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SN1 reaction
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SN1 reaction

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. 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 groupss 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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