Is there a code that states there has to be a certain distance from the back of the toilet to the wall? I know from the shower to the toilet it is 15".

If you're asking for the rough in dimensions of toilet it is 12" inches from the wall line to the center of the closet flange. This will put that back part of the toilet tank almost touching the wall. The plumbing code does not address the distance between the tank and the wall. If this doesn't answer your question or is not what you're asking. Just click on back. Regards, Tom

I have a new home and my master bath toilet sits about 2 1/4 " from the wall. How would I measure this to see if it was installed properly? I have another bath room and the toilet sits close to the wall as you describe originally.

The caps that cover closet bolts that secure the bowl to the  floor are  the center that you measure from.  You should have 12",  give or take 1/2",  from the center of the closet bowl caps to the wall.  It sounds like you have 14 1/2" out from the wall.  To remedy this the toilet will have to come up, then the old toilet flange removed and a 2" offset toilet flange installed.  This error should have been caught by the contractor if not by the plumber who did the work.  Call your contractor and have him contact the plumbing company to correct this error.   This should be done at no charge to you, the home owner.   Good luck,  Tom