hey can anybod help how to decide abt leaving groups in nucleophilic substitution reactions?

Answer 1

At a simple level, leaving groups are typically stable ions or molecules

Answer 2

My O.Chem book has entire chapters devoted to explaining this. One can't fully explain it with a few sentences. It basically comes down to determining if compound will bound more strongly with the carbocation formed during Sn1 and Sn2 reaction. Think of it this way: whichever compound can hold on the very electropositive carboaction will stick to the molecule and the loser will be the leaving group. So sometimes the potential leaving group will not be removed since there would be no adequate replacement. It gets more complicated when dealing with solvents (aprotic v protic), hydride and carbocation shifts, complex leaving groups, substitution reaction v elimination (E2 or E1) reactions, etc. (which is why there are entire chapters devoted to explaining it)
Don't be a bum. Read the chapter and get a strong foundation. I'm assuming you're doing an organic chemistry class, and it all goes uphill after learning the substitution reactions.