WordNet (r) 3.0 (2006):

unrealizable
    adj 1: impossible to achieve; "an unattainable goal" [syn:
           unachievable, unattainable, undoable,
           unrealizable]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "unrealizable":
   beyond one, impracticable, impractical, infeasible, inoperable,
   insuperable, insurmountable, too much for, unachievable,
   unattainable, uncompassable, undoable, uneffectible, unfeasible,
   unnegotiable, unperformable, unsurmountable, unworkable