alter statement in sybase

I wrote some SYbase sql to check if a column exists. If it does not exists I create a new column in the table. It almost works but something is wrong with my exec statement at line 3. When I try and run the code code block below I get an error message --> stored procedure '"ALTER TABLE dbo.customers ADD CustLoyalty VARCHAR(20)"' not found.
 So can someone tell me what is wrong at line 3 below?

1  if not exists (select 1 from syscolumns where id= object_id('dbo.customers') and name = 'CustLoyalty')
2   begin
3  exec "ALTER TABLE dbo.customers ADD CustLoyalty VARCHAR(20)"
4  end
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It looks like exec is looking for the name of a stored procedure.  Have you tried running it without the exec and quotes and just using alter table directly?

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