The SN1 reaction is an organic reaction in chemistry. "SN" 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 SN1 reaction takes place in two steps:
- Formation of a carbocation by separation of leaving group (molecule A) from the carbon.
- 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:
- (CH3)3CBr → (CH3)3C+ + Br-
- (CH3)3C+ + H2O → (CH3)3C-OH2+
- (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.
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