Cycloaddition reactions onto graphene were studied by means of first-principle calculations. We found that for all reactions studied there is a cooperative behavior responsible for dramatically increasing the reaction energies. This result explains the reason why the 1,3 dipolar cycloaddition of azomethine ylides is both feasible and slow. The cooperative effect decreases in the following order: [2 + 2]-addition of benzynes >1,3-dipolar cycloaddition >[2 + 1]-cycloaddition of NH. This cooperative behavior can open new avenues to fine-tune the physical and chemical properties of graphene. Our results indicate that it is possible to fine tune the band gap of graphene from 0 to 2 eV by varying the number of azomethine groups attached. (c) 2012 Elsevier B.V. All rights reserved.