jail vs put behind bars

jail is a noun but put behind bars is not a noun.

Word	Noun	Adjective	Verb	Adverb
jail	Yes	No	Yes	No
put behind bars	No	No	Yes	No

As verbs, jail and put behind bars are synonyms defined as:

Other synonyms of jail include gaol, immure, imprison, incarcerate, jug, lag, put away, remand.

jail (noun)	put behind bars (noun)
a correctional institution used to detain persons who are in the lawful custody of the government (either accused persons awaiting trial or convicted persons serving a sentence)

jail (verb)	put behind bars (verb)
lock up or confine, in or as in a jail	lock up or confine, in or as in a jail