Description and General Mechanism


The SN1 mechanism is a nucleophilic substitution. A group more electronegative than carbon (X) is bonded to a carbon in the organic molecule and a nucleophile (Nu) is present. The SN1 mechanism has two steps.

  • In the first step, the X-group (the leaving group) leaves with both bonding electrons. This step creates a carbocation, for which reason it is the slower of the two reaction steps.
  • In the second step of the reaction, a nucleophile attacks the carbocation, forming a covalent bond.

Back to Nucleophilic Substitution.


Rate Law

The first step of the SN1 mechanism is the rate determining step. Because this first step has only one reactant, the reaction proceeds with a rate proportional to the concentration of R—X. SN1 reactions are 1st order.

rate = k[R—X]

Factors Affecting Rate

Competition With Other Reactions

The SN1 reaction always competes with the E1 reaction.

Carbocation Rearrangement

As in other reactions that give rise to carbocation intermediates, the intermediates in the SN1 mechanism can rearrange to become more stable. This rearrangement occurs in the intermediate before the subsequent intermolecular reaction steps.

  • Benzylic, allylic, and 3o carbocations are the most stable carbocations. Because they have similar stability, they will not rearrange during the reaction; doing so will not significantly increase their stability.
  • 2o carbocations will rearrange to form benzylic, allylic, or 3o carbocations where possible.
  • 1o and methyl carbocations are not formed as intermediates.

When This Mechanism Will Occur


More Information

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