means SN1 , SN2 or E1 E2

simple dear when

substitution occurs then there will b either sn1 or sn2

for Sn1 rxn goes through carbocation n nucleophile or electrophile is not so  strong it wait for the leaving group tht it should go n then it comes to attach 

 

for SN 2 rxn carbocation is not formed becoz leaving group is not so strog n when nucleophile n electrophile attacks then it leave tht corresponding atom n for it nucleophile n electrophile is strong

 

E1 n E2 is Elimination rxn.

E1 is similar as SN1

n

E2 is similar as SN2

 i hope u got it