SN1 ) this reaction proceeds  according to these steps

1) hetrolatically cleavage of carbon halogen bond

2)formation of carbocation(rearrangment)

3)formation of new bond

eg) CH₃CH₂CH_2Cl + KOH (aq) = CH₃CH(OH)CH₃

mechanism

   CH₃CH₂CH2Cl  =  CH₃CH_{2CH2⁺       ( 1^O CARBOCATION)}

 1^o carbocation rearranges to 2^o by hydride shift , to gain stablity ...

    CH₃CH⁺CH₃ + OH^- (neucleophile) = CH₃CH(OH)CH₃

order of reactivity  = benzyl>allyle>3^o >2^o>1^o                  (on the basis of carbocation stablity))

SN2)

in this reaction breaking of bonds & formation of new bonds take place samultaneously during a transition state...

in this state neuclephile(OH^-) attacks from back side and at the same time carbon halogen bond breaks and product is formed...

this transition state is pentavalent & attaking neuclophile ,halogen are  along axial bond ......no rearrangment occures...

eg)

     CH₃CH₂CH₂Cl + OH^- => CH₃CH₂CH₂OH

this reaction favoures  aprotic solvents & strong base ......

order of reactivity = 1⁰>2^o>3⁰              (less staric hindered alkyle halide will react fast )