Hey substitution reaction occurs when an atom or group from an added reagent substitute for one in the reactant

Answer 1

Substitution: The action of a "nucleophile" replacing a "leaving group electrophile."

Refer to the acidity trend to determine the best leaving groups.

(To the right and down and periodic chart, Iodine is the most acidic in terms of Organic Chemistry, and therefore Iodine is an INCREDIBLE leaving group.)

The best nucleophile is a negatively charged carbon. Then Nitrogens, then Oxygens, and finally Halogens.

This means, it is often the case, that Nitrogens can replace oxygens and halogens. Yet, oxygens only replace halogens. And Halogens don't replace many things, without special reactions.

Hope this helps. <3