why is my 2005 jeep liberty smoking under the hood when we drive with the ac on

Hi, I have a 2005 jeep liberty sport. When we got the car around 5 years ago
the ac would only blow on two speeds. Within the last year or so the car has
probably been driven a total of 15 times. We are now trying to sell it so we
took it out for a test ride yesterday, the ac wasn't great and would only work
while you were driving. Today I drove it around the block and tried to test out
the heating and cooling and hardly anything came out of the vent, but once I
got home and parked smelly white smoke started coming from under the
hood. now you can't have the ac turned on while driving or the car will
smoke.