ORA-00933: SQL command not properly ended

I have Oracle v10g on Windows system.

SQL> startup mount;
ORACLE instance started.

Total System Global Area  293601280 bytes
Fixed Size                   788908 bytes
Variable Size             154138196 bytes
Database Buffers          138412032 bytes
Redo Buffers                 262144 bytes
Database mounted.
SQL> ALTER DATABASE CONVERT TO PHYSICAL STANDBY;
ALTER DATABASE CONVERT TO PHYSICAL STANDBY
                       *
ERROR at line 1:
ORA-00933: SQL command not properly ended

Please help

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