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SN2: kinetics, mechanism and stereochemistry

Reaction kinetics of the nucleophilic substitution of (R)-2-bromooctane with hydroxide is second order.

Energy profile of the SN2 reaction of 2-bromooctane with its transition state

v = k[C8H17Br][HO−] mol/L·s

A reaction following such 'second order' kinetics is called bimolecular because both substrate and nucleophile are involved in the 'transition state' (TS) of the rate-limiting step. The nucleophile attack and the leaving group expulsion must be synchronized. This synchronization is explained by a TS in which the carbon under attack is momentarily pentacoordinated, in sp2 hybridization.

Animation of the SN2 mechanism: the nucleophile attacks from the side opposite the leaving group and the configuration is inverted
Beginning of the SN2 attack: the nucleophile orbital approaches the small lobe of the carbon sp³ orbital

The attack begins... The nucleophile approaches from the face opposite the leaving group. An occupied orbital of the nucleophile starts interacting with the small lobe of the sp3 orbital of the carbon still bonded to the leaving group. Only this approach leads to an effective collision.

SN2 transition state: carbon rehybridized sp² with partial bonds to the nucleophile and the leaving group

The activation energy is overcome and the system reaches the TS. The attacked carbon is transitorily pentacoordinated, in sp2 hybridization. Its remaining p orbital keeps partial bonds with the nucleophile and the leaving group. The R groups are on a plane.

End of the SN2 reaction: the leaving group departs and the carbon recovers sp³ hybridization with inverted configuration

The strain of the TS is released as the leaving group is expelled. The carbon gradually recovers its initial sp3 hybridization. The configuration of the attacked carbon ends up inverted.

The ease of an SN2 reaction depends on:

  • The nature of the leaving group.
  • The reactivity of the nucleophile.
  • The steric factors exerted by the R groups.