In strong acid there cannot be any strong bases and in strong bases there cannot be any strong acids. For example, it is impossible to have hydroxide anion in sulfuric acid, and it is impossible to have hydronium in hydroxide solution (they would be neutralized immediately). In neutral conditions there are no strong acids or strong bases. Carbocations will not form under basic conditions because the base is likely to attack the substrate quickly before a carbocation can form (e.g. in the E2 process).