A group which departs from the substrate in a nucleophilic substitution reaction is called a leaving group. The ease of departure is determined by the acidity of the conjugate acid of the leaving group; higher the acidity, better the leaving group. The correct order of reactivity of the following compounds in a given nucleophilic reaction is

$$\mathrm{R-Cl>R-OCOCH_3>R-OSO_2CH_3>RI}$$

$$\mathrm{R-OSO_2CH_3>R-Cl>R-OCOCH_3>ROH}$$

$$\mathrm{R-I>RNH_2>R-OCOCH_3>R-OSO_2CH_3}$$

$$\mathrm{R-Br>R-OSO_2CH_3>R-OCOCH_3>ROCH_3}$$