More stable intermediate or product, more will be formed.

1. More stable intermediate or product, more will be formed.

             A <=================> B

             x%                                               y%

If B is more stable than A ==>  y > x

If A is more stable than B ==>  x > y

CH₃—CH₂—Cl  <=========>  CH₃—C⁺H₂  +  Cl^-

Reactant-I                                            Intermediate-I

(CH₃)₃C—Cl     <=========>    (CH₃)₃C⁺  +  Cl^-

Reactant-II                                          Intermediate-II

(CH₃)₃C⁺  is more stable than  CH₃—C⁺H₂  So, Intermediate-II is more formed than Intermediate-I and (CH₃)₃C—Cl  is more reactive than CH₃—CH₂—Cl 

2. Reactivity of reactant is directly proportional to the stability of intermediate.

    Reactivity  ∝  Stability of intermediate

   More stable intermediate, more reactive corresponding reactant.

Reactivity order of Alkyl halide [SN¹ reaction]

a.      CH₃—Cl     b.  CH₃—CH₂—Cl   c.  (CH₃)₂C—Cl   d.  (CH₃)₃C—Cl

Their Carbocation and stability order:-

a.      CH₃⁺       <    b.  CH₃—C⁺H₂   <    c.  (CH₃)₂C⁺   <    d.  (CH₃)₃C⁺

a.      Methyl carbocation  <  b. 1’C  <  c. 2’ C  <  d. 3’C

So reactivity order:-

a.      CH₃—Cl    <   b.  CH₃—CH₂—Cl   <   c.  (CH₃)₂C—Cl   <   d.  (CH₃)₃C—Cl