Question
Arrange the nucleophilicity (rate of SN2 reactivity) of
(i) H2O, OH–, CH3COO- and CH3O-
(ii) NH3 and PH3.
Solution
(i) When the nucleophilic site has the same atom (here an O), nucleophilicity follows basicity. Thus
(ii) When the attacking atoms are different but in the same periodic family, the one with the largest atomic mass is the most reactive. Therefore, PH3 > NH3. This order is the reverse of basicity.
(ii) When the attacking atoms are different but in the same periodic family, the one with the largest atomic mass is the most reactive. Therefore, PH3 > NH3. This order is the reverse of basicity.
