prosaic vs commonplace

prosaic is not a noun while commonplace is a noun.

Word	Noun	Adjective	Verb	Adverb
prosaic	No	Yes	No	No
commonplace	Yes	Yes	No	No

As adjectives, prosaic and commonplace are synonyms defined as:

Other synonyms of prosaic include humdrum, unglamorous, unglamourous.

prosaic (noun)	commonplace (noun)
	a trite or obvious remark

prosaic (adjective)	commonplace (adjective)
not challenging; dull and lacking excitement	not challenging; dull and lacking excitement
lacking wit or imagination	repeated too often; overfamiliar through overuse
not fanciful or imaginative	completely ordinary and unremarkable