The word prearranges means to plan or organize something beforehand. Its antonyms are words that denote the opposite meaning. One such antonym is "improvise," which means to make things up as one goes along, without prior planning or preparation. Another antonym is "spontaneous," which implies a natural and impulsive action or decision-making. Other words that are antonyms of prearranges include "disorganize," "disrupt," and "disturb," which denote chaos or lack of order. In essence, the antonyms of prearranges all imply a lack of planning or structure, highlighting the importance of organization and prior preparation in achieving success.