In nucleophilic substitution reactions (SN2), one group, called the leaving group, is exchanged for another, called the nucleophile.


[1] Substrate - Species containing the leaving group
[2] Nucleophile - Lewis base capable of attacking atoms with positive polarity (or charge)
[3] Reaction product
[4] Leaving group - Species that leaves the substrate, being replaced by the nucleophile

The SN2 (bimolecular nucleophilic substitution) is a concerted reaction, that is, it occurs in a single step.

mecanismo de la sn2
The mechanism consists of the nucleophile attacking the carbon containing the leaving group. This carbon presents an important positive polarity, due to the electronegativity of the halogen. At the same time that the nucleophile attacks, the carbon-halogen bond is broken, obtaining the final product.

The energy diagram represents the potential energy against the reaction coordinate, and gives us the energy change suffered by the reactants when transformed into the corresponding products.


During the course of the reaction, the reactants increase their energy until they reach the transition state, forming the activated complex. At that point the carbon-nucleophile bond is half formed and the carbon-leaving group half broken.

Nucleophile attack on the carbon containing the leaving group can occur in two different ways. In the first case, the nucleophile can approach the substrate from the side on which the leaving group is located. This approach is called frontal attack, in which the nucleophile takes the place of the leaving group, producing retention in the configuration .


The leaving group has the mission of leaving the substrate at the same time that the nucleophile attacks. The best leaving groups are the less basic species, since they bond weakly to carbon. In the periodic table the best leaving groups are to the right and bottom.


outgoing group 01

[1] Fast

[2] Slow

Nucleophiles are Lewis bases that attack a carbon, displacing the leaving group. Ionic nucleophiles are common, but there are also numerous examples of neutral nucleophiles. The general characteristic of all nucleophiles is the presence of lone pairs on the attacking atom.

nucleophile 01

[1] Slow

[2] Fast

There is a significant speed difference between the substrates; methyl bromide, ethyl bromide, isopropyl bromide and tert-butyl bromide when reacted with a nucleophile under the same conditions.

substrate 01

As you move up the list of reactions, the speed increases.

Stressed cyclic substrates react more slowly via the SN2 mechanism than acyclic substrates. Thus, isopropyl bromide reacts faster with the acetate ion than does cyclopropyl bromide.