The growth of graphene nanoribbons has been widely investigated on metal surfaces in an ultrahigh vacuum. Here, we re-investigate the growth of graphene nanoribbons obtained by thermal annealing of 9,9'-bianthryl derivatives on a Cu(111) surface by using scanning tunnelling microscopy. On the basis of our results, we propose to complete the reaction mechanism commonly accepted in the literature by adding an intramolecular hydrogen atom transfer from the 2,2'-positions to the 10,10'-positions as a key-step in the formation of (3,1)-graphene nanoribbons on a Cu(111) surface.