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SN2 - Second-order Nucleophilic Substitution

Influence of the Solvent on Nucleophilic Substitutions

The solvent has a great influence on nucleophilic substitutions. Aside from the substrate structure, it largely depends on the solvent's character whether a nucleophilic substitution will display an SN2 or an SN1 mechanism. Furthermore, due to the different stabilization of the transition state by the solvent, the reaction rate of nucleophilic substitutions is considerably controlled by the solvent. According to their physical properties, solvents may be classified into several classes. For instance, it may be distinguished between polar and apolar solvents. A measure of polarity is the dielectric constant ε. The higher the dielectric constant and thus the polarity of a solvent is, the more efficiently electrically charged species, such as the transition states of SN1 and SN2 reactions, are stabilized through their interaction with the solvent molecules. Apolar solvents, such as hydrocarbons, possess a low dielectric constant. Consequently, they are hardly capable of stabilizing electric charges. As a result, salts and (highly) polar molecules are almost insoluble in such solvents. Polar solvents, such as ethanol, DMSO (dimethyl sulfoxide) and water, show a high dielectric constant. They can therefore stabilize electrically charged species by solvation quite efficiently. As a result, salts and (highly) polar molecules are readily soluble in such solvents.

Tab.1
Dielectric constants of some common solvents.
Aprotic solventsDielectric constant (ε, at 25 °C)Protic solventsDielectric constant (ε, at 25 °C)
Hexane1.9Acetic acid6.2
Benzene2.31-Methyl-2-propanol11
Diethyl ether4.3Ethanol34.3
Chloroform4.8Methanol33.6
Hexamethylphosphoric triamide(HMPT) 30Formic acid(methanoic acid) 58.0
Dimethyl formamide (DMF)38Water80.4
Dimethyl sulfoxide (DMSO)48

Furthermore, it may be distinguished between protic and aprotic solvents, as well. Protic solvents possess at least one hydrogen that is bound to a more electronegative atom, such as oxygen or nitrogen. Due to the significant difference in electronegativity between hydrogen and oxygen or nitrogen, respectively, O-H and N-H bonds are appreciably polarized. As a result, protic solvents can form hydrogen bridge bonds. Aprotic solvents, on the other hand, possess no acidic hydrogens. Consequently, they cannot form hydrogen bridge bonds.

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