Students are known to have various difficulties in dealing with organic reaction mechanisms. A systematic classification of these difficulties appears necessary to design appropriate support. This paper presents insights into whether and how Bloom's revised taxonomy can be used to classify student difficulties in dealing with organic reaction mechanisms. We conducted an interview study with 12 undergraduate chemistry students using problem-solving tasks on nucleophilic substitution and elimination reactions to provide examples to test the classification. In our attempt at systemization, student difficulties are perceived as unachieved learning objectives. The classification reveals that student difficulties pertain to different cognitive process and knowledge dimensions. Specific major difficulties occurred within each cell of Bloom's revised taxonomy and for individual students. Our analysis suggests that general support for dealing with reaction mechanisms might be less beneficial for some students and that more adapted support is needed. Our approach of using Bloom's revised taxonomy to classify student difficulties might also benefit other domains to better understand student difficulties and evaluate appropriate support.