immediately vs straightaway

immediately is not a noun while straightaway is a noun.

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

As adverbs, immediately and straightaway are synonyms defined as:

immediately and straightaway: without delay or hesitation; with no time intervening

Other synonyms of immediately include at once, directly, forthwith, instantly, like a shot, now, right away, straight off.

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

immediately (adjective)	straightaway (adjective)
	performed with little or no delay

immediately (adverb)	straightaway (adverb)
without delay or hesitation; with no time intervening	without delay or hesitation; with no time intervening
near or close by
bearing an immediate relation