#S_(N)1# 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^-#
And as the #"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-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......
