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 take place on the first order, that is it happens with only one molecule. 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:

  1. Changing the carbon centre:
    1. Formation of a carbocation: due to the separates of leaving group (molecule A) from the carbon.
    2. Nucleophilic attack: a nucleophile (molecule B) joins onto the carbon.
  2. Deprotonation: Removal of a proton on the protonated nucleophile by a molecule (sometimes the same species as A) in the solvent, usually protic.

An example reaction:

  1. C(CH3)3Br --> C(CH3)3+ + Br-
  2. C(CH3)3+ + OH- --> C(CH3)3OH

This gives the overall reaction:

C(CH3)3Br + OH- --> C(CH3)3OH + Br-

SN1 occurs where the central carbon atom is surrounded by bulky groupss. 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 cation, R+, is planar, an optically active substrate gives a racemic mixture because nucleophilic attack can occur from either side.

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