impromptu vs extemporaneous

impromptu is a noun but extemporaneous is not a noun.

Word	Noun	Adjective	Verb	Adverb
impromptu	Yes	Yes	No	Yes
extemporaneous	No	Yes	No	No

As adjectives, impromptu and extemporaneous are synonyms defined as:

impromptu and extemporaneous: with little or no preparation or forethought

Other synonyms of impromptu include ad-lib, extemporary, extempore, off-the-cuff, offhand, offhanded, unrehearsed.

impromptu (noun)	extemporaneous (noun)
an extemporaneous speech or remark
a short musical passage that seems to have been made spontaneously without advance preparation

impromptu (adjective)	extemporaneous (adjective)
with little or no preparation or forethought	with little or no preparation or forethought

impromptu (adverb)	extemporaneous (adverb)
without advance preparation