Aliphatic SN1 reaction is carried out in two steps. In first step carbocation is formed and its formation is based on the stability of carbocation,
C6H5
+
C
H2>CH3−
+
C
H−CH3>CH3—
+
C
H2
In second step, nucleophile is attracted towards carbocation to give final products. Hence, order of SN1 reaction is
C6H5
+
C
H2>CH3−
+
C
H−CH3>CH3—
+
C
H2
The aryl halides e.g.f chlorobenzene are less reactive as compared to alkyl halides towards nucleophilic reagents in either SN2 or SN1 reaction because carbon-halogen bond in the aryl halide is strong (due to its double bond character).