brain vs mental capacity

brain and mental capacity both are nouns.

Word	Noun	Adjective	Verb	Adverb
brain	Yes	No	Yes	No
mental capacity	Yes	No	No	No

As nouns, brain and mental capacity are synonyms defined as:

Other synonyms of brain include brainpower, learning ability, mentality, wit.

brain (noun)	mental capacity (noun)
that part of the central nervous system that includes all the higher nervous centers; enclosed within the skull; continuous with the spinal cord	mental ability
mental ability
that which is responsible for one's thoughts, feelings, and conscious brain functions; the seat of the faculty of reason
the brain of certain animals used as meat
someone who has exceptional intellectual ability and originality

brain (verb)	mental capacity (verb)
hit on the head
kill by smashing someone's skull