Tacking as onto the end of where predictably has an effect on the meaning of its root word. But unlike disinterested and uninterested, you can’t really ever get away with using whereas in a situation that calls for where—or vice versa—without confusing your audience.
Because of the prevalence and versatility of the word where, English speakers generally do a good job of using it correctly without too much thought. If you’re inquiring about the location of a bathroom, you probably wouldn’t accidentally say, “Whereas is the bathroom?” If you’re talking about location at all, the term you want is almost definitely where.
So where should you use whereas? As Grammarist explains, whereas comes before a dependent clause that you’re trying to connect with a contrasting independent clause. If that’s a little too jargon-y to make any sense, this example should help. Let’s say you start with this independent clause: We used to go clubbing on Saturday nights. Like all independent clauses, it’s a complete sentence on its own. But maybe you want to elaborate by explaining what your Saturday nights look like now. Since there’s going to be a contrast between what you used to do and what you now do, whereas can tie the two together:
We used to go clubbing on Saturday nights, whereas now we stay home and binge-watch Doctor Who.
Whereas isn’t the only conjunction that can accomplish this type of clausal union—while, although, and even but can work, too (though you might want to reorder things to make the sentence flow better).
To make this slightly more confusing, whereas also has a history of being used to introduce legal documents in a way that basically means “considering that.” For contracts, this introductory section—which essentially states the purpose of the document—is called a recital; for constitutions, it’s a preamble. In fact, kicking off a legal document with whereas was common enough in the past that people just started using whereas as a noun that meant preamble. Feel free to try to bring that trend back in your next casual conversation about the Constitution.