the properties of a good nucleophile :
1. high electon density on central atom.
2. less sterically hindered.
e.g. OH- , RO-
the properties of a good base:
* A good nucleophile can also be a good base.
* wheather SN2 takes place or E2 depends on nature of both substrate and reagent(nucleophile),
first observe substrate- primery , sedcondry or tertiery
then if its primery , see if its hindered or not
if its hindered then surely elimination takes place (nucleophile will not substitute due to steric reasons)
if its not hindered then see if the nucleophile is hinderded or not .
if either of two (substrate or nucleophile) is hindered then E2 takes place , OR Sn2 takes place.
Secondary substrate : elimination will be major if nucleophile is good base
tertiery substrate : elimination.