# SN2 - Second-order Nucleophilic Substitution

## Mechanism of the $SN2$ Reaction

In an $SN2$ reaction, the nucleophile (Nu) attacks the substrate from the side opposite to the leaving group (L). This is called a back-side attack.

Fig.1
$SN2$ reaction.

While approaching the substrate, the nucleophile's n orbital (lone electron pair) interacts with the antibonding σ* orbital of the C-L bond.

With increasing overlapping of the nucleophile's n orbital and the substrate's σ* orbital, the antibonding σ* orbital becomes more and more occupied. As a result, the C-L bond is weakened and extended. To the same extent, the C-Nu bond is strengthened and shortened. As the spatial position of the three remaining substituents is largely determined by their steric interactions with the other substituent(s), they begin to turn away from the entering nucleophile towards the leaving group.

Fig.2
Orbital interactions during an $SN2$ reaction.

In the transition state, the C-L bond is partially cleaved, while the C-Nu bond is partially formed, and the remaining three substituents are arranged trigonal planar around the central carbon. The central carbon is $sp2$-hybridized, whereat the p orbital is used for the partial C-L and C-Nu bond. The negative charge that has initially been located on the nucleophile is now distributed between the nucleophile and the leaving group. The C-L bond is then finally completely cleaved, while the C-Nu bond is completely formed and the negative charge is then entirely transferred to the leaving group.

Fig.3
Energy-reaction coordinate diagram of an $SN2$ reaction.

As a result of the $SN2$ mechanism, the three remaining substituents are rejected. This is comparable to an umbrella's inversion in a windstorm. This inversion is called Walden inversion, named after Paul Walden (1863-1957). As a result of the Walden inversion, the absolute configuration at the central carbon is also inverted if the central carbon is a chirality center. If the nucleophile has the same CIP priority with respect to the remaining three substituents as the leaving group, a molecule with R configuration is converted into a molecule with S configuration, and the vice versa. However, if the nucleophile's and the leaving group's CIP priorities are different with respect to the remaining three substitutents, it depends on the position of the nucleophile and the leaving group in the CIP priority sequence of the central carbon's substituents whether or not the absolute configuration is changed. Nevertheless, the absolute configuration is always actually inverted during an (pure) $SN2$ reaction. The differences in the naming of absolute configration arise from the fact that the product and the starting product are completely different compounds whose absolute configurations have to be individually determined according to the CIP priorities of their substituents.

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