immediate vs straightaway

immediate is not a noun while straightaway is a noun.

Word	Noun	Adjective	Verb	Adverb
immediate	No	Yes	No	No
straightaway	Yes	Yes	No	Yes

As adjectives, immediate and straightaway are synonyms defined as:

Other synonyms of immediate include prompt, quick.

immediate (noun)	straightaway (noun)
	a straight segment of a roadway or racecourse

immediate (adjective)	straightaway (adjective)
having no intervening medium	performed with little or no delay
very close or connected in space or time
performed with little or no delay
of the present time and place
immediately before or after as in a chain of cause and effect

immediate (adverb)	straightaway (adverb)
	without delay or hesitation; with no time intervening