Nucleophilic Substitution (SN2) Reactions

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This Demonstration discusses the main features of nucleophilic substitution reactions of alkyl halides, which take place in a single concerted stage. These are known as SN2 reactions.


Select "SN2 mechanism" to show the kinetics of the reaction: the nucleophile approaching the organic molecule forms a bond as the halide moves away, gradually breaking its bond. Select the organic molecule from among "Chloromethane," "Chloroethane," "(S)-2-Chlorobutane," "2-Chloro-2-methylbutane" and "1-Chloro-2, 2-dimethylpropane."

The carbon to which the halogen is attached varies from primary to tertiary, causing the reaction to occur with increasing difficulty. Steric hindrance hampers the formation of a bond with the nucleophile.

The steric hindrance also exists with 1-Chloro-2, 2-dimethylpropane, which, even for a primary carbon, finds it difficult to react because of the bulky substituent tert-butyl [1, 2].

At the top right are shown the orbitals of the during the reaction. For the hybrid orbitals associated with the three bonds that do not change, the angle between them increases in the first half of the reaction, as the hybridization transitions from to . The reverse transformation occurs in the second phase. The orbital used in the formation of the bond with the halogen initially turns into a orbital and then an orbital again, but showing a 180° displacement with respect to the initial position.

Select "reaction rate" to show the slope of the nucleophilic substitution reaction rate as a function of the RCl and nucleophile Nu concentrations: rate of formation of . For more information, see [1, 2].

Select "comparing SN1 and SN2 reactions" to see how the same parameters affect the two different types of reactions.

Select "reactivity of alkyl halides" to see how increasing electronegativity leads to an increase in the activation energy, making the reaction more difficult.


Contributed by: D. Meliga, V. Giambrone, L. Lavagnino and S. Z. Lavagnino (January 2023)
Open content licensed under CC BY-NC-SA



Snapshot 1: C-Cl bond is about to break, with early formation of the C-Nu bond

Snapshot 2: slopes of the reaction rates are different for the two types of reactions

Snapshot 3: increasing electronegativity increases the activation energy of the reaction


[1] H. Hart, L. E. Craine and D. J. Hart, Organic Chemistry: A Short Course, 10th ed., Boston: Houghton Mifflin, Co., 1999.

[2] S. V. Lavagnino. SN1 e SN2 [Video]. (Aug 23, 2022)

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