So, Alchemist usually gets Brew Potion at level 1. Big point for the class. However some Archetypes trade it away for another power or skill. Currently I'm doing this(Taking crypt breaker: trade Brew potion for trapfinding). This is okay, I'll just pick it up at level 3...,
And then I recall the problem of Alchemists aren't technically casters for the sake of feats and other things. So while I can't find it(My google-fu is weak late at night), is this true? That Alchemists can't pick Brew Potion if they trade it away?
The class feature is necessary to bypass the normal prerequisites of the feat, and to allow you to use extracts in the place of spell slots. So yes, if you trade off the Brew Potion class feature you cannot just buy it back as a feat.