# SN2 - Second-order Nucleophilic Substitution

## Stereochemistry of $SN2$ Reactions

The stereochemical result of a reaction is the consequence of its reaction mechanism. Therefore, by investigating the stereochemistry of the starting products and products, important details about a reaction mechanism and, in particular, the structure of its transition state are often obtained. In the case of $SN2$ reactions, the answers to several questions concerning the mechanism can be found through observation of the reaction's stereochemistry. Some things to be taken into consideration, for example, may be as follows: From which direction does the nucleophile attack the substrate? In which direction does the nucleofuge leave the substrate? How are substrate and nucleophile arranged in the transition state?

Retention
If the stereochemistry of the substrate remains in the product - that is, if the attacking nucleophile has the same spatial position in relation to the other substituents in the product, as the leaving group has in the substrate - this is called retention of configuration, or just retention.
Inversion
If the absolute configuration of the reaction center is inverted - that is, if the attacking nucleophile has a completely opposite spatial position in relation to the other substituents in the product, as the leaving group has in the substrate - this is called inversion of configuration, or inversion.

In reality, a mixture of retention and inversion is often found. If both processes occur to the same degree in a reaction with an asymmetric reaction center, the racemate is obtained. This is the case in pure $SN1$ reactions. However, in the investigation of the $SN2$ reaction mechanism, it must be ensured that the reaction is actually a pure $SN2$ reaction, which does not have any $SN1$ characteristics.

If a pure enantiomer is applied to an $SN2$ reaction, three different stereochemical results are conceivable:

• The initial spatial arrangement of the reaction center's substituents remains (retention).
• The initial substituent's spatial arrangement is inverted (inversion).
• Retention, as well as inversion takes place. If retention and inversion occur to the same degree, the reaction yields a racemate (racemization).
Tab.1
Fig.1
Fig.2

Certainly, the reaction can only yield a stereochemically distinguishable product when a chiral substrate that contains an asymmetric reaction center is applied.

Many experiments have been conducted to investigate the stereochemistry of $SN2$ reactions. These experiments have proven that in pure $SN2$ reactions always complete inversion occurs.

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