なくちゃ is the short informal of なくてはいけません but why is there なくちゃいけないの rather than なくちゃの?

Question

なくちゃ is the short informal of なくてはいけません but why is there なくちゃいけないの rather than なくちゃの?

For me, なくちゃいけないの sounds like double negative being used here.

Answer 1

なくちゃ is the short informal of なくてはいけません

Well, なくちゃ is a contracted form of なくては. It has no "counterpart" for the part いけません in the formal sense.

For your information,

なくてはいけません -> なくちゃいけません -> なくちゃいけない -> なくちゃだめ -> なくちゃ

is epitomising the process of (possible) contraction, but you should be careful in resorting to the final (rightmost) step. If you are talking with someone whom you are familiar with, there is little room for confusion/misunderstanding, but remember that なくちゃ without いけません (or いけないの or whatever) is very colloquial and casual.