S_(N)1SN1 reactions are thus presumed to occur thru the intermediacy of a 3-coordinate carbocation:
CR_1R_2R_3X stackrel("rate determining step")rarr""^(+)CR_1R_2R_3 +X^-CR1R2R3Xrate determining step−−−−−−−−−−−−→+CR1R2R3+X−
And as the "RDS"RDS, this is the slow step, i.e. the formation of a carbocation.....and the formation of a bond between the three-coordinate carbocation and a nucleophile is usually the fast step......
R_1R_2R_3C^(+) + Nu^(-) stackrel("fast")rarrR_1R_2R_3C-NuR1R2R3C++Nu−fast−−→R1R2R3C−Nu
Because the carbocation is three-coordinate it cannot (normally) support optical activity, and an optically active substrate would give rise to a racemic product......
