SQL & PL/SQL

I want to display only part of a column value in sql select. I use rtrim as following:

select rtrim(SESS_NAME, ' to ') src
from S_SESSION;

suppose the column value is 'A to B', I want to see

src
--------
A

but instead, I got the original string 'A to B'. why is that?
I tried replacing SESS_NAME with a dummy string, then rtrim works. However, I do need to parse from SESS_NAME.
Please help!