How to convert empty spaces into null values, using SQL Server?

I have a table and the columns on this table contains empty spaces for some records. Now I need to move the data to another table and replace the empty spaces with a NULL value.

I tried to use:

REPLACE(ltrim(rtrim(col1)),' ',NULL)

but it doesn't work. It will convert all of the values of col1 to NULL. I just want to convert only those values that have empty spaces to NULL.