Is there a place for closed, well-structured problems in PBL?

The use of open-ended, ill-structured problems is often listed as one of the essential characteristics of Problem Based Learning. While the use of problems of that sort seems eminently reasonable in the context of professional disciplines, in which practitioners are often faced with complex problems, their use in the basic Sciences is more problematic. There are excellent examples of the use of open-ended problems in introductory level Science courses, but it is not so clear how to apply them in some more focused, advanced level courses. Does it make sense to attempt to devise open ended problems when dealing with topics in which ‘real life’ problems have only one correct solution? If the use of closed problems seems appropriate to the subject material, are any of the major benefits of using PBL lost? Can the curriculum, which is often content heavy in the later years of Science programmes, be properly ‘covered’ using PBL? These issues will be discussed in relation to a 3rd year course in ‘Mechanism in Organic Chemistry’. The evolution of the PBL methodology used, the type of problems used, and the nature of the ‘scaffolding’ provided in this course will be described. Closed problems, aligned with the intended learning outcomes, are now used, and it will be argued that they can be very effective. The case will be made that PBL can play a very valuable role in advanced level Science education, and that educators should not be deterred from adopting it by the (over) emphasis on using openended problems.