Antenna CPW internal layer
Is it possible to place a CPW in an internal layer? if so what should
  I take in consideration?


I'm assuming that this is intended as a transmission line feed to an antenna and not the actual antenna.

If so, it becomes a stripline like this: -



And you can use various calculators to get answers. I took the above picture from here. There are plenty of on-line calculators that can give answers to your problem.

If you don't have a ground plane either side but have component tracking on the top layer it can get messy.