why is there a difference in a reaction when it takes place in a neutral or acidic or basic medium instead of it’s normal state?

@ akshat 
in all the 3 medium every reaction cannot happen , and each reaction have its specific temp and ph requirment .. 
so, they all are differ in their ph value , the ph value will affect adversely in the reaction . 
refer to the examples below . 
Equation Reduction of KMnO4 in acidic medium:
8H(+) + MnO4(2-) + 5e(-)  ---------->  Mn(2+) + 4H2O
in neutral medium 
2H2O + MnO4(-) + 3e(-) ------------> MnO2 + 4OH(-)
in besic medium 
MnO4(-) + e(-) ------------> MnO4(2-)
so, they all have diffrent mechanism in all the three medium .
HOPE IT CLEARS YOUR DOUBT