria. A change in the solvating ability or dielectric constant can thus influence the acidity or basicity.

Solvents effects on reaction rates

Solvents can affect rates through equilibrium-solvent effects that can be explained on the basis of the transition state theory. In essence, the reaction rates are influenced by differential solvation of the starting material and transition state by the solvent.

Other effects of solvents

The solvent used in substitution reactions inherently determines the nucleophilicity of the nucleophile. As such, solvent conditions significantly affect the performance of a reaction with certain solvent conditions favouring one reaction mechanism over another. For SN1 reactions the solvent’s ability to stabilise the intermediate carbocation is of direct importance to its viability as a suitable solvent. The ability of polar solvents to increase the rate of SN1 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.

SN1 reactions

The SN1 reaction is a substitution reaction in organic chemistry. ‘SN’ stands for nucleophilic substitution and the ‘1’ represents the fact that the rate-determining step is unimolecular. The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary alkyl halides under strongly basic conditions or, under strongly acidic conditions, with secondary or tertiary alcohols.

SN2 reactions

The SN2 reaction is a type of reaction mechanism that is common in organic chemistry. In this mechanism, one bond is broken and one bond is formed synchronously. SN2 is a kind of nucleophilic substitution reaction mechanism.

The case for SN2 reactions is quite different, as the lack of solvation on the nucleophile increases the rate of an SN2 reaction.

The reactions involving charged transition metal complexes (cationic or anionic) are dramatically influenced by solvation, especially in the polar media.

© WOC Article

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