Solvent Polarity in SN1 and SN2 Reactions

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This Demonstration shows how the solvent polarity influences the nucleophilic substitution in both SN1 and SN2 reaction mechanisms [1].


Whether a substitution reaction occurs under kinetic or thermodynamic control is determined by the reactant and transition state energies. You choose between them using "kinetic/thermodynamic control."

The control "reaction mechanism" selects between the SN2 and SN1 mechanisms, showing the corresponding reaction graphs. In the SN2 case, the plot will show only a single maximum, while in the SN1 case, it will show two maxima. The fundamental parameters associated with kinetic control are the activation energies and the difference in activation energies. Increase in solvent polarity is shown by the red curve, while polarity minimum is shown by the green curve [2].

To highlight the molecule-ion interaction with polar solvents, the reactants are enclosed in rectangles. The magnitude of the interaction with the polar solvent is indicated by the intensity of the background color. A transition state or a molecular ion can interact with the polar solvent, with the resulting effect of a lower energy. So the higher the interaction, the higher the background color intensity. When there is no background color, no interaction happens between the species and the solvent. In formulas, only the the part in blue is influenced by the solvent polarity.

Vary "solvent polarity" to make the curves change. When charges in the transition state increase and are more localized, this causes a rise in the activation energy () and, as a consequence, there will be a lower reaction rate. Conversely, if the charge of the transitional state decreases or is more dispersed, the reaction rate increases ().

To show these effects, the generic molecular structure of reactants and transition states is shown.

Three different types of compounds, are selectable through "examples SN2."

For SN1 reactions, a single type of a single transition state is observed.


Contributed by: D. Meliga, V. Giambrone, L. Lavagnino and S. Z. Lavagnino (October 12)
With additional contributions by: F. Calcagno
(ITIS A. Artom, Asti)
Open content licensed under CC BY-NC-SA


Snapshot 1: in an SN2 reaction, increasing solvent polarity changes the interactions between molecules of the reactants and the transition state; on the left an increase in the activation energies causes a decrease in the reaction rate; on the right, the opposite occurs

Snapshot 2: in an SN1 reaction, we can observe two maxima, showing the presence of two reaction pathways; only the first one is relevant to the reaction rate (bottleneck approximation) [1]

Snapshot 3: for the cases considered, the change in polarity does not appreciably affect the heat exchanged in the reactions


[1] S. Z. Lavagnino. Effetto solvente, nucleofilia reattività alogenuri alchilici [Video]. (Sep 14, 2023) _Gib&index=49.

[2] E. V. Anslyn and D. A. Dougherty, Modern Physical Organic Chemistry, Sausalito, CA: University Science, 2006.


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