Your question is not clear. Do you mean "Can I use the word 'whichever' in a sentence instead of the word 'rational'"?
Note my amendments to your post in red. It is important to follow the rules of written English:
- Start every sentence with a capital letter.
- End every sentence with a single appropriate punctuation mark.
- Always capitalise the word "I".
- Do not put a space before a full stop, comma, question mark or exclamation mark.
- Always put a space after a full stop, comma, question mark or exclamation mark.