When the grammar in a sentence calls for a noun clause — e.g., direct object, object of the infinitive, object of the preposition — the clause has to begin with whether, not if.
…I don’t know WHETHER he will go.
…He doesn’t remember WHETHER she was there.
…We want to ask WHETHER she is coming.
…I am not sure WHETHER I can attend.
…She is not aware WHETHER he is alive or dead.
…He is not certain WHETHER she knows that.
In not one of these sentences is the word if correct.