Tips on Unscrewing Kitchen Sink Waste Pipe?

I'm replacing my old kitchen sink and all the plumbing underneath it. I'll probably use all plastic/PVC for the drain pipes. Anyway, everything has gone reasonably well until I tried to unscrew the metal waste pipe that runs from the trap to the main sewer pipe in the wall. I'm sure it's been in there since the house was built in 1959 and I can't get it to budge, even with a 24" pipe wrench.
This particular one looks like very tarnished copper, about 7" long and 1 1/2" in diameter. It appears to be sweat soldered on to a 2" bushing or threaded male adapter, which is threaded into the main pipe in the wall. They used a lot of pipe joint compound on the threads, and it's hardened. I've squirted Liquid wrench on in 3 times, but since it's running horizontially I don't think it's penetrating very far.
Any of you professional or experienced amateur plumbers got any tips on how to deal with it?
I've been using the pipe wrench near the trap, so I won't accidentally damage the new hot & cold water plumbing. Also, I've got more room to work out there. Would I be able to apply more torque closer to the wall (connection)? Not as much room to work close to the wall, because of the other plumbing.
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On Tue, 22 Sep 2009 21:13:31 -0700 (PDT), Josh

I'm an amateur and not even an amateur plumber, just a dilettante of all trades. I usually forget to consider a propane torch, but I too think it might be a big help. I think it might not just soften the stuff, but actually break it down. Maybe put a sheet of metal against anything that could be damaged by the heat, although if you're careful that shouldn't be necessary.
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Thanks guys.....That's exactly what the guy who installed my new countertop (minus sink) recommended I do this morning. I found one like you referenced at Lowe's. It says it's for 1 1/2 inch pipes of similar or dissimilar material but it seems a lot bigger than that. Hopefully it will tighten up snug around the cut off copper waste pipe and the new PVC from the trap.
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A friend of mine is a social worker. He describes what he calls "vertical thinking" as: if it isn't working, do it harder. "Horizontal thinking" is if it isn't working, try something different.
You just provided a perfect example of horizontal thinking. And, I do greatly respect that.
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Christopher A. Young
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Please check your shipping crate. Your coupling is missing an o, and an i. Might be in the bottom of the package.
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I've seen internal pipe wrenches for smaller pipes. Be nice if he could get internal pipe wrench, and then impact wrench to rattle it loose.
The sawzall and cuplr idea is a good one, too.
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Christopher A. Young
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