The S_N1 reaction is an organic reaction in chemistry. "S_N" stands for substitution nucleophilic and the "1" represents the fact that the rate-determining step is unimolecular. Increasing or decreasing the concentration of that kind of molecule in a closed system changes the rate of the reaction in a direct relationship.

The S_N1 reaction takes place in two steps:

1. Formation of a carbocation by separation of leaving group (molecule A) from the carbon.
2. Nucleophilic attack: a nucleophile (molecule B) joins onto the carbon.

If the nucleophile B is a neutral molecule (very often it is a solvent molecule), a third step is required to complete the reaction.

3. Deprotonation: Removal of a proton on the protonated nucleophile by a nearby ion or molecule

An example reaction:

1. (CH₃)₃CBr → (CH₃)₃C⁺ + Br^-
2. (CH₃)₃C⁺ + H₂O → (CH₃)₃C-OH₂⁺
3. (CH₃)₃C-OH₂⁺ + H₂O → (CH₃)₃COH + H₃O⁺

This gives the overall reaction:

(CH₃)₃CBr + H₂O → (CH₃)₃COH + HBr

SN1 occurs where the central carbon atom is surrounded by bulky groups and produces a stable carbocation. The rate depends only on the concentration of the substrate.

rate = k[RX]

Nucleophilicity is irrelevant in the determining step's rate, in which only the substrate is crucial.

Because the intermediate carbocation, R⁺, is planar, an optically active substrate gives a racemic mixture because nucleophilic attack can occur from either side.

External links

• Diagrams: Frostburg State University
• Exercise: the University of Maine