straightaway vs immediately

straightaway is a noun but immediately is not a noun.

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

As adverbs, straightaway and immediately are synonyms defined as:

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

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

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

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

straightaway (adverb)	immediately (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