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) CH3CH2CH2Cl + KOH (aq) = CH3CH(OH)CH3
mechanism
   CH3CH2CH2Cl  =  CH3CH2CH2+       ( 1O CARBOCATION)
 1o carbocation rearranges to 2o by hydride shift , to gain stablity ...
    CH3CH+CH3 + OH- (neucleophile) = CH3CH(OH)CH3
order of reactivity  = benzyl>allyle>3o >2o>1o                  (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>2o>30              (less staric hindered alkyle halide will react fast )