Is r600a a replacement for r22?

It is an alternative to mostly banned CFCs (most infamous of which is R22), and to be phased out in the future HFCs (R410a, R134 and similar). That being said the general consensus is that R600a (isobutane) requires compressors/systems specifically designed for it. The most suitable HC or natural refrigerant substitute for R22 is R290, which although shunned by the industry will perfectly replace it with an efficiency increase of about 20% without any retrofitting or other adjustments.