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SN2 Mechanism with Transition State

The SN2 reaction is a bimolecular nucleophilic substitution reaction that involves a negative nucleophile generally paired with a cation. In the final product, the nucleophile sits in place of the leaving group and has the opposite stereochemical designation.

A Few Important Things About SN2 Reactions

  • The SN2 reaction works with zeroth (0º), primary (1º), and secondary (2º) leaving groups but NOT tertiary (3º).

  • It inverts stereochemistry, so watch out if you have a 2º leaving group because you could have a chiral center!

    • If the leaving group is on a chiral center, the stereochemical designation will be inverted—i.e. an R-designation will become an S-designation

    • Even if it’s not a chiral center, it’s still good practice to indicate inversion by switching the wedge/dash information

  • SN2 reactions tend to occur at room temperature or in the cold

  • Bulky bases do not work for SN2 reactions

  • And finally, SN2 reactions are not subject to rearrangement because there is no carbocation to rearrange

The SN2 Mechanism

Secondary (2º) leaving group and negative nucleophile
The starting material has an S-designation and secondary (2º) leaving group. The reagent is a negative nucleophile

In this example, the Z represents the leaving group, NU represents the nucleophile, and sodium is used as the cation but it could be potassium, lithium, or something else. The starting material used here has an S-designation.

bimolecular nucleophilic substitution mechanism
The nucleophile forms performs a backside attack, kicking off the leaving group

The mechanism is a single-step reaction with two arrows. In what's called a backside attack, the nucleophile forms a bond to the carbon to which the 0º, 1º, or 2º leaving group is attached and kicks the leaving group off the carbon. In other words, the nucleophile forms a bond to the carbon holding the leaving group. Carbon can only have four bonds, so it breaks its bond to the leaving group upon the nucleophile’s attack.

bimolecular nucleophilic substitution transition state
Transition state showing bonds forming and breaking and the nucleophile's and the leaving group's partial negative charges.

In the transition state, the nucleophile is forming a bond to carbon and the leaving group's bond to carbon is breaking. Notice that at this point the leaving group and nucleophile both have partial negative charges as well as partial bonds, which are indicated by the dashed lines.

bimolecular nucleophilic substitution inversion of stereochemistry
The final product has the nucleophile attached with inverted stereochemistry

In our final product, the nucleophile is attached to carbon, stereochemistry at the reactive site has been inverted—in this case from S to R—and of course the leaving group is off to the side floating around in solution, probably paired with that cation that we had in the beginning.

SN2 Mechanism with Transition State and helpful table
Here it is in all its glory! I hope this table helps too

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