# SN2 - Second-order Nucleophilic Substitution

## Summary: $SN2$ Reaction

Primary and secondary alkyl halides react with nucleophiles in an $SN2$ mechanism.

• $SN2$ reactions are second-order reactions, as their reaction rate depends on both the substrate's and the nucleophile's concentration.
• The nucleophile attacks the substrate from the opposite side of the leaving group, which is called a back-side attack. The absolute configuration at the reaction center of the substrate is inverted during an $SN2$ reaction. This inversion is known as the Walden inversion.
• The higher the reaction center's number of substituents (not hydrogen) is, the lower the reaction rate of the $SN2$ reaction is, as well. The $SN2$ reaction is then superseded by the $SN1$ reaction or elimination more and more.
• Substituents in the position adjacent to the reaction center reduce the reaction rate.
• The more effective the nucleophile is, the higher the reaction rate is. Soft bases that do not contain bulky substituents are usually good nucleophiles.
• Weak bases are often good leaving groups.
• Protic solvents reduce the reaction rate, as they shield the nuceophile by solvation.
• Polar aprotic solvents do not reduce the reaction rate.
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