SQL> show parameter BLANK_TRIMMING;

NAME                     TYPE    VALUE
------------------------------------ ----------- ------------------------------
blank_trimming               boolean     FALSE


SQL> alter system set BLANK_TRIMMING=true scope=spfile;

System altered.

Then bounce the database.

SQL> startup force;

SQL> update tblchar set col1=TRIM(col1);

SQL> alter table tblchar modify col1 char(16 byte);

Table altered.

ALTER TABLE t
   ADD (col2 CHAR (16));

UPDATE t
   SET col2 = TRIM (col1);

ALTER TABLE t
   DROP (col1);

ALTER TABLE t
   RENAME COLUMN col2 TO col1;