Question

With proper reasoning, arrange the following resonance structures in order of decreasing stability.
I. `H_(2)C = overset(+)(N)=N^(-)` II. `H_(2)overset(+)(C)-N = N^(-)` III. `H_(2)C^(-) - overset(+)(N) -= N` IV. `H_(2)C^(-) - N = N^(+)`

Answer 1

In structure (I and III), each atom has an octent of electrons but in structure (II), C has a sextet of electrons while in structure (IV), N has a sextet of electrons. Thus, structures I and III are more stable than structures II and IV. Among structures I and II, structure (I) has a negative charge on the more electronegative N atom while structure (III) has a negative charge on the less electronegative C atom. Therefore, structure (I) is more stable than structure (III). Among structures (II and IV), structure (IV) carries a +ve charge on the more electronegative N atom while structure (II) has a +ve charge on the less electronegative C atom. Therefore, structure (II) is more stable than structure (IV). Thus, the overall stability decreases in the order : `I gt III gt II gt IV`.