然而 is a conjunction, while 反而 is an adverb. So 然而 may be placed before a subject, but 反而 may only be placed before a predicate. For example:
“然而” is only used as a transition, often in concert with “虽然”, “反而”not only is used as a transition,but also expresses the meaning of exceeding one’s expectation or going against the common sense. Furthermore, it also means to develop to the opposite direction, so it often used in concert with“不但不/没”、“不仅不/没”.