If there is a small amount of freon, it is possible to run the AC, but it is not recommended. The AC has been set to run with a certain amount of freon, and without it's impossible for the AC to operate as it should. It restricts the heat the air conditioner can absorb along the evaporator coil before the coil freezes over. The ice will keep growing until the AC is destroyed. A lack of freon will also lead to damage to the compressor, which will cause it to overheat.