Antonyms of adj effectual

2 senses of effectual

Sense 1
effective (vs. ineffective), effectual, efficacious -- (producing or capable of producing an intended result or having a striking effect; "an air-cooled motor was more effective than a witch's broomstick for rapid long-distance transportation"-LewisMumford; "effective teaching methods"; "effective steps toward peace"; "made an effective entrance"; "his complaint proved to be effectual in bringing action"; "an efficacious law")


Sense 2
legal, sound, effectual -- (having legal efficacy or force; "a sound title to the property")

INDIRECT (VIA valid) -> invalid -- (having no cogency or legal force; "invalid reasoning"; "an invalid driver's license")

Similarity of adj effectual

2 senses of effectual

Sense 1
effective (vs. ineffective), effectual, efficacious -- (producing or capable of producing an intended result or having a striking effect; "an air-cooled motor was more effective than a witch's broomstick for rapid long-distance transportation"-LewisMumford; "effective teaching methods"; "effective steps toward peace"; "made an effective entrance"; "his complaint proved to be effectual in bringing action"; "an efficacious law")
       => hard-hitting, trenchant -- (characterized by or full of force and vigor; "a hard-hitting expose"; "a trenchant argument")
       => impelling -- (markedly effective as if by emotional pressure; "impelling skill as a teller of tales"; "an impelling personality")
       => impressive, telling -- (producing a strong effect; "gave an impressive performance as Othello"; "a telling gesture")
       => rough-and-ready -- (crude but effective for the purpose at hand)
          Also See-> efficacious#1; efficient#1; potent#3, strong#4, stiff#4; powerful#1; useful#1, utile#1

Sense 2
legal, sound, effectual -- (having legal efficacy or force; "a sound title to the property")
       => valid (vs. invalid) -- (well grounded in logic or truth or having legal force; "a valid inference"; "a valid argument"; "a valid contract")