I replaced a kitchen sink and the p-trap. I had to make some minor adjustments to the configuration as the new sink drain was too close to the outlet pipe for the old p-trap to fit. Now when I turn on the disposer, it backs up into the opposite side of the sink. Any ideas?

the new sink drain was too close to the outlet pipe for the old p-trap to fit Let's hear more about that. If the old trap wouldn't hook up what did you install in place of it?

when I turn on the disposer, it backs up into the opposite side of the sink.You forgot to mention the sink was full of water and wouldn't drain. When both tubs have water turning on the disposal will act like a centrifugal pump and pump from the disposable to the other tub. Does your sink drain or do you have a blockage? Give us more information. Back to you, Tom