Would I be allowed to buy a bottle of scotch at frankfurt airport?

Question

And carry it with me on the plane to the US?

Answer 1

If you buy it inside security (i.e. at Duty Free), you can carry it on to the plane to the US.

However, if you're connecting to another flight in the US, you'll have to move it to your checked luggage when you go through customs.  Even if the airport shop puts it in their special clear sealed bag with the receipt, showing that you bought it in the airport, you still have to check it before your onward US flight (You can put it in your luggage when you claim it at customs).  When you clear security again after customs, they will not let you carry it through.