code: public static IAtomContainerSet getAllReactants(IReaction reaction) {
          IAtomContainerSet moleculeSet = reaction.getBuilder().newInstance(IAtomContainerSet.class);
          IAtomContainerSet reactants = reaction.getReactants();
          for (int i = 0; i < reactants.getAtomContainerCount(); i++) {
              moleculeSet.addAtomContainer(reactants.getAtomContainer(i));
          }
          return moleculeSet;
      }
docs: get all reactants of a IReaction
  
  @param reaction  The IReaction
  @return The IAtomContainerSet