Question

The correct order of stabilities of the following resonance structures is :
(I) `H_2 C = overset (oplus) N = overset (Ө) N`
(II) `H_2 overset (oplus) C - N = overset (Ө) N`
(III) `H_2 overset (Ө) C - overset (oplus) N -= N`
(IV) `H_2 overset (Ө) C - N = overset (oplus) N`.
A. `(III) gt (I) gt (IV) gt (II)`
B. `(I) gt (III) gt (II) gt (IV)`
C. `(I) gt (II) gt (IV) gt (III)`
D. `(II) gt (I) gt (III) gt (IV)`

Answer 1

Correct Answer - B
Both (I) and (III) have more number of covalent bonds than `(II)` and `(IV)`. But (I) is the most stable as it has negative charge on more electronegative N atom while (III) has negative charge on less electronegative C atom. In both (I) and (III), the octet of every atom is complete. Thus, (III) is the second most stable.
Both (II) and (IV) have equal number of covalent bonds but (II) is more stable as it has negative charge on more electronegative N while (IV) has negative charge on less electronegative C atom. Note that in (II), the octet of C is incomplete while in (IV), the octet of N is incomplete. Thus, the correct stability order is
`IgtIIIgtIIgtIV`