The SN1 reaction
is an organic reaction
" 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:
- Changing the carbon centre:
- Formation of a carbocation: due to the separates of leaving group (molecule A) from the carbon.
- Nucleophilic attack: a nucleophile (molecule B) joins onto the carbon.
- 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:
- C(CH3)3Br --> C(CH3)3+ + Br-
- 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]
is irrelevant in the determining step's rate, in which only the substrate
Because the intermediate cation, R+, is planar, an optically active substrate gives a racemic mixture because nucleophilic attack can occur from either side.