  Our main goal in this paper is to translate the diagram relating groups,
  Lie algebras and Hopf algebras to the corresponding 2-objects, i.e. to
categorify it. This is done interpreting 2-objects as crossed modules and
showing the compatibility of the standard functors linking groups, Lie algebras
and Hopf algebras with the concept of a crossed module. One outcome is the
construction of an enveloping algebra of the string Lie algebra of Baez-Crans,
another is the clarification of the passage from crossed modules of Hopf
algebras to Hopf 2-algebras.
