Chemoselective reagents constitute a very small group. So it is very difficult to find a chemoselective reagent for an Functional group introduction in a polyfunctional conpound. In practice a functional group (FG) is protected from unwanted attack during reactions at other functions. The protection is given to the Fg by converting it to a suitable derivative. The desired FGI is then carried out in on the protected molecule and finally the protected FG is brought back to its original form. For example, if we want to carry out a nitration reaction on aniline to get p-nitroaniline, we cannot perfoem the reaction directly on aniline because undesirable products will be obtained owing to the activating effect of and coordination of a proton to amino group. Therefore, the amino group is protected before nitration. After the completion of nitration the amino group is deprotected. In this case the amino group is protected by converting it to an amide,and deprotected by hydrolysing p-nitroacetanilide as follows