put behind bars vs imprison

put behind bars and imprison both are verbs.

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

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

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

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