Defects change essentially not only the electronic properties but also the chemical properties of graphene, being centers of its chemical activity. Their functionalization is a way to modify the electronic and crystal structure of graphene, which may be important for graphene-based nanoelectronics. Using hydrogen as an example, we have simulated a chemistry of imperfect graphene for a broad class of defects (Stone-Wales (SW) defects, bivacancies, nitrogen substitution impurities, and zigzag edges) by density functional calculations. We have studied also an effect of finite width of graphene nanoribbons on their chemical properties. It is shown that magnetism at graphene edges is fragile, with respect to oxidation, and, therefore, chemical protection of the graphene edges may be required for the application of graphene in spintronics. At the same time, hydrogenation of the SW defects may be a prospective way to create magnetic carbon.