If and whether are nowadays usually considered subordinators.
Though their usages overlap, they are not totally interchangeable:
Ask him whether/if I can come. ['if' more colloquial] ['or not' may be faintly implied]
If/*Whether he is really a policeman, I'm a Chinaman.
I will come if / * whether she is going to be there. [no 'or not' implied: no choice involved]
They talked about whether / *if United still had a chance of winning
the league. ['or not' quite strongly implied: 'choice' of alternative possibilities]
I will come whether or not / *if or not she is going to be there. [not may be placed at the end of the sentence] ['choice' (ie alternatives) overtly stated]